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[Palaeontology, Vol. 56, Part 3, 2013, pp. 523–556]
WETLAND MEGABIAS: ECOLOGICAL AND
ECOPHYSIOLOGICAL FILTERING DOMINATES THE
FOSSIL RECORD OF HOT SPRING FLORAS
by ALAN CHANNING and DIANNE EDWARDS
School of Earth and Ocean Sciences, Cardiff University, Wales, CF10 3AT, UK; e-mails: [email protected], [email protected]
Typescript received 10 October 2012; accepted in revised form 5 March 2013
Abstract: Siliceous hot spring deposits form at Earth’s sur-
examples. Evidence of the partitioning of wetland hydrophytic and dryland mesophytic communities, a feature of
active geothermal areas, is provided by well-preserved and
well-exposed fossil sinter apron complexes, which record
flooding of dryland environments by thermal waters and
decline of local forest ecosystems. Such observations from the
fossil record back-up hypotheses based on active hot springs
and vegetation that suggest removal of taphonomic filtering in
hot spring environments is accompanied by an increase in ecological and ecophysiological filtering. To this end we also demonstrate that in the hot spring environment, the wetland bias
extends beyond broad ecology. We show that ecosystems preserved from the Cenozoic to the Mesozoic provide clear evidence that the dominant plants preserved in situ by hot spring
activity are also halophytic, tolerant of high pH and high concentrations of heavy metals. By extension, we hypothesize that
this is also the case in Palaeozoic hot spring settings and
extended to the early land plant flora of the Rhynie chert.
face above terrestrial hydrothermal systems, which create
low-sulphidation epithermal mineral deposits deeper in the
crust. Eruption of hot spring waters and precipitation of
opal-A create sinter apron complexes and areas of geothermally influenced wetland. These provide habitat for higher
plants that may be preserved in situ, by encrustation of their
surfaces and permineralization of tissues, creating T0 plant
assemblages. In this study, we review the fossil record of hot
spring floras from subfossil examples forming in active hot
spring areas, via fossil examples from the Cenozoic, Mesozoic and Palaeozoic to the oldest known hot spring flora, the
Lower Devonian Rhynie chert. We demonstrate that the
well-known megabias towards wetland plant preservation
extends to hot spring floras. We highlight that the record of
hot spring floras is dominated by plants preserved in situ by
permineralization on geothermally influenced wetlands. Angiosperms (members of the Cyperaceae and Restionaceae)
dominate Cenozoic floras. Equisetum and gleicheniaceous
ferns colonized Mesozoic (Jurassic) geothermal wetlands and
sphenophytes and herbaceous lycophytes late Palaeozoic
Key words: silicification, taphonomy, geothermal wetland,
epithermal Au–Ag, fossil, ecosystem.
I N general, plant fossils are preserved only where sediments accumulate. In the vast majority of cases, this is in
a wet environment where water transports and deposits
sediment. Plant preservation potential in surface environments is negatively affected by oxidation at the surface
and in the subsurface vadose zone, but increases dramatically in the presence of surface water bodies, high water
tables or with rapid burial to below the vadose zone
(Gastaldo and Demko 2011). These positive taphonomic
circumstances are met more frequently in humid climatic
environments than in arid or semi-arid environments and
where sedimentation rates are high. As a consequence,
wetland floras preserved in basinal environments during
humid climatic intervals dominate much of the plant
fossil record. This megabias is exemplified by the wellknown and widespread Late Palaeozoic clubmoss (lyco-
phyte)-dominated coal-swamp floras, which formed in
wet humid climates in lowland coastal plain environments. These contrast with relatively less well-known coeval conifer, cordaitalean, pteridosperm, fern floras
associated with dryland environments within the same
lowland setting and with floras of time periods between
coal formation with more seasonally dry climate, evaporation dominated surface environments and lower water
tables (DiMichele et al. 2010; Falcon-Lang et al. 2011).
‘T0’ is a term used to describe fossil vegetation, which
was preserved in essentially the same spatial conformation
as in life and that has undergone little or no taphonomic
filtering following plant death. Geologically instantaneous
events, such as rapid flooding or burial, are required for
the formation of T0 assemblages. Settings and contexts
that are especially favourable for their origin include
© The Palaeontological Association
doi: 10.1111/pala.12043
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PALAEONTOLOGY, VOLUME 56
volcanic ashfalls, drowning of coastal plains by rapid relative sea-level rise and, at smaller scales, rapid sedimentation associated with fluvial environments such as channel
bars, crevasse splays and distributary lobes (DiMichele
and Falcon-Lang 2011). DiMichele and Falcon-Lang
(2011), having reviewed the Pennsylvanian record of such
assemblages, noted that the vast majority of Palaeozoic T0
assemblages were formed in wetland settings at, or close
to, sea level, whereas drylands and uplands are poorly
represented. Such autochthonous plant assemblages may
preserve vegetation in situ on its growth substrate allowing unique insights into plant palaeoecology. For example, T0 assemblages can provide evidence for whole-plant
reconstructions and estimates of plant density, canopy
height, productivity, plant hydraulics, cohort dynamics,
spatial heterogeneity, ecological gradients, tree–sediment
interactions and animal–plant interactions (DiMichele
and Falcon-Lang 2011).
The observation that the plant fossil record has a broad
bias to wetland preservation and that wetland settings
host many of the most informative T0 fossil floras suggests that our knowledge of plant evolution, palaeoecology and hence reconstructions of ancient vegetation is
skewed towards inhabitants of such environments
(DiMichele et al. 2010; DiMichele and Falcon-Lang 2011;
Falcon-Lang et al. 2011). The wetland versus dryland
preservation bias and bias towards wetland T0 assemblages occur at global, regional and local (deposit) scale,
and in this study, we consider the preservation potential
of plants and the ecological signal provided by fossils in a
set of unique T0 environments, viz. those associated with
terrestrial silica-depositing hot springs. This less widespread and less common, although equally important, set
of T0 environments or assemblages has a fossil record
extending from active thermal areas at the present day to
the early land plant flora of the Lower Devonian, Rhynie
chert, and possibly beyond to microbial communities of
the Precambrian (Walter 1996).
The Rhynie chert (Lower Devonian of Aberdeenshire,
UK) is arguably the finest example of such an assemblage.
At Rhynie, the discharge of silica-rich geothermal fluids
from hot spring vents created siliceous chemical sedimentary rocks (sinter) as opaline silica (opal-A) precipitated
from the hot spring water that flowed away from vents
into areas of plant growth. Sinter deposition entombed a
diverse early land plant flora, including the earliest welldocumented plant of lycophyte affinity (Asteroxylon), rhyniophytes (e.g. Rhynia) and those of less certain relationships (e.g. Horneophyton, Aglaophyton). At Rhynie, plants
with an erect in-life habit were preserved in great numbers, anatomically in three dimensions and to the cellular
level, as the silica-laden hot spring waters permeated plant
structures and cells (Channing and Edwards 2009a and
references therein). The Rhynie assemblage provides our
best insight into early terrestrial ecosystems. It underpins
many inferences of the palaeoecophysiology of the plants
and of their taxonomic affinities, plus the broader evolutionary patterns in basal tracheophytes (Channing and
Edwards 2009a). As with other T0 environments, the Rhynie chert also captures other elements of the local ecosystem including the earliest body-fossil evidence of certain
groups of insects, continental aquatic crustaceans, arachnids, algae, lichens and fungi. Interactions between the
various elements of the biota, including early evidence of
parasitism (chytrid fungi on aquatic algae), mutualism
(mycorrhizal fungi within plants), symbioses (lichen),
herbivory (mite coprolites), saprotrophism (chytrid
fungi), detritivory (collembolans, myriapod coprolites)
and predation (trigonotarbid arachnids and centipedes),
provide a snapshot picture of a relatively complex and, in
many respects, modern looking trophic web (Habgood
et al. 2004; Channing and Edwards 2009a and references
therein).
In earlier papers (Channing 2003; Channing and
Edwards 2004, 2009a, b), we have hypothesized that,
based on plant silicification and sinter accretion rates,
plant numbers and density, aerial extent and longevity of
conditions conducive to plant preservation, the most
important environment for in situ permineralization-type
plant preservation associated with hot spring settings is
geothermally influenced wetland. Latterly (Channing and
Edwards 2009a), we have synthesized information on the
palaeoenvironments recorded in the Rhynie chert, plus
anatomical and autecological data from the preserved
plants and compared these features with active analogue
environments and flora at Yellowstone. We concluded
that many of the Rhynie plants colonized wetlands at the
low-temperature fringes of the hot spring system and
were versatile, but physiologically highly specialized, capable of withstanding osmotic and chemical stresses in a
dynamic environment, and were probably out-competed
by mesophytic vegetation elsewhere.
Here, we review the fossil record of hot spring floras
from the intervening c. 400-million-year period. We augment the limited published accounts of hot spring floras
with new, although admittedly often initial, observations
and plant identifications and include information from
mineral exploration industry grey literature and unpublished works of further research targets. We summarize
the dominant ecology of plants preserved in the hot
spring record. The plant fossils we describe are overwhelmingly those adapted to wetland settings, with lesser
evidence of dryland groups, further suggesting the influence of the broad-scale wetland megabias in hot spring
floras. Finally, we discuss physicochemical aspects of hot
spring environments and ecophysiological features common to the dominant plant groups preserved through
time by comparison with extant close relatives.
CHANNING AND EDWARDS: ECOLOGICAL FILTERING OF HOT SPRING FLORAS
HOT SPRING DEPOSITS AND T 0
CONDITIONS
Silica-depositing hot springs (Fig. 1A) are the surface
expression of terrestrial hydrothermal systems. They occur
where meteoric water descends into the subsurface,
becomes heated by magma or cooling plutons, equilibrates chemically with silicic rocks through which it is
flowing and then, as it is hot and buoyant, ascends to the
Earth’s surface.
The primary feature of a T0 plant assemblage, in situ
preservation of plant communities, occurs in the hot
spring environment because ascending water within the
hydrothermal system is rich in dissolved silica. Eruption
of water from a spring vent is accompanied by cooling
(from c. 100°C towards ambient temperatures), which
forces dissolved silica to become supersaturated. Hence,
as water flows into surface environments, amorphous silica (opal-A) precipitates. This forms siliceous chemical
rocks, which create sinter mounds and aprons around
vents (Fig. 1A). In addition, as water flows into areas of
microbe and plant growth, opal-A deposition can entomb
the local ecosystem in situ.
A second feature of T0 plant assemblages, rapid preservation, occurs in the hot spring environment because
opal-A precipitation and sinter deposition occur almost
instantaneously on eruption. Two modes of preservation
are discernible in active settings and in the fossil record.
The first, preservation of morphology as external moulds,
occurs extremely rapidly because opal-A precipitates readily on solid surfaces of materials immersed even temporarily in geothermal fluid. Plant surfaces (and microbial
communities living on them) may be rapidly encrusted
by this mechanism over periods of days to weeks, and in
active hot spring basins, it is relatively easy to find examples of still-living plants with areas of stems/branches
enveloped in sinter coatings.
The quality of plant preservation in the hot spring
environment may go beyond the level of capture seen in
most clastic and volcaniclastic settings (where compression or impression preservation of nonwoody tissues
and organs dominates) because anatomical preservation
by silica permineralization is the second major preservational mode in hot spring settings. The classic example
of this comes from blocks of Rhynie chert that contain
dense, in situ stands of Lower Devonian land plants
such as Rhynia or Aglaophyton with near-perfect preservation of the plants’ parenchymatous anatomy (Trewin
1996). Taphonomic experiments in active hot spring
pools and sinter aprons at Yellowstone (Channing and
Edwards 2004) show that permineralization of small
stems of the sedge Eleocharis occurs on the scale of
months, with complete permineralization being achieved
within 11 months.
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Preservation observed in the hot spring fossil record
ranges from large, decay-resistant organs such as ligninrich tree trunks and branches, to small herbaceous plants
comprising relatively decay-prone parenchymatous tissues,
to delicate plant gametophytes, and fungal, algal or bacterial structures. This means that, in combination, silica
encrustation and permineralization by hot spring fluids
are capable of capturing near-intact, taphonomically
unfiltered hot spring ecosystems.
The removal of taphonomic filtering in hot spring settings allows a pair of important and related observations.
Firstly, if a plant group occurred in a hot spring subenvironment where other plants are preserved, evidence of
that plant group should also be present. Conversely,
absence of a plant group from a subenvironment is a real,
rather than taphonomic, absence and more likely a result
of ecological partitioning. We will develop this idea further below by reviewing plant preservation environments
of active hot springs and the fossil record of hot spring
floras.
PLANT COMMUNITIES AND
TAPHONOMY OF HOT SPRING
SUBENVIRONMENTS
There is an inescapable requirement for the preservation
of an organism by silicification within a hot spring setting; the microbe, plant or animal has to be immersed in
silica-rich geothermal water for either mouldic- or permineralization-style preservation to progress. This means
that the presence of an organism in an ancient hot spring
sinter indicates that it was within a wet environment during the course of preservation at least. The observation
that rates of both mouldic- and permineralization-type
preservation occur on the scale of months, rather than
days or weeks, suggests that immersion has to be relatively protracted. In addition, for three-dimensional preservation of collapse-prone cells and tissues such as
parenchyma, extended periods of drying must be prevented (Channing and Edwards 2004, 2009b). Silicification of an organism does not occur in isolation and
chemical precipitation of opal-A is also ongoing, and as
distinctive sinter macro- and microfabrics form in different subenvironments of a hot spring complex (discussed
below), its fossil occurs within a matrix recording the palaeoenvironmental conditions of the site of preservation.
Observations of active thermal areas reveal three main
routes for plant material to be incorporated into sinter
deposits. Allochthonous and potentially para-autochthonous plant organs shed from local vegetation (either
within the hot spring complex or beyond the margins)
and transported into an environment of silicification
include twigs and branches, angiosperm leaves, conifer
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needles as well as reproductive structures such as cones,
seeds and pollen. These may occur within any area of the
hot spring system. Preservation state is variable and
related to subenvironment. On aprons where drying and
A
B
oxidation prevail, in general, preservation is confined to
external moulds and permineralization of degraded and
collapsed organs, whilst in lower temperature and more
frequently wet settings, anatomical preservation may be
CHANNING AND EDWARDS: ECOLOGICAL FILTERING OF HOT SPRING FLORAS
observed. The context of preservation environment for
such material is provided by matrix chert macro- and
microfabrics.
Autochthonous preservation of ‘normal’ dryland and
wetland vegetation occurs by progradation of apron and
peripheral geothermal wetland margins into areas formerly
unaffected by geothermal fluids. This process creates a distinctive sedimentary sequence that is visible in the fossil
record. Typically, bases of vertical sections comprise clastic
sediments that show evidence of flooding by thermal
waters in the form of partial to pervasive silicification.
Some contain fragments of sinter apron material indicating off-apron fluid flow. Palaeosols may be associated with
these horizons and contain well-preserved root horizons,
and top surfaces may preserve in situ sapling and tree
stumps. Initial sinter horizons may preserve degraded
plant litter and better-preserved fallen trunks, branches
and shed foliar and reproductive organs from the recently
drowned vegetation. Given prolonged flooding of the
environment, geothermal wetland conditions develop, giving rise to the third route to preservation, fossilization of
plants actually colonizing cooler areas of geothermal discharge. Typically, in this environment, small herbaceous
plants replace arborescent forms and species diversity
declines. In situ permineralization of plants with intact
roots/rhizomes and upright stems is common. Sustained
input of thermal waters normally sees the encroachment
of sinter apron environments into the geothermal wetland
and many fossil flooding sequences are capped by laminated cherts with evidence of low- to mid-temperature
sinter aprons with microbial silicification fabrics.
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mounds and lower angle apron deposits. Areas proximal
to vents aggrade more rapidly than distal, giving sinter
deposits a convex lenticular morphology, and proximal
regions of the outflow complex tend to be elevated relative to apron margins, geothermal wetlands and surrounding clastic environments.
At the deposit scale, limits to the habitability of hot
spring subenvironments for various plant groups and
facies-dependent taphonomic conditions produce a
broadly concentric (but sometimes mosaic-like) range of
variability in the quantity and quality of palaeobotanical
content of active and fossil sinters. Broadly, high temperature or topographically high and dry regions close to
vents offer poor habitat to higher plants (Fig. 1A), whilst
distal, lower-temperature, less steep areas of apron with
shallow water pools and low-lying apron margin wetlands
(Fig. 1B) offer progressively more habitable geothermally
influenced environments. In active hot spring areas of
Yellowstone, a suite of hot spring subenvironments provides habitats for communities of higher plants (those
most important with respect to potential preservation are
discussed in detail by Channing and Edwards (2009a)
and highlighted below).
Hot spring subenvironments
Vent pools. Vent water temperatures, which approach
boiling point in many examples, exclude all eukaryotic
organisms, and fluids are habitat only for archaea and extremophile bacteria (Brock 1994). The vast majority of
vent pools observed in active settings contain plant fragments from local vegetation but lack in situ plant communities. Cooler vent pools may form habitats for
communities of aquatic and semi-aquatic plants and such
a setting has been suggested for some plant preservation
within the Rhynie/Windyfield cherts (Trewin et al. 2003).
Hot spring sinter aprons (Fig. 1A–B) form around point
sources of thermal up-flow (vents), and silica deposition
is dominantly controlled by rapid, cooling-driven supersaturation of dissolved silica. As a consequence, most
precipitation occurs close to vents producing vent
Sinter aprons. Apron complexes may reach many hundreds of metres in diameter (Fig. 1B), and thicknesses in
excess of 10 m are not uncommon (Walter et al. 1996).
Depending on eruption style and frequency, apron morphology may develop various surface subenvironments,
Vent pool, sinter apron and geothermal wetland complex at Big Blue Hot Spring, Elk Park near Norris Geyser Basin, Yellowstone National Park, Wyoming, USA. A, vent pool and proximal region of sinter apron. Dryland mesophyte community (conifers and
dryland grasses) are partitioned from areas influenced by geothermal waters and occur topographically above the vent and apron complex. Vent pool (typical water temperature 65–75°C, pH 7.5–8, salinity c. 1200 ppm) is confined by a low silica sinter rim through
which three run-off streams flow. Silica precipitation from the out-flowing waters has produced a low-angled sinter apron deposit that
is habitat for mat-forming microbes (in areas of flowing water) and sparse higher plant vegetation (on interfluves). Shallow (cm scale)
sinter terraces can be seen within the run-off streams. At the lateral margin of the apron (foreground), water temperature falls to
below 30°C allowing colonization by the salinity, alkalinity-tolerant wetland plant Eleocharis rostellata. B, distal region of sinter apron
and geothermal wetland complex. Image taken in June 2011 at the beginning of the growth season of the wetland plants. Water flowing from the sinter apron enters the geothermal wetland at temperatures below 30°C (pH 8–9, salinity c. 1300 ppm) where it forms
sluggishly flowing shallow pools colonized by dense carpets of Eleocharis rostellata and less frequent patches of Triglochin maritimum.
Conifers that have been engulfed by geothermal water as the wetland margin prograded have been killed and shed their branches but
remain standing.
FIG. 1.
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most of which are dominated by bacteria. Where apron
surfaces have a shallow dip and where sheet-flow eruptions
are common, sets of shallow apron terraces (from a few
millimetres to tens of centimetres high) form (Walter et al.
1996). Water, in this area of the apron (Fig. 1A), cools to
temperatures which allow colonization by mat-forming
bacterial communities (Cady and Farmer 1996; Walter
et al. 1996). Silicification of mats forms distinctive internal
lamination and or bedding within developing sinter deposits which, because different microbial communities and
bacterial growth habits occur at different temperatures
and hydrodynamic settings, may be environment specific
(Walter et al. 1996; Jones et al. 1998). Where filamentous
microbes are entrained in the flowing water in apron environments, they become silicified to create distinctive streamer fabrics that are indicators of local flow direction
(White et al. 1989). When these become silicified, they
form strongly laminated sinter deposits, which are commonly preserved in the rock record and have distinctive
microfabrics that aid their recognition.
Dry areas of sinter aprons offer a poor substrate for
higher plant growth (Channing and Edwards 2009a, b).
They are essentially a hard, but relatively porous, monomineralic rock that proves difficult for roots to penetrate
(Channing 2001). As the only organic matter present in
these areas comprises low volumes of microbial material,
they have poor water retention properties and low nutrient availability (Channing and Edwards 2009a). In Yellowstone, colonization of abandoned areas of aprons is a
slow process, bryophytes and lichens are early colonizers
and alkali- and salinity-tolerant bunch grasses are not
uncommon (Channing and Edwards 2009a, fig. 2d).
Aprons generally remain essentially barren on the scale of
decades rather than years with trees eventually becoming
established on old, weathered and eroded apron surfaces
(Jones and Renaut 2003). Plant taphonomic potential is
low on frequently dry apron areas because sinter accretion, plant encrustation and permineralization rates are
too slow and commonly outpaced by oxidation and
microbial decay (Channing and Edwards 2009a, b).
Wet areas of sinter aprons may also be hostile environments for higher plant growth and high water temperatures in areas proximal to vents prevent colonization
(Channing and Edwards 2009a). However, where water
temperatures drop below c. 40°C, higher plants can colonize apron settings, and in Yellowstone (Fig. 1A–B), two
wetland species Triglochin maritimum (seaside arrow
grass) and Eleocharis rostellata (beaked spikerush) form a
sparse low-diversity vegetation (Channing and Edwards
2009a, fig. 1d, e). Plant preservation is controlled in this
setting by frequency of flooding by thermal waters, flow
rates and water depth. Deep, cool apron pools may have
relatively high preservation potential but shallow areas of
flow have much lower potential.
Geothermal wetlands. At the periphery of sinter aprons
(Fig. 1B), where water temperatures drop below c. 35–
40°C, extensive tracts of thermally influenced wetland
develop (Channing and Edwards 2009a, fig. 1d). These
provide habitat for large numbers of emergent aquatic
plants, typically in Yellowstone Triglochin maritimum and
Eleocharis rostellata. Relative to apron surfaces, plant
numbers and density are high; however, species diversity
remains low (Channing and Edwards 2009a, b). High initial plant numbers and constant immersion of plants during life and immediately following death mean that
geothermal wetlands are sites of widespread in situ preservation. In Yellowstone, they are by far the most important environment for in situ, permineralization-style
preservation (Channing and Edwards 2009a, b).
Mesic environments. Sharp boundaries (Fig. 1A–B)
between mesic (forested and grassland) and stressed
(aquatic and/or geothermally influenced) vegetation types
are evident around hot spring areas (Channing and
Edwards 2009a). At Yellowstone, Pinus contorta (lodgepole pine) forest surrounding geothermal areas is adapted
to growth on low-nutrient, monomineralic soils derived
from weathering of volcanic rocks. The species is a colonizer of abandoned sinter aprons. However, it is intolerant of high root temperatures and flooding and is
commonly killed in geothermal areas by both processes
(Fig. 1B; Channing and Edwards 2009a, fig. 2a–c). The
progradation of sinter aprons and geothermally influenced wetlands across mesic environments causes drowning of broad areas of lodgepole forest and meadows
(parkland). When inundation of forest areas occurs,
drowned trees may remain standing (Fig. 1B) and bases
of trunks and root systems may become silicified by permeating silica. However, geothermal wetland environments quickly develop between stumps (Channing and
Edwards 2009a, fig. 2c).
The biosedimentary features created by silicification of
microbial and higher plant communities create environmentally diagnostic sinter microfabrics. Identification of
such features in fossil hot spring deposits allows interpretation of palaeoenvironments and by linking lateral and
vertical facies changes, reconstruction of physical (e.g.
temperature) and chemical (e.g. pH, salinity) gradients or
changes over time. Excellent examples of publications
illustrating the concepts of hot spring facies interpretation
and palaeoenvironmental reconstruction, which underpin
the observation and interpretation of plant preservation
and environment of growth that follow, include those
from the Cenozoic (Campbell et al. 2001, 2003; Hampton
2002; Hinman and Walter 2005; Lynne et al. 2005, 2008;
Ertel 2009; Lynne 2012), Mesozoic (Guido and Campbell
2009, 2011, 2012; Guido et al. 2010; Channing et al.
2011) and Palaeozoic (Walter et al. 1996, 1998).
CHANNING AND EDWARDS: ECOLOGICAL FILTERING OF HOT SPRING FLORAS
Postdepositional processes
In clastic T0 environments, increasing accommodation
space is viewed as a prerequisite for rapid burial and
increased preservation potential. In the hot spring setting,
rapid burial may have an opposite taphonomic effect as
geothermal gradients in thermal areas may be extremely
steep leading to removal of organic structures by overpressured water in the form of steam and/or superheated
liquid. Ground temperatures at Yellowstone may exceed
70°C (Channing and Edwards 2009a), and down-hole
temperatures observed within sinter horizons encountered
at the top of research drill-holes have been observed at
80–90°C at 10–11 m below the surface (Bargar and
Beeson 1981; Guidry and Chafetz 2003).
Loss of biological information has been observed in
cores drilled in Yellowstone geothermal basins where even
woody plant materials buried only a few metres below the
current surface are preserved as external moulds or
‘ghosts’ of plants with few organic anatomical tissues persisting (Guidry and Chafetz 2003). Additionally in the
shallow subsurface, secondary opaline silica plus other
minerals including calcite, iron and magnesium oxides,
zeolites and clay minerals may be deposited filling former
porosity and overprinting primary textures. The effect of
overprinting on fidelity of preservation and ease of plant
identification is variable.
Most silica deposited by hot springs at the surface is
in the form of hydrated X-ray amorphous opal-A
(Herdianita et al. 2000). It converts over timescales of
tens of thousands of years to progressively more crystalline forms via opal-C, opal-CT to chalcedony, quartz
(Herdianita et al. 2000). This transformation process may
destroy plant tissues or obliterate features required for
plant identification. Sinter deposits preserved from within
this maturation window are often opaque, and whilst
containing evidence of plants, anatomical details are hard
to discern (Fig. 2A).
Finally, the lenticular nature of sinter deposits, with
thickly developed apron cores and thinner apron and
geothermal wetland margins, biases preservation during
erosion towards the resistant apron cores. Numerous sinter deposits we have investigated retain well-preserved,
although generally, plant-poor aprons, the marginal geothermal wetland environment having been largely
destroyed and only present as blocks preserved on deflation surfaces or in drift.
THE FOSSIL RECORD OF HOT SPRING
FLORAS
A worldwide hunt for epithermal and hot spring Au/Ag/
Hg mineral deposits and geothermal energy resources has
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led to a dramatic expansion of the number of verified
fossil hot spring deposits and hot spring ecosystems. Geothermal energy exploration uses the presence of relatively
young fossil sinters as guides for potentially concealed
high-temperature geothermal systems, and mineral exploration uses the presence of sinters as a guide to potentially economic breccia and/or vein mineralization in the
shallow subsurface. As both resources are most easily
identified in regions of active or geologically recent volcanism, the world’s tectonically active plate margins have
been a focus for major exploration activity, but increasingly palaeo-orogenic belts are coming under scrutiny
from mineral exploration companies. Further examples of
hot spring deposits have reached the academic literature
as the result of astrobiological and Precambrian research
aimed at unravelling the formation and subsequent longterm preservation of microbial fossils and their biofabrics.
Reviews including information on the geographical and
temporal distribution of hot spring deposits include: Nelson (1988); Walter (1996); Nakanishi et al. (2003); and
Sillitoe and Hedenquist (2003).
Mining exploration activity has led to the creation of
ever more complex ore-deposit models, which aid deposit
discovery and recognition. From these it can be seen that
sinter deposits occur almost exclusively associated with a
class of epithermal deposit know as low-sulphidation
deposits. These are created by deeply circulating meteoric
fluids as opposed to high-temperature magmatic fluid
and, at depth in the hydrothermal system, have an
alkali- and chloride-rich water chemistry rather than an
acid- and sulphate-rich character (Panteleyev 1996a; cf.
Panteleyev 1996b). Waters flowing from sinter-depositing
hot springs above these systems have a high dissolved silica content and also relatively high sodium and chloride
content and carry a suite of heavy metals and metalloids
(Channing and Edwards 2004, 2009a, b and references
therein). As we discuss later, this has a profound effect
on vegetation that is growing around active springs and
also on vegetation preserved in the fossil record.
CENOZOIC HOT SPRING FLORAS
Tectonically and volcanically active regions and Cenozoic
volcanic provinces host the majority of the world’s known
epithermal gold and silver deposits and represent vast
geothermal energy resources. As sinter deposits are the
surface expression of subsurface geothermal circulation
and potential mineralization and hence a target in exploration for these resources, reports of fossil hot spring
deposits are relatively frequent in Cenozoic (particularly
Neogene) strata. Most important amongst the major
Cenozoic epithermal provinces are those of the ‘Ring of
Fire’ that encircles the Pacific Ocean, which are associated
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with extensional areas of island and continental margin
arcs and back-arc basins. Further examples are associated
with continental rifts and extensional continental margins
(Sillitoe and Hedenquist 2003) and hot spot volcanism
(Saunders et al. 2008).
Mining and geothermal energy company grey literature
(e.g. technical reports, investor presentations and corporate websites) from these areas abound with references to
surficial sinter deposits and, as the presence of plant fossils is a major criterion for sinter identification confirming deposition at the surface, in many instances makes
reference to preserved plants and plant moulds. Description of plant material present in the vast majority of these
deposits extends only as far as terms for plant organs
(branches, roots, stems, leaves) or broad identifications of
plant groups such as sedges, reeds or rushes. As few palaeontological investigations have yet been conducted,
these floras (Table 1) represent an untapped palaeobotanical resource.
These younger hot spring deposits are very instructive;
fossil plants present are for the most part from extant
species and genera allowing robust comparisons of ecological and ecophysiological characteristics and habitat
preferences with living analogues. The deposits also provide clear evidence of what habitats and typical plant
ecologies are favourable to preservation and incorporation
into the rock and fossil record.
fossil sinter deposits, for example Roosevelt Hot Springs,
Colorado, Holocene (Lynne et al. 2005), Beowawe,
Nevada, Holocene–Pleistocene (Rimstidt and Cole 1983)
and Steamboat, Nevada, Pleistocene–Pliocene (Lynne
et al. 2008), Rainbow Rock, Imperial County, California,
Pleistocene (Pigniolo 1995). Miocene examples include
Buckhorn Mine, Nevada (Plahuta 1987); Baxter Project,
Nevada (Kizis 2006); Cordero Project, Nevada (Carew
2006); Hycroft Deposit, Nevada (Harris 2011); Jabo Prospect, Nevada (Keith et al. 1987); Bodie Bluff, California
(Herrera et al. 1993); Red Butte, Oregon (Zimmerman
and Larson 1994); and Summitville, Colorado (Gray and
Coolbaugh 1994).
Older sinter deposits from the Western USA include
examples from the Oligocene associated with the Crooked
River caldera, Oregon (McClaughry et al. 2009). In the
same region, the richly fossiliferous Eocene Clarno chert,
which contains angiosperms plus aquatic ferns and horsetails, is suggested to have originated from hot spring
activity around a lake environment (Arnold and Daugherty 1963; Brown 1975). In the Republic Graben, Washington, Eocene epithermal gold vein systems and surficial
sinter deposits are associated with marginal lacustrine
environments that contain abundant and well-preserved
plants (Gaylord et al. 2001). Examples of sinter deposits
containing fossil floras for which plant identifications
have been provided include Holocene to Miocene examples from Yellowstone, Nevada and California.
Western USA
Yellowstone National Park, Wyoming, Holocene – Late
Pleistocene. In Yellowstone, sinter apron complexes and
geothermally influenced wetlands are revealed by downcutting of the Gibbon River at Elk Park (Channing 2001;
Channing and Edwards 2009a). The sedimentary succession at the locality shows a typical prograding hot spring
apron sequence. The base comprises silicified sandstones
with meshworks of unidentified silicified rootlets. Wetland conditions are marked by two distinct lithological/
plant associations. Dark organic-rich beds with a massive
opaline matrix contain extensive root horizons, which
may represent geothermal water flooding of a pre-existing
wetland. Identified roots, rhizomes, tubers and basal
By far the most well-characterized and extensively
explored geothermal province in the world, the Western
USA, hosts active to Eocene-aged hot spring deposits
associated with the Coast Ranges of California, extensional rift settings (e.g. Nevada Rift, Oregon-Idaho
Graben) and the volcanism of the Yellowstone hot spot.
To date, plant descriptions related to most sinter
deposits are rudimentary, often limited to the fact that
plants are present or indicating broad affinity (e.g. reeds
or rushes). Examples of these deposits include recently
active geothermal systems with Holocene to Pliocene-aged
Plants preserved in Holocene, Pleistocene and Pliocene hot spring deposits. A, Pleistocene hot spring sinter from Ohakuri,
Taupo Volcanic Zone, NZ (NMW 2009.43G.1). Sinter is partially transformed from opal-A to more ordered phases (opal-C/Ct) and
appears opaque white. Plant anatomy is poorly discernible. Scale bar represents 1 mm. B–C, wetland root horizons, Holocene–Late
Pleistocene, Yellowstone National Park, Wyoming, USA. B, basal stems (with distinctive triangular cross section) and rhizomes/tubers
of the cyperaceous genus Scirpus (NMW 2001.29G.29). Scale bar represents 1 mm. C, geothermally influenced wetland sinter containing abundant stems of Eleocharis rostellata with distinctive lacunae visible in transverse sections (NMW 2001.29G.25). Scale bar represents 1 mm. D, base of early Holocene apron sinter horizon at Potts Basin, Yellowstone with numerous rosettes formed by silica
encrustation of in situ but wilted stems and leaves of bunch grasses. Scale bar represents 10 cm. E, conifer needle litter horizon preserved in sinter apron sinter at Potts Basin. Scale bar represents 10 mm. F, anatomically preserved stems of Eleocharis sp. preserved
within geothermally influenced wetland environment at the Pliocene McGinness Hills hot spring deposit, Nevada (NMW 2009.43G.2).
Scale bar represents 1 mm. NMW, National Museum of Wales, Cardiff, UK.
FIG. 2.
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TABLE 1.
Paleocene
*
*
Age and geographical distribution of Cenozoic fossil hot spring sinter deposits.
Eocene
*
*?P
Oligocene
*?P
Miocene
Pliocene
Pleistocene
Holocene
Region
*
*
*P
*P
*
*P
*
*P
*
*
*
*P
*P
*P
*
*P
*P
*P
South America
Central America
North America
China
Japan
Philippines
New Zealand
Africa
Iceland
*P
*
*
*
*P
*P
*P
*
*
*P
*P
Compiled from mining and geothermal energy company grey literature plus Sillitoe and Hedenquist (2003), Nelson (1988), and
Nakanishi et al. (2003).
*, sinter deposits reported; P, plant fossils reported; ?, not verified.
stems within the deposit belong to the Cyperaceae genera
Scirpus (Fig. 2B) and Eleocharis (Fig. 2C). Stabilization of
geothermal wetland conditions is marked by lighter-coloured sinter dominated by in situ clumps of Eleocharis
basal stems surrounded by microbial mat fabrics. Top
surfaces of these beds have a thick litter of randomly orientated Eleocharis stem fragments. Sinter apron encroachment on the wetland environment is evidenced by
laminated sinter with well-preserved microbial mat
fabrics.
Elsewhere in Yellowstone, examples of fossil vegetation
of sinter apron surface environments are preserved. Holocene sinter deposits on the shore of Yellowstone Lake at
Potts Basin contain rosette-like structures (Fig. 2D) created by in situ silica encrustation of wilted stems and
leaves of alkali and salinity-tolerant ‘bunch grasses’ (e.g.
Puccinellia nuttalliana). In the same area, lenses of wellpreserved conifer needles represent para-autochthonous
material transported into shallow pools on the apron surface (Fig. 2E).
Older hot spring deposits identified in Yellowstone
include a Middle to Upper Pleistocene sinter apron core
complex at Artists Point (Hinman and Walter 2005) and
areas of scattered laminated sinter boulders and cobbles
in the Norris Geyser Basin/Elk Park area (White et al.
1988; Channing 2001). These deposits, which contain
abundant evidence of microbial fabrics, exemplify the low
volume of plant material incorporated into sinter apron
environments because they have yielded only rare unidentifiable higher plant specimens with poor straw-like preservation that lacks cellular replication (Powell 1994).
Beyond Yellowstone, members of the Cyperaceae dominate the flora at two other Holocene – Late Pleistocene
hot spring deposits. Erwin and Schorn (1996) reported
cyperaceous species in Holocene hot spring deposits associated with the volcanic vent lake at Soda Lake, Nevada.
At Clearlake, California Taylor et al. (2006) reported rhizomes and basal stems of a Cyperus-like species within
wetland sediments that also contained algal filaments and
diatoms.
Nevada, Miocene–Pliocene. The oldest thoroughly investigated hot spring deposit in the region with identified
plants occurs at the McGinness Hills, Nevada (Ertel
2009). This Pliocene-aged hot spring system is largely
intact and is well-exposed. The locality preserves a sinter
apron core complex that is up to 24 m thick and approximately 500 m in diameter. The sinter apron provides evidence of multiple vents and coalesced apron deposits
exhibiting microbial fabrics, which represent a typical
down-apron zonation of progressively cooler subenvironments. Sinter-chip breccias and geothermal wetlands
located at the periphery of the deposit are the only two
subenvironments from which plants have been reported.
Only sparse plant fragments occur in the sinter-chip breccias. In contrast, abundant and often well-preserved
plants are preserved in the adjacent geothermal wetland
setting. Ertel (2009) provided the general description
‘marsh vegetation consisting of reeds, grasses and roots’
and identified some roots and woody stems as belonging
to dicots. The ‘reeds’ at the deposit occur as small matchstick-like fragments of aerial stems that form litter on
bedding top surfaces, a feature that is directly comparable
with active and subfossil wetland surfaces at Yellowstone.
Vertical sections through the deposit reveal in situ vertical
bunches of stems that radiate from a common origin.
Our observations of the wetland vegetation reveal the
presence of monocot roots and rhizomes and that the
dominant plant present in both subenvironments is a species of Eleocharis (Fig. 2F), the most commonly preserved
plant in active and subfossil deposits of Yellowstone.
Roots and basal stems are preserved in stacked rootmat horizons within massive dark-brown to black chert
that occurs in float blocks at the margins of an in situ
sinter apron core complex at the Miocene Hasbrouck
Mountain epithermal deposit at Tonopha, Nevada (Gra-
CHANNING AND EDWARDS: ECOLOGICAL FILTERING OF HOT SPRING FLORAS
ney 1987). Roots (Fig. 3A) have anatomy (large cortical
air spaces traversed by small vascular strands) that are
similar to Scirpus roots preserved in Holocene environments of Yellowstone. The top surfaces (wetland sediment
surfaces) of some float blocks preserve fragments of small
decorticated tree or shrub branches.
Iceland
Jones and Renaut (2007) and Jones et al. (2007) investigated the stratigraphy and chronology of Holocene siliceous sinters associated with the active Geysir hot spring
in Haukadalur, Iceland. In contrast to most other hot
spring deposits in the literature, plant material preserved
within the deposit is dominated by leaf litter created by
shed leaves of shrubby dicots, most notably Betula
(Fig. 3B). Preservation of leaf anatomy is extremely common and extends to preservation of parenchymatous cells
of the palisade layer and spongy mesophyll, indicating
rapid silicification despite transport to the site of preservation. The genus is still present within the active hot
spring area and occurs as low-growing, shallow-rooting
plants that are present within the sinter-forming environment where thermal run-off channels traverse and cut
into areas of remnant peaty soils.
In situ plants occur in a widespread wetland horizon
that occurs on the hillside above the current area of hot
spring activity. These slopes were considered by Jones
et al. (2007) to be the site of vents responsible for the formation of the oldest sinter horizons in the Geysir sinter
apron deposit. Here, basaltic basement is overlain by silicified tephra/volcaniclastics with upper horizons containing
sinter chips and Betula roots. Stratigraphically above
these, wetland sinter contains upright axes of Equisetum
that grow through surfaces covered with horizontal Betula
twigs and leaf mats (Fig. 3C). An eroded vent mound
within the outcrop of geothermal wetland sinter appears
to represent the point source of geothermal fluids. Additional fossil species recorded include abundant bryophytes
(Fig. 3D), unidentified grasses, stems and leaves of Carex
and the dicot Plantago maritima. The latter species is a
common component of the present flora of the area and
one of the many salt tolerant species that lives in areas of
thermal run-off. Other species present in active Icelandic
sinter deposition areas include Triglochin palustre, Agrostis
stolonifera, Carex nigra and Carex maritima, species that
often occur as part of Boreal salt-marsh communities.
New Zealand
The North Island of New Zealand, like the Western USA,
is a site of active geothermal activity with an extensive
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record of low-sulphidation epithermal mineralization and
fossil hot spring deposits, which extends back to the Miocene. Fossil plants occur associated with most reported
hot spring deposits including the Holocene (8.5 ka)
Mangatete sinter (Brathwaite 2003), Late Pleistocene
(c. 15–40 ka) Umakuri (Campbell et al. 2001), Tahunaatara (Campbell et al. 2003), Ohakuri (Henneberger and
Browne 1988) sinters and older (60–140 ka) Otamakokore and Whirinaki (Holland, 2000; Rodgers et al. 2004)
sinters within the active Taupo Volcanic Zone. In the
Northland region, plants are reported from Late Pleistocene (c. 40 ka) sinters at Omapere (Herdianita et al.
2000; Pastars, 2000) and Pliocene (3–4 Ma) sinters at
Whenuaroa (Hampton 2002). Sinters of Pliocene to Miocene age occur associated with the Coromandel Volcanic
Arc in the Hauraki goldfield, Coromandel Peninsula.
Examples occur at Waitaia near Kuaotunu (Edbrooke
2001), Broken Hills, Ohui, Onemana and Ascot Mine
(Brathwaite and Christie 2000).
Taupo Volcanic Zone, Pleistocene. The most thoroughly
investigated Late Pleistocene hot spring sinters occur at
Tahunaatara (Campbell et al. 2003) and Umakuri (Campbell et al. 2001). At both localities, sinter apron and associated geothermal wetland and clastic sediments are
revealed in cliff sections allowing detailed observations of
stratigraphy and palaeoenvironments.
The Tahunaatara sinter (14–20 ka) is a well-preserved
but partially eroded hot spring deposit with preserved
mid to distal areas of a sinter apron complex, peripheral
geothermally influenced wetland and intercalated ashfall
and fluvial deposits (Campbell et al. 2003). Geothermal
wetland settings of the deposit are dominated by anatomically preserved stems of a member of the Restionaceae,
Apodasmia (Leptocarpus) similis (Fig. 3E). The plant
occurs in great numbers in monospecific stands preserved
in growth position and surrounded by a litter of fallen
aerial stems. Up to 50 per cent of rock volume in wetland
horizons is plant material. The same species occurs in,
and growing through, intercalated tuffaceous siltstone
lenses. Less common wetland plants include an unidentified member of the Cyperaceae (Fig. 3F) preserved as
leaves and seeds. Wetland sediments dominate the
Tahunaatara sequence, and the stacked plant horizons are
repeatedly colonized by the same species.
Laminated/bedded low-temperature apron sinter within
the deposit contains much less plant material (up to
about 10 per cent of rock volume). Infrequent in situ
plants are preserved as clusters of basal stems and
mouldic preservation of scattered plant material lying on
bedding dominates. Coarser silicified clastic and volcaniclastic sediments at the locality, in contrast to peripheral
apron and wetland horizons, contain more diverse plant
types and organs. Preserved material is dominated by
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woody organs, such as twigs and branches, and isolated
small leaves. Occasional leafy shoots exhibit good preservation of lignified tissues but lack preservation of parenchyma. Less frequent monocots within the detrital
horizons include compressed stems of Apodasmia. Plants
preserved in life position are less common than in wetland horizons.
The Umukuri sinter has a minimum age of 27 ka and
is considered to preserve vegetation of either the last
interglacial oxygen isotope stage III, 20–30 ka (Campbell
et al. 2001) or oxygen isotope stage V between 80 and
120 ka (Rodgers et al. 2004). As at Tahunaatara, the hot
spring subfacies preserved at Umukuri represent mid- to
low-temperature sinter apron and slope areas, and distal
geothermally influenced wetland. Thinly laminated and
palisade apron sinter fabrics and plant-rich sinters dominate. Campbell et al. (2001) considered that the thinly
laminated microfacies at Umukuri formed from silicification of filamentous bacterial mats at low temperatures
(c. 35°C) under sheet-flow conditions. Palisade fabrics
also formed at similarly low temperatures although under
different hydrodynamic conditions. Closely spaced, lateral
and vertical intercalation of various microfacies in outcrop implies changing local flow and temperature conditions.
At Umukuri, most facies contain some plant matter
although evidence for in situ preservation is less frequent
than at Tahunaatara. Most plants are preserved as external moulds formed around plant fragments that were
lying on bedding surfaces. Campbell et al. (2001) compared the plant-rich microfacies at Umakuri to active
environments at the nearby Orakei Korako thermal area,
where they observed low-density areas of living rushes
being actively silicified along moist, outflow-channel
interfluves and found plant moulds formed in sinter.
Anatomically preserved plant organs at Umakuri are
dominated by roots or rhizomes of wetland monocots
with large air spaces and stem fragments of Apodasmia
(Fig. 4A; Campbell et al. 2001, fig. 13). Less frequent larger plant moulds formed around woody twigs and
branches and poorly preserved small dicot leaves appear
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to represent material transported into the apron/wetland
environment.
A third Late Pleistocene (c. 40 ka) hot spring deposit
at Lake Omapere, Northland, is exposed sufficiently to
view apron complex and wetland facies in plan view
(Pastars 2000; Rodgers et al. 2004). The sinter apron
environments preserved all contain fabrics typical of
low-temperature (<35°C) depositional hot spring environments. Massive to diffusely bedded, vitreous, black
plant-rich sinter, comparable with that at Tahunaatara,
occurs in marginal areas of the deposit and below apron
sinter in cores. Peat horizons intercepted in cores indicate apron development adjacent to a lacustrine environment. Plants preserved in situ are dominated by reeds
with tissue organization comparable with Apodasmia
preserved elsewhere (Herdianita et al. 2000, fig. 3h). Less
frequently preserved plants include unidentified grasses
and leaves and the basal stems of Cordyline australis
(Rodgers et al. 2004).
All three Late Pleistocene deposits are dominated by a
low-diversity wetland plant assemblage. Apodasmia similis,
the most commonly preserved species, is endemic to New
Zealand and occurs most commonly at present in coastal
settings such as estuaries, salt marshes, dunes and sandy
flats and hollows. The species also occasionally grows
inland in gumland scrub, along lake margins, fringing
peat bogs or, as in the fossil record, surrounding hot
springs.
Northland, Pliocene. The Puhipuhi geothermal field,
Northland hosts Pliocene (3–4 Ma) fossiliferous sinter
deposits. Hampton (2002) investigated the stratigraphy
and microfabrics of sinters at Mt Mitchell and Plumduff
and reconstructed palaeoenvironments within two discrete
apron complexes. Recorded hot spring subenvironments
range from near-vent apron with columnar stromatolites
(T > 70°C), to medium-temperature apron with conical
(tufted) and shrubby microfabrics (c. 40–60°C) to
lower-temperature apron with palisade fabrics. Plant-rich,
low-temperature facies were found to be concentrated
around the distal margins of the aprons although small
Plants preserved in Holocene, Pleistocene and Miocene hot spring deposits. A, root/rhizome of unidentified wetland plant
from the Miocene Hasbrouck Mountain hot spring deposit, Nevada (NMW 2009.43G.3). Cortical regions of the organ have large air
spaces traversed by root traces. Scale bar represents 1 mm. B, anatomically preserved Betula leaf from dense leaf litter within a Holocene sinter apron deposit preserved below the active apron of Geysir hot spring, Haukadalur, Iceland (NMW 2009.43G.4). Scale bar
represents 10 mm. C, Three transverse sections of Equisetum stem (fractured at, or adjacent to, stem nodes) and a collapsed, beddingparallel stem fragment viewed on the underside of wetland sinter block from fossil vent complex adjacent to the Haukadalur geothermal area (NMW 2009.43G.5). Scale bar represents 5 mm. D, bryophyte material from Holocene sinter, Iceland (NMW 2009.43G.6).
Scale bar represents 1 mm. E, several transverse sections of Apodasmia stem in wetland sinter matrix from the Late Pleistocene, Tahunaatara sinter, TVZ, New Zealand (NMW 2009.43G.7). The epidermal regions of the stems have distinctive dark radial cells (pillar
cells) that line areas of parenchyma cells. Stem centres have variably preserved parenchymatous pith. Scale bar represents 1 mm. F,
leaves of an unidentified member of the Cyperaceae from Tahunaatara (NMW 2009.43G.8). Scale bar represents 1 mm. NMW,
National Museum of Wales, Cardiff, UK.
FIG. 3.
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pockets of silicified plants occurred across many of the
lower-temperature apron surfaces. Most plant organs are
preserved as moulds and casts considered to represent
fallen and transported branches and twigs and in situ
reeds. Vertical sections through the base of the Mt Mitchell apron record lateral facies changes from high-temperature settings in eastern outcrops (columnar structures) to
low-temperature in the west (abundant silicified reeds).
At the site of reed silicification, overlying sinter horizons
indicate progradation of the sinter apron over the wetland
setting and a progressive reduction in plant numbers. At
Plumduff, distal areas of the apron complex record the
flow of silica-rich water down apron slope into marshy
ground on the edge of a lake, where reeds and other
small plants became silicified. Within both wetland areas,
stem casts and moulds are preserved in multiple layers as
randomly organized litter and upright stems set in an
indurated black to grey quartz matrix. Generic identifications of species present have not been made; however, the
tissue organization of better-preserved stems illustrated by
Hampton (2002, fig. 5.1) appear to confirm the presence
of abundant wetland monocots.
Coromandel Peninsula, Pliocene–Miocene. Miocene sinters occur associated with two epithermal mineralization
events at c. 16.3–10.8 and 6.9–6.0 Ma on the Coromandel
Peninsula (Mauk et al. 2011). Most fossiliferous sinter
deposits are highly dissected and preserved as in situ erosional remnants or boulder fields. Some plant-rich cherts
(e.g. those in the Waihi area) occur only as clasts in
hydrothermal explosion breccias or transported river
cobbles.
Perhaps the most intact hot spring deposit occurs at
Waitaia where sinter occurs as seven discontinuous boulder strewn fields, distributed over an area of 4.5 9
1.5 km. Sinter fabrics representative of low- to moderatetemperature sinter apron and plant-rich marsh facies can
be distinguished in the area (Rodgers et al. 2004). In
places the sinter apron deposits are interbedded with
swamp deposits and where in situ dissected sinter masses
537
occur at Black Jack Hill, Kuaotunu, Rowland and Sibson
(1998) described plant-rich sinter at the base of sinter
apron deposits as containing ‘sedges’.
A similar series of sinter outcrops and boulder fields
occurs at Kohuamuri Stream, south of Whitianga (Harvey
1981). Apron sinter present in the boulder field of the
deposit contains abundant transported leaves and twigs.
Tissue preservation at the deposit is extremely poor, and
most plant material is preserved as external moulds infilled with a later generation of clear quartz (Fig. 4B).
Some leaves (Fig. 4C) appear comparable to small, linear
parallel (elliptical)-shaped leaves of Podocarpus with constricted leaf bases and a strong mid-rib illustrated by Pole
(1992) from the Miocene of New Zealand. The poorquality preservation of samples and matrix chert fabrics
suggest preservation of allochthonous plant material on
apron surfaces.
Plant-rich chert cobbles discovered by amateur collectors in the Waihi area contain abundant roots and basal
stems of wetland monocots with a distinctive central stele
and large cortical air canals defined by thin radial strands
of parenchyma. Further examples of plant-rich sinters of
Pliocene to Miocene age are reported from Broken Hills
(Handley and Campbell 2011), Ascot Mine, Mackaytown
(Edbrooke 2001) and Waitekauri valley in the Waihi area
(Simpson and Mauk 2011). Species identifications have
not been made for any of these occurrences.
Japan
Hot spring sinter deposits of Japan range in age from
active to Miocene (Nelson 1988; Nakanishi et al. 2003).
To date, only two reports of preserved plant material
have been published. A Holocene sinter deposit (Ozaki
1972) associated with the Hakone caldera in Kanagawa
Prefecture preserves a diverse (39 species) leaf, seed and
cone-scale flora, which records local coniferous (Abies)
and broadleaf woodland vegetation. A single species of
Carex is reported. Belhadi et al. (2002) illustrated a dicot
F I G . 4 . A, poorly preserved stem of Apodasmia from apron sinter environment of the Pleistocene, Umukuri hot spring deposit, TVZ,
New Zealand (NMW 2009.43G.9). Little organic material remains within the stem. Anatomy is only visible due to voids once occupied
by cell walls remaining free of later phases of infilling opal-A. Scale bar represents 0.5 mm. B, external moulds of small ?Podocarpus
leaves from the Miocene, Kohuamuri Stream sinter deposits, Coromandel, New Zealand. All organic structures have been lost from
within the moulds and a later phase of quartz fills the open space (NMW 2009.43G.10). Scale bar represents 1 mm. C, a ?Podocarpus
leaf external mould revealed by fracturing along a sinter lamina (NMW 2009.43G.11). Shrunken remnants of the leaf that created the
mould remain. Scale bar represents 1 mm. D, transverse section of anatomically preserved Equisetum thermale stem from geothermally
influenced wetland of the Jurassic San Agustın hot spring deposit, Santa Cruz, Argentina (Accession MPM-PB 2029). Scale bar represents 0.5 mm. E, gleicheniaceous fern stipe (oblique TS) from geothermal wetland, San Agustın (Accession MPM-PB 2029). Regions of
cortex with thin-walled parenchyma cells forming an open aerenchyma are heavily degraded. Scale bar represents 1 mm. F, in situ
stump of conifer preserved in geothermal wetland sinter formed during flooding of dryland environment at San Agustın. Scale bar
represents 10 mm. NMW, National Museum of Wales, Cardiff, UK; MPM, Museo Padre Jesus Molina, Rio Gallegos City, Santa Cruz
Province, Argentina.
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PALAEONTOLOGY, VOLUME 56
leaf within Pliocene sinter, which caps epithermal vein
deposits in the Hoshino area, Hohi volcanic zone,
Fukuoka Prefecture.
Minerals); and Valle del Cura, Argentina (Malbex).
Reports of plant fossils from these regions are at present
limited to a report of fossil ‘reed casts’ from the El
Gallardo sinters associated with the Pliocene El Dorado
epithermal gold district, El Salvador (Richer et al. 2009).
Africa
Detailed accounts of fossil Holocene to Pleistocene hot
spring deposits of the East African Rift system are scant.
The complexities of the lakeshore/spring complex palaeoenvironments, typical of active examples, coupled with a
mix of cool-, warm- and hot spring–fed environments,
plus potential chert formation via routes other than deposition from hot springs (e.g. magadiite formation) make
interpretations of fossil examples difficult (Owen et al.
2004, 2008). Notwithstanding these caveats, Holocene to
Pleistocene (7 ka to c. 500 ka) sinter deposits are
reported from volcanic centres of the northern Kenya Rift
Valley (Sturchio et al. 1993; Darling and Spiro 2007) and
Pleistocene sequences of the Afar Rift (Moussa et al.
2012). A single fossiliferous hot spring deposit of Pleistocene age is reported from the Stratex International owned
Megenta Project, Afar Depression (Hall 2012), which contains in situ stands of ‘reeds’.
A species of Equisetum is reported from Holocene–
Pleistocene hot spring sinter deposits located close to
extinct volcanic craters, near Ambohidratrimo, Madagascar (Baron 1889).
PRE-CENOZOIC RECORDS OF HOT
SPRING FLORAS
The Lincang superlarge germanium deposit contains siliceous hydrothermal sedimentary structures interpreted
(rather controversially) as sinter, which are interbedded
with Neogene lignites. The presumed sinter contains abundant but as yet unidentified permineralized plants
(Qi et al. 2004). Three fossil sinters of Pleistocene age (the
oldest dated at 380 ka) occur in the Tengchong (Rehai)
area and are reported to contain abundant casts of plant
stems, roots and leaves (Meixiang and Wei 1987).
Whilst many of the world’s orogenic belts have evidence
for the former presence of hot spring activity in the form
of epithermal mineralization, only a handful of epithermal provinces retain evidence of surficial sinters. Estimates of the number of epithermal deposits created
during the Phanerozoic suggest that about 83 per cent
have been removed completely by erosion leaving approximately 63 000 deposits to be discovered (Kesler and
Wilkinson 2009). Hot spring environments as surficial
deposits associated with fault-related settings within tectonically and volcanically active terranes are even more
susceptible to erosion and removal from the rock record
(Gray et al. 1997). This problem of long-term preservation is highlighted by the scarcity of pre-Cenozoic examples of sinter deposits.
Exploration activity in the Mesozoic magmatic-arc and
back-arc environments of the American Cordillera and
Western Pacific and Palaeozoic and Mesozoic regions of
the Tethyan–Himalayan and Ural–Mongolian tectonic
belts has discovered increasing numbers of deposits preserving near surface vein systems and hydrothermal breccias. At present, sinter deposits are represented within
most geological periods (Table 2). Hot spring deposits
that lack information on the presence of plants include
Cretaceous cherts presumed to have formed in the hot
spring environment from the American Cordillera of
Alaska (Bundtzen and Miller 1997) and La Rioja, Argentina (Fiorelli et al. 2011); Jurassic examples preserved in
the Solenker epithermal district of south-east Mongolia
(Xanadu Mines Ltd 2011) and Silurian examples of the
Botwood Basin, Newfoundland (Wardle 2007).
Central and South America
Mesozoic hot spring floras
Cenozoic sinter deposits are reported from along the
length of the Central and South American Pacific Margin.
Examples from mining company grey literature include
Monte Cristo, Mexico (Paramount Gold and Silver);
Ludavina, Mexico (Cardero Resource); Cerro Blanco,
Guatemala (Goldcorp); Camporo, Honduras (First Point
Minerals); Santa Rosa, Panama (Greenstone Resources);
Ganarin, Ecuador (Nortec Minerals); Ccaccapaqui, Peru
(Alturas Minerals); Puchuldiza, Chile (Southern Legacy
Mesozoic hot spring floras are recorded from just two
palaeogeothermal provinces (Table 2).
China
China. The Sipingsang epithermal gold deposit, Heilongjiang Province, NE China, occurs in the Late Jurassic–Early Cretaceous Wandashan Fold Belt part of the
circum-Pacific tectonic belt. The sinter deposit occurs
above an Upper Jurassic rhyolite. Mineralization has been
dated as Upper Cretaceous, c. 87 Ma (Huang et al. 2011).
CHANNING AND EDWARDS: ECOLOGICAL FILTERING OF HOT SPRING FLORAS
TABLE 2.
Silurian
539
Age and geographical distribution of Mesozoic and Palaeozoic fossil hot spring sinter deposits.
Devonian
Carboniferous
Permian
*?
Triassic
Jurassic
Cretaceous
Region
*Pc
*?e
*?
*?d
*Pf
North America
South America
Asia
Europe
Australia
a
b
*Pg
*Pi
*P
*Ph
j
*, sinter deposits reported; P, plant fossils reported; ?, not verified. a, Botwood Basin epithermal prospects, Newfoundland (Wardle
2007); b, south-western Alaska, Late Cretaceous Early – Cenozoic (Bundtzen and Miller 1997); c, Deseado Massif, Argentina (Guido
and Campbell 2011) and Fruta del Norte, Ecuador (Henderson 2010); d, La Rioja, Argentina (Fiorelli et al. 2011); e, Solenker epithermal district, SE Mongolia (Xanadu Mines Ltd 2011); f, Heilongjiang Province, China (Huang et al. 2011); g Rhynie chert, Scotland
(Trewin and Rice 1992); h, Massif Central, France (Marcoux et al. 2004); i, Drummond Basin, Queensland, Australia (White et al.
1989); j, Drummond Basin, Queensland, Australia (Uysal et al. 2011).
The deposit comprises silica-cemented breccias and gravels overlain by a 3 to 15 m thick sequence of laminated
chert. The bulk of the in situ cherts appear to represent
sinter apron environments, although blocks of organicrich chert with a mottled to massive matrix suggestive of
geothermal wetland settings also occur. To date, identifications of plants are limited to fragments associated with
apron surfaces and appear to be disseminules of adjacent
forest vegetation. Identification is hampered by a high
degree of tissue loss, with most plant organs being preserved as silica encrusted tubes with poorly preserved
anatomy. Transverse sections of many tubes are reminiscent of conifer needles. Only the most recalcitrant organic
compounds remain intact; fungal structures, spores and
pollen (including bisaccate forms) and wood fragments
are relatively common.
Argentina. The Jurassic strata of the Deseado Massif,
Santa Cruz Province, Argentina, host the largest concentration of epithermal systems with preserved hot spring
deposits in the world, after the Cenozoic of the Western
USA. In this region, Late Jurassic volcanism, extension
and a high thermal gradient in a back-arc setting produced
hydrothermal mineralization including epithermal deposits
and their surface expression, hot spring deposits (Guido
and Campbell 2011). To date, about 25 examples of silica
sinter and carbonate (travertine) hot spring systems have
been identified (Guido and Campbell 2011, 2012).
Of the silica sinter deposits discovered, eight have associated silicified floras. All deposits are dissected to some
degree by postdeposition erosion. Some deposits (La Marciana; Guido and Campbell 2009) are sinter apron core
complexes preserving much evidence of microbial fabrics
but few, and only poorly preserved, plants (Guido and
Campbell 2009, fig. 3n). At other deposits, such as Flecha
Negra (Channing et al. 2007), Ca~
nad
on Nahuel and La
Bajada (Guido and Campbell 2011), apron complexes are
poorly represented or absent, and plant-rich cherts with
geothermal geochemical signatures occur in close proxim-
ity to underlying epithermal vein systems and/or hydrothermal breccias.
By far the best-preserved and best-exposed silica-depositing hot spring complex occurs at San Agustın (Guido
et al. 2010). Hot spring sinters and cherts occur here as
part of the Bahıa Laura Group (Middle – Upper Jurassic),
within an area of extensive La Matilde Formation (Upper
Jurassic) lacustrine sediments. These filled fault-related
Jurassic valleys within a terrain composed dominantly of
Chon Aike Formation ignimbrites. Siliceous sinter deposits developed above faults, which focussed geothermal
fluid up-flow (Guido et al. 2010).
The San Agustın deposit represents a laterally extensive
and near-intact geothermal landscape in which hot spring
and ‘normal’ terrestrial/lacustrine deposits are exposed
side by side. Excellent exposure allows observations of
original lateral and vertical relations amongst the various
hot spring subfacies. It also enables the recognition of
palaeoflow directions from hot spring vent areas, across
sinter aprons to geothermal wetlands, and from there into
either peripheral terrestrial or lacustrine clastic depositional environments (Guido et al. 2010).
Features of plant colonization and preservation visible
in active and Cenozoic hot spring complexes are readily
discernible at San Agustın. Apron complexes, which constitute the most extensive features at the deposit, provide
abundant evidence of microbial communities in mid- to
low-temperature sinter apron settings but lack identifiable
plants.
Plant preservation within the apron environment is
limited to silica-rimmed low terraces on the outermost
sinter apron, which enclosed localized supra-apron geothermal ponds (10–20 cm deep, 5–10 m diameter). These
provided habitat for dense monospecific carpets of the
sphenophyte Equisetum thermale (Channing et al. 2011).
Geothermal wetlands forming beyond the apron are dominated by the same species (Fig. 4D) and less common
gleicheniaceous ferns with clear adaptations to wetland
conditions as indicated by well-developed aerenchyma
540
PALAEONTOLOGY, VOLUME 56
(Fig. 4E). Other plant organs within the wetland deposits
include infrequent transported branches, leafy shoots and
seed scales of conifers from nearby forested environments.
Vertical sections through the larger sinter apron deposits reveal typical apron development sequences comparable with Cenozoic examples. Bases of sections have
silicified clastic or volcaniclastic sediments and palaeosols
with silicified roots and sometimes small sinter fragments.
Basal cherts contain in situ conifer stumps (Fig. 4F) and
rotted fallen trunks in a mottled matrix containing conifer branches, leafy shoots, cones and cone scales. Successive chert horizons contain the typical geothermal
wetland species. The top of the sequence is capped by
cherts preserving evidence of low- to mid-temperature
apron environments with lenticular higher-temperature
fabrics indicating outflow channels.
In other outcrops, thermal discharge entering lacustrine
environments via shore-line stands of shrubby conifers
and their seedlings is evident. Here, abundant Classopollis
pollen indicates the presence of members of the Cheirolepidiaceae, which have been considered to be xerophytic,
drought-tolerant and thermophilic and are commonly
(although not exclusively) associated with oligohaline to
saline environments (Alvin 1982; Mendes et al. 2010).
Plant remains within the lake shoreline facies are enveloped by dense, ‘felted’ microbial linings, suggesting wet,
marshy conditions during preservation (Guido et al.
2010).
Nonsilicified lacustrine deposits associated with the San
Agustın hot spring complex (Zamuner et al. 2011) contain plants from seven genera (14 morphospecies) including the corystosperm Pachypteris, four species of the
bennettitalean genus Dictyozamites, three forms of araucarian seeds and Pagiophyllum/Brachyphyllum foliage. Lycopsids (Isoetites) and stems of Equisetum comparable with
E. thermale are also present. Whilst species and morphogeneric diversity of basal cherts related to initial flooding
of terrestrial settings are comparable in diversity with
these sediments, the diversity relative to demonstrable
geothermal wetland species is high. It appears here, as in
active environments, that the onset of geothermal wetland
conditions is marked by a reduction in ‘dryland’ species.
These are replaced by species with obvious anatomical,
and by analogy with extant species, physiological adaptations to oligohaline wetland conditions (Channing et al.
2011).
At the La Claudia exploration project, a broad suite of
hot spring environments is preserved including both travertine and sinter aprons. Plants have not been identified
in the apron settings of the deposit, and geothermal wetlands are poorly represented. Root horizons (Fig. 5A)
dominate plant-rich cherts and these contain abundant
evidence of shrubby conifers (leafy shoots and small
branches) and much less frequent evidence of Equisetum
and gleicheniaceous ferns.
The La Bajada and Ca~
nad
on Nahuel deposits (Guido
and Campbell 2011) have not been fully investigated sedimentologically or palaeontologically, but initial observations indicate at both sites a mixed flora comparable with
those preserved during initial flooding of terrestrial settings at San Agustın. In situ permineralized stumps and
well-preserved roots occur in variably silicified volcaniclastics at the base of sections along with thickets of wellpreserved osmundaceous ferns. In situ conifer stumps,
sapling stems and osmundaceous ferns also occur in basal
chert horizons (Fig. 5B) in association with large decorticated sections of conifer trunks and fallen branches. Smaller fragments of conifer leafy shoots, branches, cones and
seed scales, which are well-preserved, occur in the extremely plant-rich chert above. The wetland genera Equisetum and gleicheniaceous ferns present at San Agustın
occur within the same horizon and in life position within
the massive, but generally plant-poor, chert that caps
chert beds.
Lenticular chert lenses at the Laguna Flecha Negra
deposit (Channing et al. 2007) similarly contain large volumes of variably preserved wood fragments, in situ roots,
rootlets and young stems of conifers plus shed conifer
shoots with scale leaves of Brachyphyllum and Pagiophyllum morphology, cycadophyte (cycad or bennettitalean)
A, roots of an unidentified plant in wetland chert from the Jurassic La Claudia hot spring deposit, Santa Cruz, Argentina
(Accession MPM-PB 2029). Other organs in the same chert include foliar axes of conifers and rare stems of Equisetum and gleicheniaceous ferns. Scale bar represents 2 mm. B, base of Osmunda-like fern preserved in dark plant-rich chert at the Jurassic Ca~
nad
on Nahuel deposit, Santa Cruz (Accession MPM-PB 2029). As with conifer stumps at San Agustın, such ferns can be seen to colonize dryland
settings and are incorporated into sinter during initial flooding by geothermal waters. Scale bar represents 10 mm. C, poorly preserved
transverse section of Sphenophyllum stem from the early Permian Meillers sinter, Autun, France (NMW 2009.43G.12). Scale bar represents 1 mm. D, Partial transverse section of herbaceous lycopsid axis from the late Devonian Wobegong sinter, Drummond Basin,
Queensland, Australia (QML1441). Central (cog-shaped) xylem is surrounded by a thin inner cortex, which is separated from the
outer cortex be a large air space. Scale bar represents 1 mm. E, longitudinal section of the same lycopsid axis with sporophylls (left)
and vascular traces crossing the air spaces of the middle cortex either side of the xylem (QML1441). Scale bar represents 1 mm. F,
poorly preserved leaves and roots and/or stems of possible cordaitaleans/ferns in massive chert from the Upper Carboniferous Twin
Hills sinter deposit, Queensland (QML1444). Scale bar represents 1 mm. MPM, Museo Padre Jesus Molina, Rio Gallegos City, Santa
Cruz Province, Argentina; NMW, National Museum of Wales, Cardiff, UK; QML, Queensland Museum, Brisbane, Australia.
FIG. 5.
CHANNING AND EDWARDS: ECOLOGICAL FILTERING OF HOT SPRING FLORAS
A
B
C
D
E
F
541
542
PALAEONTOLOGY, VOLUME 56
leaves and conifer cones and seeds (Channing et al.
2007). Intercalated sandstones within the chert beds of
this locality appear to indicate frequent disruption of sinter formation in an alluvial depositional environment.
Palaeozoic hot spring floras
Palaeozoic hot spring floras have been recorded from
only three palaeogeothermal provinces (Table 2).
Massif Central, France. The Massif Central is well known
for Carboniferous and Permian floras preserved in cherts
(Galtier 2008). The majority of cherts are considered to
be related to early preservation of plant materials in
lacustrine, fluvial or swamp settings by silica derived from
dissolution of intercalated volcanic and volcaniclastic
horizons (Rex 1986; Matysova et al. 2010). Many plantrich cherts from the region occur as isolated float blocks
or as clasts within tuffs or conglomerates and suffer a lack
of spatial context. An exception to this is provided by the
Meillers ‘quartzite’ of the Autun Basin, which, based on
geochemical, structural and textural evidence, has been
re-interpreted as a hot spring sinter formed c. 295 Ma in
the early Permian (Marcoux et al. 2004). Detailed sedimentological and palaeoenvironmental investigations of
the deposit required to identify hot spring subenvironments have yet to be conducted. However, the stratigraphy of the deposit, which shows intercalations of
laminated ‘sinter facies’ and clastic sediments within a
general sequence that progresses from mottled, diffusely
bedded cherts to laminated cherts with vuggy beddingparallel porosity and ‘pillar and stall’ microbial mat fabrics (Marcoux et al. 2004), appears to show an apron
complex prograding over peripheral wetland environments. Palaeontological data from the deposit are limited.
Marcoux et al. (2004) reported Psaronius, ‘fructiform
spikes’ and silicified ostracods in the upper sinter horizon
of the deposit. Freytet et al. (2000) recorded various algal,
cyanobacterial and stromatolitic fabrics and pollen, silicified woods, possible Cordaites leaves, vertical traces, which
may be stems in their living position and possible moulds
of roots. Hand specimens with matrix fabrics comparable
with low-temperature sinter apron and wetland environments reveal abundant plant material (pers. obs.). Initial
observations suggest the dominance of the sphenophyte
Sphenophyllum in these environments (Fig. 5C).
Elsewhere in the Massif Central, suspected hot spring
deposits with epithermal geochemical signatures (anomalous levels of As, Au, Ag and Sb) are reported from the
Upper Carboniferous at St-Priest-en-Jarez near St-Etienne
and hydrothermally silicified fluvial conglomerates and
sandstones at Lugeac (Copard et al. 2002). The St-Priest
deposit contains freshwater algae and stromatolitic horizons (Freytet et al. 2000).
Drummond Basin, Queensland, Australia. The Drummond
Basin, Queensland represents a major north–south trending back-arc extensional system located at the inboard
margin of the northern New England Orogen. It contains
an early rift infill sequence comprising Upper Devonian
to Lower Carboniferous volcanic, volcaniclastic and sedimentary rocks (Henderson et al. 1998).
Numerous epithermal deposits with proven or probable
associated hot spring sinter deposits occur within the
Basin (e.g. Bimurra (Wood et al. 1990; Ewers 1991), Durrah Creek (White et al. 1989; Ewers 1991) and Pajingo
(AC pers. obs.)). The best-preserved and exposed hot
spring complexes located at Wobegong (near Conway
Station north of Mount Coolon) and Verbena (south-east
of Mount Coolon) respectively occur within the Bimurra
and Silver Hills Volcanics. New SHRIMP U–Pb zircon
ages indicate deposition of these units in the latest Devonian (Famennian) at around 360 Ma (Cross et al. 2008).
Latest radiometric dates for mineralization with associated
sinters and silicified lacustrine sediments at Twin Hills
Mine (Uysal et al. 2011) suggested a mineralization age
of 313 6 Ma placing this deposit in the latest
Carboniferous.
The geology, geochemistry, sedimentology and palaeontology of the Conway and Verbena sinters have been
studied in great detail (Cunneen and Sillitoe 1989; White
et al. 1989; Ewers 1991, Walter et al. 1996, 1998). Walter
et al. (1996) recorded 13 different microfacies in field
investigations of the two deposits and classified these
groups into four main facies associations, which broadly
reflect down-apron temperature and topographical gradients. Very high-temperature microfacies (>73°C) occur in
vent zones and comprise geyserite and vent wall and pool
deposits. High- (60–73°C) to mid-temperature (35–59°C)
microfacies represent proximal to distal areas of run-off
channels and supra-apron terrace pools and ponds.
Down-slope and down the temperature gradient (35°C to
ambient) towards the periphery of the apron complex,
low-temperature terrace-slope, terrace-pool and marsh
environments occur (Walter et al. 1996, 1998).
Plant preservation within apron environments at the
deposits is dominated by silica sinter encrustation of lycopsid stems and their external moulds are preserved
lying on the bedding planes of thin-bedded cherts. Walter et al. (1996, 1998) considered that these plants were
growing on interfluves between run-off streams on the
apron. In other areas of the apron complexes, lenses and
tabular beds of chert contain stems with orientations
that vary from horizontal to vertical. Commonly, stems
occur within thick encrusting concentric sheaths of chert
CHANNING AND EDWARDS: ECOLOGICAL FILTERING OF HOT SPRING FLORAS
containing evidence of filamentous microbial fossils.
Many of the vugs within this microfacies have circular
cross sections and were interpreted as moulds of lycophyte stems and roots (rhizoliths). Walter et al. (1996,
1998) considered these deposits to be analogous to environments in active spring systems where bushes and
trees in growth position are engulfed by newly formed
ponds and become encrusted with opaline silica. Walter
et al. (1998) described three vegetative axis morphotypes
from the Drummond basin sinters and a single strobilus.
Two of the axes types and the strobilus represent herbaceous lycophytes and the third axis a possible sphenophyte.
In our field investigations of the Drummond Basin sinter deposits, we specifically focused on plant-rich microfacies. At Wobegong, in addition to the microfacies
described by Walter et al. (1996, 1998), we found horizons of grey to red-brown massive to mottled or diffusely
laminated/bedded chert characterized by a matrix rich in
microcoprolites. These microcoprolite-rich sediments are
recorded at San Agustın in geothermal wetlands and at
Rhynie. At all three deposits, in addition to exceptionally
preserved higher plants, algae and aquatic invertebrates
occur. At Conway, anatomically preserved examples of
the lycophyte described by Walter et al. (1998) occur in
monotypic, stacked horizons above ‘root horizons’ that
are also enclosed within microcoprolite-rich chert. Field
relationships and anatomical features of the lycophyte,
most notably the substantial air spaces within stems
(Fig. 5D–E), are consistent with life in wetland conditions. This single species appears to dominate the full
spectrum of geothermally influenced environments represented by the Conway and Verbena sinters.
As at San Agustın, local clastic and volcaniclastic environments contain evidence of plants beyond the limit of
thermal activity. Tuffs adjacent to the Conway locality
contain large (10–20 cm diameter) arborescent lycopsid
stems. Fluvial sediments of the Bimmura Formation at
Conway Station (approximately 2 km from the apron
complex) contain a diverse compression flora comprising,
herbaceous, plus arborescent, lycophytes (stigmarian
roots, stems, branches, cones and dispersed sporophylls),
sphenophyte stems and fragments of pteridosperm foliage.
No examples of the arborescent groups have been discovered in geothermally influenced environments of the
Upper Devonian Drummond Basin sinters suggesting partitioning of the local flora.
The Upper Carboniferous Twin Hills sinter deposit
(Uysal et al. 2011) comprises broad areas of laminated
sinter within lacustrine sediments some of which are silicified. Plants preserved at the deposit occur in massive
chert that appears laminated due to bedding parallel collapsed and partially permineralized plant material. Collapsed arborescent lycophyte stems and/or branches up to
543
70 mm in diameter cover top surfaces of these chert outcrops. Three-dimensionally preserved plant organs in the
bed include fragments of possible cordaitaleans or ferns
and unidentified leaves preserved as silica-infilled external
moulds (Fig. 5F).
Rhynie, Aberdeenshire, Scotland. The Rhynie chert,
Lower Devonian of Aberdeenshire, Scotland, contains by
far the most thoroughly investigated Palaeozoic hot
spring ecosystem (Trewin and Rice 2004). An extensive
literature, produced over nearly a century of research
(Kidston and Lang, 1917; Haug et al. 2012), documents
the microbial (Krings et al. 2007), fungal (Taylor et al.
2004), algal (Edwards and Lyon 1983; Kelman et al.
2004) and higher plant (Edwards 2004; Kerp et al. 2004)
flora, plus associated fauna (Anderson and Trewin 2003).
Unlike the majority of hot spring deposits described
above, the Rhynie chert system does not crop out and
occurs only as float or where intersected by trenching and
research boreholes (Powell, 1994; Powell et al. 2000). Drill
core recovered from the deposit intersected 45 in situ
chert beds, most of which contained permineralized
plants, although a few were barren (Powell 1994). In situ
plants occurred in 29 of the 45 chert horizons (Powell
et al. 2000 reporting Rhynie drill-core 19C) and in situ
rhizomes with aerial axes were recorded for Rhynia, Aglaophyton, Horneophyton and Asteroxylon. Only Nothia and
Trichopherophyton were not found as in situ rhizomes,
and the least common permineralized Rhynie plant, Ventarura, was not recorded at all. Compared with the
majority of younger hot spring floras, species diversity at
Rhynie is relatively high, comprising seven species of
sporophytes, four of which also have identified gametophytes. We suspect this is an observation bias, as in reality, four species (Rhynia, Aglaophyton, Horneophyton and
Asteroxylon) might be considered to dominate the flora.
Mega-fossil floras from coeval, but geographically separate, clastic settings are not dramatically more diverse
than the chert flora, containing eleven species (Channing
and Edwards 2009a, table 4). However, when viewed relative to the dispersed spore assemblage of the broader
Rhynie drainage basin (which yields over 40 distinctive
dispersed spore types) and the total dispersed spore diversity of coeval sedimentary sequences (over 100 distinctive
spore types), the chert flora is depauperate (Wellman
2006). Wellman (2004, 2006) considered that the plants
preserved within the hot spring cherts were the only elements of the regional flora that were capable of survival
in the hot springs environment and that other elements
of the regional flora simply could not tolerate this inhospitable environment. Comparisons of coeval macrofossil
floras from more ‘normal’ sedimentary environments,
which are dominated by groups of trimerophytes and zosterophylls that are not recorded within the chert, appear
544
PALAEONTOLOGY, VOLUME 56
to provide further evidence of this ecological partitioning
and exclusion of plants that might be considered typical
Lower Devonian mesophytes from hot spring influenced
settings (Channing and Edwards 2009a).
Wellman (2004, 2006) also considered the possibility of
endemism and hot spring specialization of the Rhynie
chert plants in relation to the dispersed spore record of
the Rhynie drainage basin and broader geographical areas
of the Old Red Sandstone continent. He observed that
the distinctive spores of the Rhynie chert plants Horneophyton lignieri (Emphanisporites cf. decoratus), Aglaophyton major (Retusotriletes sp. A; Wellman, 2004) and
Rhynia gwynne-vaughanii (Apiculiretusispora plicata) are
palaeogeographically widespread and abundant elements
of coeval dispersed spore assemblages. This suggests that
the parent plants inhabited more normal environments
throughout the remainder of the Rhynie drainage basin
and were also common and widespread components of
the regional flora (Wellman 2004). As the plants were not
confined to hot spring environments, Wellman (2004,
2006) considered that it was likely that the three plants
were pre-adapted to survive in hostile conditions rather
than being highly adapted plants specialized for survival
only in geothermally influenced environments. The true
distribution of the remaining four chert plants, unfortunately, is masked due to the relatively simple morphology
of their spores (e.g. various species comparable with
Retusotriletes). Potential habitats for pre-adaptation of the
Rhynie chert plants include those identified above for
more recent hot spring plant species (e.g. coastal or
inland salinity-influenced environments). However, it is
possible that water-stressed environments were more
common and widespread during the Late Silurian to
Lower Devonian as terrestrial surfaces had less welldeveloped soils and were more sparsely vegetated by
plants of lower stature.
The Rhynie chert plants rarely appear in coeval macrofossil assemblages. A single specimen assigned to the
genus Horneophyton (Edwards and Richardson 2000) is
recorded from the Lochkovian of the Anglo-Welsh Basin.
Additionally, some species of the compression fossil, form
genus Salopella, with fusiform sporangia terminating
naked, isotomously branched axes, have been linked with
Aglaophyton based on sporangium morphology (Edwards
and Richardson 1974). Given the dispersed spore record,
this absence most likely arises from taphonomic removal
of the parenchymatous Rhynie plants from most clastic
environments (Wellman 2004, 2006).
Sinter apron environments are not well-represented at
the Rhynie deposit, whilst plant-rich cherts are extremely
abundant. The interpretation of environments of plant
growth and preservation at Rhynie rely fully on identification of chert matrix fabrics and associations with other
enclosed elements of the biota (e.g. aquatic fungi, algae,
crustaceans suggesting wetland settings and terrestrial arthropods dryland). Various palaeoenvironments have
been suggested for plant growth and preservation. Many
chert lenses at Rhynie overlay silicified silt or sandstone
sediments or contain sediment intercalations, some of
which represent silicified plant litter or soil horizons
(Powell 1994, Powell et al. 2000). Plant growth associated
with these horizons indicates that at least some of the
plant communities that were eventually preserved by sinter accumulation and permineralization were not necessarily submerged throughout their life. Such clastic
sedimentary settings that were colonized by plants and
subsequently flooded by hot spring waters were favoured
in early accounts of the chert. El-Saadawy (1966) considered that the Rhynie plants grew in and around ephemeral pools within a barren landscape. Trewin and Rice
(1992) considered that the plants generally grew on sandy
substrates or sinter surfaces and close to pools on the fluvial floodplain prior to inundation by hot spring water.
Powell (1994) envisaged the cherts as fossilized subaerial
sinters in the vicinity of hot spring vents, with laminated
cherts typical of deposition on sinter terraces and massive
and lenticular cherts containing the charophyte Palaeonitella and aquatic crustaceans as pond deposits. Trewin
et al. (2003) and Trewin and Wilson (2004) suggested
that plant preservation occurred on a low-angled hot
spring outwash apron in a distal region of sinter deposition (a geothermally influenced wetland). We favour this
latter interpretation and discussed the evidence for, and
implications of, growth of the Rhynie plants in geothermally influenced environments in Channing and Edwards
(2009a).
ECOLOGICAL FEATURES OF FOSSIL
HOT SPRING FLORAS, MEGABIAS AND
ECOLOGICAL BIAS
Observation of the Cenozoic and Mesozoic record of hot
spring floras suggests that widespread wetland species,
with broad specific and generic geographical ranges, are
preserved rather than hot spring specialists or endemics.
Mesophytic species are captured and preserved by initial
flooding by geothermal waters and as transported organs.
In better-preserved and exposed hot spring deposits, these
‘innocent bystanders’ can be identified with some certainty from field relationships and chert macro- and microfabrics.
Broadly, in hot spring settings, plant preservation environments are confined to those where plants undergo an
early and protracted period of immersion in silica-rich
fluid. The boundary between preservation and taphonomic loss of plants may be as sharp as the position of a
stem relative to the local water table. Plants growing in
CHANNING AND EDWARDS: ECOLOGICAL FILTERING OF HOT SPRING FLORAS
alternately wet and dry environments (e.g. sinter aprons)
are rarely or poorly preserved. Plants growing in wetland
environments, conversely, are commonly preserved anatomically. These two end-member taphonomic settings,
again, are readily observed in subfossil and fossil deposits.
Sinter apron outcrops are essentially barren having rare
prostrate plant organs preserved as external moulds. Wetland outcrops contain abundant in situ plants that are
anatomically preserved. Even in wetlands, however, plant
preservation is seen to be influenced by the local water
table, and basal stem preservation dominates over preservation of areas of the stem situated above the water table.
Investigations of active silica-depositing hot spring systems reveal a strong bias towards in situ and permineralization-style preservation of plants in geothermally
influenced wetlands (Channing and Edwards 2009a, b).
This bias is clearly present in Cenozoic fossil floras, as
members of the wetland family Cyperaceae and, in the
Southern Hemisphere, ecologically analogous members of
the Restionaceae dominate plants preserved in situ in
Holocene (e.g. Yellowstone), Pleistocene (e.g. Taupo Volcanic Zone, New Zealand) and Pliocene to Miocene (e.g.
Nevada) hot spring deposits. Based on ecological evidence
from closely related extant relatives and anatomical features such as aerenchyma and extensive lacunae, wetland
ecology can be demonstrated for the Mesozoic Equisetum
and gleicheniaceous fern-dominated floras of the Deseado
Massif, Argentina (Channing et al. 2011). The same lines
of evidence, plus comparison with ecological preferences
of coeval vegetation found in more widespread wetland
settings, support a bias to wetland species in the Palaeozoic herbaceous lycophyte dominated flora of the Drummond Basin, Australia (Walter et al. 1998).
A number of morphological and biological features
characterize the fossil record we have observed. Plants are
small and herbaceous with photosynthetic stems and few
or reduced leaves. In Cenozoic deposits, graminoid forms
with reduced leaves are typical, and microphyllous groups
characterize Mesozoic and Palaeozoic deposits. Shallow
rooting is common and plants use rhizomatous and/or
stoloniferous growth to cover available substrates. Vegetative reproduction appears to be more common than sexual reproduction (Channing et al. 2011). These latter two
features lead to broad areas of monotypic vegetation.
Despite growth in an environment with a high water
table and ample water supply, where physical drought is
unlikely, plants preserved in the geothermal wetland environment show a range of xeromorphic characters and
anatomical adaptations that we suggest would provide
high water use efficiency (Channing and Edwards 2009a).
These include, as already mentioned, small size, photosynthetic stems and reduced leaves, plus in various species, well-developed cuticle (Eleocharis, Apodasmia,
Equisetum), hypodermis (Eleocharis), thickened outer
545
walls of epidermal cells (Eleocharis, Equisetum), low numbers or reduced size of stomata (Cyperaceae, lycophytes),
stomata protected by cover cells and/or silica pilulae
(Equisetum) and stem hairs (Apodasmia). A final feature
links all of the plants observed as dominant geothermal
wetland species at each deposit (or at least their closest
extant relatives): in life, they all take up, accumulate and
biomineralize silica (Hodson et al. 2005).
In combination, the observation that life, death and
anatomical fossilization of wetland plants is commonplace
in the record of hot spring environments and the observation of characteristics typical of high water use efficiency lead us to a final area of discussion. Is taphonomic
bias replaced by ecological and ecophysiological bias
throughout the 400-million-year history of hot spring
ecosystems? We suggest that the major bias towards wetland ecology is amply demonstrated. The question of ecophysiological bias arises because silica precipitating hot
springs have tightly constrained hydrochemistry (Brock
1971, Channing and Edwards 2009a, tables 1 and 2 and
discussion therein) and during life geothermal wetland
inhabitants are immersed in fluids that have high pH,
brackish salinity and contain trace concentrations of phytotoxic elements including heavy metals and metalloids
(e.g. arsenic (As), mercury (Hg), antimony (Sb), thallium
(Tl)). They are therefore required to have tolerance mechanisms for multiple physicochemical stresses.
Tolerance of these stresses is readily demonstrated for
vegetation of Cenozoic geothermal wetlands. In the case of
salinity stress, species (or genera) that are recorded in
Cenozoic fossil geothermal wetlands of Yellowstone, Western USA, Iceland and New Zealand including Eleocharis
(and various other members of the Cyperaceae), Apodasmia and Equisetum may all be observed not only in active
geothermal wetlands but also in more widespread (usually
oligohaline) salinity-stressed environments including
coastal marshes, estuaries, saline seeps and salt lakes
(Channing and Edwards 2009a). Analogy with extant species of Equisetum, which are common members of communities associated with saline soils and oligohaline to saline
wetlands (Williams 1991; Boggs 2000; Funk et al. 2004,
Purdy et al. 2005; Van der Hagen et al. 2008), appears to
allow inclusion of Equisetum thermale, which dominates
Mesozoic geothermal wetlands in the Deseado Massif
within this list of halophytes (Channing et al. 2011).
In a similar vein, heavy metal and metalloid tolerance
is demonstrable in the Cenozoic and Mesozoic hot spring
floras by either direct observation of active geothermally
influenced wetlands, other natural or anthropogenic
metal-stressed environments or analogy with extant close
relatives. Members of the genus Eleocharis, for example,
naturally colonize or are transplanted into wetlands associated with abandoned metal mine workings (FloresTaviz
on 2003; Ha et al. 2009a, b, 2011a, b; Lottermoser
546
PALAEONTOLOGY, VOLUME 56
and Ashley 2011; Sa’ad et al. 2011; Adams et al. 2012; Olmos-Marquez et al. 2012; Alarc
on-Herrera et al. 2013)
where, because they are accumulators or hyperaccumulators of elements such as silver (Ag), arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe),
indium (In), manganese (Mn), nickel (Ni), lead (Pb),
antimony (Sb), tin (Sn) and zinc (Zn), they are used in
phytoremediation. Other genera of the Cyperaceae (e.g.
Cyperus, Scirpus, Schoenoplectus, and Fimbristylis) also
include species that occur commonly in metal-stressed
settings and are used in phytoremediation (Rai 2009;
Adams et al. 2012). Transplanted Apodasmia similis grow
successfully on tidal wetland sediments affected by elevated As concentrations (Thomsen et al. 2005). Extant
species of Equisetum are commonly found in environments with high concentrations of heavy metals and grow
on metal-polluted soils where they tolerate high concentrations of elements including As, cobalt (Co), Cu, Hg,
Zn and Sb (Siegel and Siegel 1982; Siegel et al. 1985,
Barghigiani et al. 1989, Hozhina et al. 2001, Deng et al.
2004, Chang et al. 2005, Cornara et al. 2007; Mir et al.
2007; N€arhi et al. 2012).
Salinity, alkalinity and metal stress tolerance of the
extinct herbaceous lycophytes of the Late Devonian
Drummond Basin wetlands are less easy to justify purely
on the basis of living analogues. However, living and fossil lycophytes appear to indicate a long history of tolerance of the typical stresses present in hot spring settings.
The living pantropical species Lycopodiella cernua (Lycopodium cernuum), which is often reported in coastal settings
(Given 1993), also occurs in geothermal areas in New
Zealand (Given 1980) and New Britain (Paijmans 1973).
The species is often observed in extremely close proximity
to steam vents (Powell et al. 2000) and is generally associated with acidic soils (Burns and Leathwick 1995) but
also occurs in semi-aquatic settings associated with apron
complexes developing around alkali-chloride hot springs
(Channing 2001). Lycopodium deuterodensum and L. volubile are also reported from lower temperature (c. 20°C)
thermally altered and heated soils from New Zealand geothermal areas (Burns and Leathwick 1995). Various other
species amongst the three extant families of lycophytes
(Isoetaceae, Lycopodiaceae and Selaginellaceae) may be
found in saline and alkaline settings and growing in
metal-stressed habitats. Salinity tolerance is exhibited by
numerous species of Isoetes. Isoetes flaccida is an obligate
brackish or salt marsh species (Light et al. 2007), I. coromandelina and I. riparia grow in saline coastal and estuarine environments (Swain and Kearsley, 2001; Shukla
et al. 2002), and I. muelleri tolerates brackish to saline
conditions in ephemeral lakes with salinities up to
4000 mg/L (Hart et al. 1991). Species of Isoetes colonize
vernal pools where they are subjected to increasing salinity stress during drying phases (Keeley and Bowes 1982).
Several species of Isoetes that are components of oligotrophic lake floras tolerate high pH, examples include
I. lacustris, pH 8.6–8.9 (Szmeja et al. 1997), and I. bolanderi, pH 9.1 (COSEWIC 2006). Vernal pool species (Keeley 1990; Keeley and Zedler 1998) are subjected to
seasonal and diurnal fluctuations of pH. During diurnal
cycles, typical predawn pH ranges from 6 to 7. This
increases by two to four pH units by early afternoon
(Keeley 1990). Species of Selaginella and Lycopodium are
reported as accumulators (rather than hyperaccumulators)
of toxic metals and metalloids including arsenic (Meharg
2003) and aluminium (Hutchinson 1943) and are common members of plant communities associated with
metal-polluted environments such as mine sites where
they tolerate high concentrations of As, Cd, Cu, Hg, Mn,
Ni, Pb, Sn, Tl and Zn (Zhang et al. 2000; Visoottiviseth
et al. 2002; Ha et al. 2009b, 2011b; Ashraf et al. 2011).
Lycopodiella cernua is considered to have potential in phytoextraction for remediation of Pb, Cu, Zn, As and Sn
(Ashraf et al. 2011).
The Palaeozoic to Mesozoic record of fossil lycophytes
includes evidence of salinity and alkalinity stress tolerance
in both arborescent and herbaceous forms. At East Kirkton,
West Lothian, Scotland, Lower Carboniferous arborescent
lycophytes, represented by Lepidophloios sp., Lepidophylloides sp. and stigmarian root mats (Scott et al. 1994), inhabited the wetland margins of an alkaline, hot spring–
influenced lake (McGill et al. 1994). Whilst most arborescent lycophytes are interpreted to have been intolerant of
salt water (DiMichele and Philips 1985), arborescent lycophytes occupying salinity-stressed near-shore and marginal
marine sites are recorded as in situ Stigmaria in bioclastic
carbonates of the Lower Carboniferous, Battleship Wash
Formation, Arizona (Gastaldo 1986). Hartsellea, a Lower
Carboniferous cormose herbaceous lycopsid, which inhabited back-barrier marshes (Gastaldo et al. 2006) is considered to have been tolerant of brackish to saline waters.
Other small, cormose forms, such as Chaloneria, have been
interpreted as capable of living in coastal marsh-like habitats (Greb et al. 2006) as well as more normal settings. Isoetalean lycophytes such as Pleuromeia from the Triassic are
regarded by many authors as halophytes that grew on salt
marshes in coastal or ephemeral semi-desert environments
(Retallack 1975, 1997), coastal mangroves (Krassilov and
Karasev 2009) and as submerged aquatic plants of oligotrophic lakes and ponds (Retallack 1997).
Analogy with specific living (or fossil) plants is not
applicable to the unique early land plants of the Rhynie
chert, because (perhaps with the exception of the putative
lycopsid, Asteroxylon) they lack credible living close relatives. However, based on the ecology of extant plants living around silica-depositing hot springs and the many
examples of younger fossil floras containing stresstolerant plants preserved by ancient hot spring activity
CHANNING AND EDWARDS: ECOLOGICAL FILTERING OF HOT SPRING FLORAS
detailed here, it appears quite plausible to extrapolate and
suggest similar tolerance back to the Early Devonian.
Anatomical adaptations amongst the various Rhynie
plants to reduce evapotranspiration and increase water
use efficiency were reviewed by Channing and Edwards
(2009a). Rhynia and Aglaophyton both possess stomata
with a substomatal channel that widens into a substomatal cavity with thickening and probable cutinization of
the walls of hypodermal and cortical cells adjacent to the
channel. The hypodermal cells partially underpin or cradle the guard cells, which possess pronounced stomatal
ledges. The deeply seated substomatal chamber is lined
with parenchymatous cells, which often possess extensions
such that there is an extensive intercellular space system
forming a tissue that is inferred to have been the site of
photosynthesis. These hypodermal and outer cortical cell
adaptations are thought to have reduced transpiration as
would have the very low stomatal densities (also noted in
Nothia and Horneophyton). A further adaptation to
reduce water loss is seen in the sunken stomata of Asteroxylon, which are borne only on stems with short spiny
and distal leaves in the fertile zone (Edwards et al. 1998).
Intercellular space systems of various configurations are
observed in Rhynia, Asteroxylon and the zosterophylls
Ventarura and Trichopherophyton, although none are as
well-developed as the large aerating systems of the younger plants described above. Nothia and Horneophyton have
large, modified epidermal cells (‘giant cells’) that have
been interpreted to have a water storage function. The
aerial axes of Nothia are characterized by a ‘multilayered
epidermis’ of thick-walled cells that may have reduced
transpiration. Some axes of Rhynia possess rhizoids
around the entire periphery of their basal regions, which
may be involved in absorption of atmospheric moisture.
These adaptations, putatively to reduce water loss and
hence increase water use efficiency or for water storage
and absorption of atmospheric water, all suggest that
either water was not consistently present in the substrate
(i.e. drought), or if present may not have been readily
available to the higher plants. The observation that in situ
growth, early and rapid permineralization and therefore
anatomical preservation were extremely common within
the Rhynie hot spring environment suggests to us that
the latter situation is more likely.
A 400-million-year record of abiotic stress amelioration by
silicon?
Silicon-mediated alleviation of abiotic stress is a relatively
well-known and widely documented phenomenon
amongst living plants (Ma 2004; Liang et al. 2007; Cooke
and Leishman 2011) against excessive heat (Agarie et al.
1998; Linjuan et al. 1999; Wang et al. 2005), drought
547
(Trenholm et al. 2004; Hattori et al. 2005; Eneji et al.
2008; Nolla et al. 2012), salinity (Yeo et al. 2002; Liang
et al. 2003; Zhu et al. 2004; Kaya et al. 2006; RomeroAranda et al. 2006; Xu et al. 2006; Tuna et al. 2008; Chen
et al. 2010; Karmollachaab et al. 2013; Mateos-Naranjo
et al. 2013) and toxic concentrations of metals and metalloids (e.g. Al (Cocker et al. 1998); Cu, Sn, Fe (Neumann
and zur Nieden 2001); As (Guo et al. 2005); Mn (Liang
et al. 2007); Zn, Cd (da Cunha et al. 2008); Sb (Huang
et al. 2012); Pb (Li et al. 2012) and Cr (Ali et al. 2013)).
To date, such studies have concentrated on agricultural
species of angiosperms (rice, wheat, barley, maize, sugar
cane, sorghum, cucumber, tomato) and turf grasses. However, Cooke and Leishman (2011), in a recent review of silicon amelioration of plant stress, concluded that ‘it seems
inevitable that silicon will also prove to be an important
element in abiotic stress alleviation in a range of taxa in
natural plant communities’. Indeed, a recent study of the
salt marsh monocot Spartina densiflora has revealed that
silicon plays a significant role in salinity tolerance in this
wetland halophyte (Mateos-Naranjo et al. 2013).
There are numerous suggested mechanisms by which
silicon alleviates stress. Silicification of leaf and stem surfaces (biomineralization of opal phytoliths) is considered
to significantly decrease water loss by transpiration thus
reducing both real drought (water deficiency) and salinity-induced physiological drought stress (Ma 2003). The
key mechanisms of silicon-mediated alleviation of toxic
element stresses (reviewed in Ma 2004; Liang et al. 2007;
Channing and Edwards 2009a; Cooke and Leishman
2011) include (1) stimulation of antioxidant systems in
plants; (2) complexation or coprecipitation of toxic metal
ions with Si; (3) immobilization of toxic metal ions in
the soil; (4) modification of element uptake processes;
and (5) compartmentation of metal ions within plants.
Close extant relatives of the plants we find preserved in
geothermal wetlands from the Cenozoic (Cyperaceae,
Restionaceae, Equisetaceae), Mesozoic (Equisetaceae and
ferns allied to the Gleicheniaceae) to Late Devonian (lycophytes) accumulate silicon in life and biomineralize to
form phytoliths (Prychid et al. 2004; Briggs and Linder
2009; Mazumdar 2011) and inter- and intracellular silica
bodies in tissues other than the epidermis (Prychid et al.
2004). Silicon is considered an essential element for Equisetum growth, whilst for other higher plants, it is considered only a beneficial element (Chen and Lewin 1969;
Hoffman and Hillson 1979). In a meta-analysis of mean
relative shoot Si concentration of 735 plant species (Hodson et al. 2005), members of the genera Eleocharis (Cyperaceae), Equisetum (Equisetaceae) and the lycophytes
Lycopodium and Selaginella ranked as some of the highest
accumulators of silicon amongst higher plants. A likely
general consequence of in-life deposition of silicon is to
increase plant preservation potential.
548
PALAEONTOLOGY, VOLUME 56
We do not believe that it is coincidence that hot spring
floras both extant and fossil are dominated by silicon
accumulators and that the list of stresses that silicon has
been demonstrated to alleviate encompasses all of the
major stresses associated with growth in hot spring wetlands. We suggest instead that a long-standing ecophysiological resolution to the stresses inherent to geothermal
wetlands is provided by the silicon-rich hydrochemistry
of the environment itself.
bell, Juan Garcia Massini, Pilar Moreira, Sun Ge, Hong-He Xu,
James Wheeley, Jackie Timms and reviewers Charles Wellman
and Barry Lomax. Funding for this research was provided by
The Leverhulme Trust (Grant Number F/00 407/S), NERC
(Grant Number NE/F004788/1) and Royal Society international
travel grants.
Editor. Patrick Orr
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Hot spring habitats are strongly affected by ecosystem
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