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Palaeogeography, Palaeoclimatology,Palaeoecology 127 (1996) 1 20
Coordinated stasis: An overview
Carlton E. Brett a, Linda C. Ivany b, Kenneth M. Schopf b
a Department of Geological Sciences, University of Rochester, Rochester, NY14627, USA
b Museum of Comparative Zoology and Department of Earth and Planetary Sciences, Harvard University. Cambridge,
MA 02138, USA
Received 14 September 1995; accepted 13 December 1995
Abstract
Coordinated stasis, as defined herein, represents an empirical pattern, common in the fossil record, wherein groups
of coexisting species lineages display concurrent stability over extended intervals of geologic time separated by episodes
of relatively abrupt change. In marine benthic fossil assemblages, where the pattern was first recognized, the majority of species lineages (60 to more than 80%) are present in their respective biofacies throughout timespans of 3-7
million years. Most lineages display morphological stasis or only very minor, typically non-directional, anagenetic
change in a few characters throughout a prolonged time interval; evidence for successful speciation (cladogenesis) is
rare, few lineages (< 10%) become extinct, and very few new immigrant taxa become established within a region or
province during such intervals. Moreover, species associations (biofacies) are nearly constant during an interval of
stability, showing very similar taxonomic membership, species richnesses, dominance-diversity patterns and guild
structure throughout. Conversely, during the intervening episodes of rapid change, many species (generally 70% or
more) become extinct, at least locally, some lineages undergo rapid speciation and/or anagenetic change, and new
immigrant taxa become successfully (semi-permanently) established. All (or most) biofacies arrayed across an
environmental gradient display rapid and nearly synchronous changes in various aspects, including species composition,
richness, dominance and guild structure. These intervals of abrupt evolutionary and ecological change typically
represent only a small fraction (< 10%) of the duration of the stable units. The resulting stable blocks of species
separated by turnover events comprise "ecological-evolutionary sub-units" in the Appalachian Basin type example,
and are considered to be components of the longer, more generalized ecological evolutionary units (EEUs) recognized
by Boucot, Sheehan, and others.
Causes of coordinated stasis and of regional ecological crisis/reorganization remain poorly understood. Tracking
of spatially shifting environments appears to be the rule, rather than adaptation to local change. Incumbent species
appear to have a very strong advantage and may exclude potential immigrants, as evidenced by temporary incursions
of exotic taxa ("incursion epiboles"); this suggests a role for ecological and biogeographic factors in maintaining
paleoecological stability. Stabilizing selection may be critical for producing morphological stability in individual
lineages. Episodic crises appear to involve environmental perturbations that were too pervasive and/or abrupt to
permit local tracking of environment to continue. Some faunal turnovers associated with unconformities may be
partially an artifact of stratigraphic incompleteness. Others, however, seem to occur within conformable successions
and were evidently rapid. Widespread anoxia, changes in current patterns, and/or climatic change associated with
major marine transgression are common correlates of faunal turnovers in marine habitats in the Appalachian Basin.
The phenomenon of coordinated stasis has been noted, albeit not fully documented, in a number of ancient marine
and terrestrial ecosystems. An important goal for evolutionary paleoecology should be to document the patterns of
0031-0182/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved
PH S0031-0182 (96)00085-5
2
C.E. Brett et al./Palaeogeography, Palaeoclimatology, Palaeoecology 127 (1996) 1-20
stability and change in common and rare members of fossil assemblages in order to discern the relative frequency of
coordinated stasis in the rock record, to evaluate the mechanisms by which such apparent evolutionary and ecological
stability might be produced, and to seek clues (e.g., paleobiologicaland stratigraphic patterns, geochemicalanomalies)
as to causes of abrupt pulses of faunal change.
Keywords: evolution; ecology/paleoecology;coordinated stasis; speciation; extinction; bioevents
1. Introduction
Nearly a century ago Herdman F. Cleland
(1903) captured the essence of coordinated
stasis - - the near absence of change in species and
in regional fossil assemblages through geologically
long time spans. In the summary to his monograph
on the fossil faunas of the Middle Devonian
Hamilton shales of central New York, Cleland
made the following remarks:
"In a section such as that of the Hamilton
formation at Cayuga Lake... if the statement
natura non saltum facet is granted, one should,
with some confidence, expect to find many - - at
least some - - evidences of evolution. A careful
examination of the fossils of all the zones, from
the lowest to the highest, failed to reveal any
evolutional changes, with the possible exception
of Ambocoelia praeumbona. The species are as
distinct or as variable in one portion of the section
as in another. Species varied in shape, in size, and
in surface markings, but these changes were not
progressive. The conclusion must be that, so long
as the conditions of sedimentation remain as uniform as they were in the section under consideration, the evolution of brachiopods, gastropods,
and pelecypods either does not take place at all or
takes place very seldom, and that it makes little
difference how much time elapses so long as the
conditions of environment remain unchanged."
(pp. 90-91 )
The term "coordinated stasis" was introduced
by Brett and Baird (1992) to denote an empirical
pattern of concurrent near-stasis coupled with
synchronous abrupt change in a number of lineages
and assemblages of fossils. The pattern was initially
recognized in middle Paleozoic marine faunas of
the Appalachian basin, particularly in Ontario,
New York State, and Pennsylvania. Within the
60-million-year interval from the Early Silurian to
the Middle Devonian (approximately 438-378
Ma), Brett and Baird (1995) discern fourteen
blocks of coordinated stasis, each spanning some
3-7 m.y. These intervals include the well-known
Hamilton-Tully fauna of the Middle Devonian
Givetian Stage (Boucot, 1990a; Brett et al., 1990;
Brett and Baird, 1995), the stable nature of which
was already recognized by Cleland (and Williams,
1903), as indicated above.
Within each block of stability in the
Appalachian Basin, faunas of marine invertebrates
(including 60-350 species of brachiopods, corals,
molluscs, echinoderms, and trilobites) display a
high degree of persistence from the stratigraphically lowest to highest available occurrences of
their appropriate facies (Fig. 1). Many species
lineages display little or no morphological change
over prolonged intervals, very few lineages become
extinct, and few new species are added via either
speciation or immigration. Furthermore, most
common to moderately rare species (those making
up 90% or more of individuals within assemblages)
recur in predictable associations ("paleocommunity types" of Bennington and Bambach, 1996)
or biofacies throughout numerous sedimentary
cycles. These associations are characterized by
similar species lists, guild structures, richness,
dominance-diversity, and rank abundance of
common taxa. The restructuring events that form
the boundaries of stable units occur within geologically brief intervals and involve local extinction of
widespread and abundant species, rapid evolutionary turnover, and successful immigration of
"exotic" taxa.
Recently, a number of researchers have
expressed interest in coordinated stasis and what
it may signify (Miller, 1993; DiMichele, 1994a;
Brett, 1995; Morris et al., 1995; Morris, 1995; Roy
and Wagner, 1995). The pattern, if found to be
widespread, carries with it a number of interesting
C.E. Brett et al./Palaeogeography, Palaeoclimatology, Palaeoecology 127 (1996) 1 ~ 0
implications for both evolutionary and ecological
theory. Because of this, and in the context of this
theme issue, we feel that a broad overview of
coordinated stasis and a discussion of its defining
characteristics is especially needed at this time.
Below, we propose an operational definition that
can be consistently applied to other studies, suggest
how best to test for the pattern in a stratigraphic
section, review work done to date that suggests
the widespread nature of the phenomenon, and
discuss its macroevolutionary and ecological
implications.
2. Definitions
Coordinated stasis, in its simplest form, is an
expression of the bimodality of evolutionary and
ecological change. This general phenomenon has
been emphasized by Boucot (1975, 1978, 1983,
1986, 1990b), who recognized that patterns of
stability and change in the fossil record are hierarchically organized. While his concept of Ecological
Evolutionary Units (EEUs) is widely known, he
has also acknowledged that these large-scale
(107-10 s m.y.) units are in turn composed of
smaller scale subunits that display consistent
groupings of species or lineages (Boucot, 1990c,
1994). Although Boucot's (1994) EE subunits are
global and often substantially longer than the
regional blocks of stasis termed EE subunits by
Brett and Baird (1995), several of the subunit
boundaries in the Silurian-Devonian interval of
the Appalachian Basin match those of Boucot
rather closely. Coordinated stasis, however, was
first documented and is expressed as a regional
phenomenon, with intervals of stability lasting on
the order of several millions of years.
The pattern of concurrent stasis and abrupt
change (coordinated stasis) in mid-Paleozoic
faunas of the Appalachian Basin provides the basis
for recognition of many of the local stages. We
draw upon these stable faunas as the type example
for coordinated stasis, and suggest, based upon
them, the following set of defining characteristics.
We concede that the details of this definition are
in some ways arbitrary, but see the value of having
some specific set of properties against which to
3
compare other concepts and data within the theme
of bimodal turnover. To ensure meaningful comparisons, the definition should include the percentage of taxa persisting throughout a given interval,
the proportion of taxa retained across turnover
events, and the amount of time (relative and
absolute) encompassed by stasis and turnover;
taxonomic scale should also be carefully
considered.
It is extremely unlikely that any two fossil
assemblages, like any two modern communities,
will ever be exactly the same, hence we must decide
how much paleoecologic variability to include
under the aegis of coordinated stasis. Based on the
Appalachian Basin example, we tentatively define
blocks of coordinated stasis as intervals, generally
exceeding one million years in duration, during
which 60% or more of species-level lineages persist
from oldest to youngest samples of appropriate
biofacies, with only minor and typically non-directional evolutionary changes. In contrast, relatively
few lineages (fewer than 40% of lineages, and
typically less than 20% in Appalachian Basin
examples), persist across bounding intervals of
evolutionary restructuring. This definition recognizes that minor morphological changes in one or
a few characters may occur within lineages (e.g.,
reduction of the number of eye files from 18 to 16
in "Phacops rand', but with no accompanying
directional change in any other morphological
character, as documented by Eldredge, 1972, and
discussed by Eldredge and Gould, 1972). However,
these changes represent either anagenesis or possibly cladogenesis in which the parent species
rapidly became extinct, and thus result in no net
increase in diversity. In saying "non-directional"
we further acknowledge that morphological
changes may be reversed in time. Such morphological reversal within lineages has for example been
documented by Stanley and Yang (1987) and
Sheldon (1987, 1990, 1996) and appears to be
common within the Middle Devonian Hamilton
Group (see Lieberman et al., 1995, for a quantitative example). The result can be termed "net
stasis".
Again drawing on the Appalachian Basin example, we further suggest that a block of coordinated
stasis be defined as an interval of faunal stability
4
C E. Brett et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 127 (1996) 1-20
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C.E. Brett et al./Palaeogeography,
Palaeoclimatology, Palaeoecology 127 (1996) 1-20
persisting at least 10 times longer than the pulse
of change that marks its boundary. Although
absolute time is difficult to measure in many
ancient stratigraphic settings, relative durations of
stability may be interpreted on the basis of stratigraphic scale. For example, substantial change in
a fauna can be said to be abrupt if it occurs on
the scale of a single biostratigraphic subzone or
small-scale sedimentary cycle while adjacent faunas
appear stable over 10 or more subzones or sedimentary cycles of a similar scale. A sequence
stratigraphic framework can be especially useful
for this purpose (e.g., Brett and Baird, 1995).
Appalachian Basin faunas persist on the order of
5
3-7 m.y., while turnovers, where best constrained,
occur within at most a few hundred thousand
years (i.e., one fourth order cycle; Brett and
Baird, 1995).
An important aspect of the coordinated stasis
pattern is the pronounced bimodality of turnover
rates. Speciation (cladogenesis), extinction, and
successful immigration rates are all very low within
stable blocks but peak sharply at interval boundaries. This is particularly so for common to abundant and widespread species, a general feature of
the fossil record emphasized by Boucot (1990b,
1994), but at least in the Appalachian Basin it
appears also to be true of many rare and/or
Fig. 1. Stratigraphic ranges of Middle to earliest Late Devonian fossil species of two broadly defined biofacies in the Hamilton
Group of west-central New York (Cayuga to Owasco Lake outcrops). Symbols for range lines: thick line =species common to
dominant; thin line = species present; arcuate curves to left imply species migrated temporarily from study area down slope (south);
arcs to right indicate species migrated temporarily upslope (north) tracking favored environment; • =incursion epibole, brief
incursion of species not normally present; o = outage, temporary absence of normally common species from appropriate biofacies.
These data encompass a small subset of available Hamilton Group taxonomic ranges, and are presented from a limited geographic
perspective so as to illustrate faunal tracking. Note that most Hamilton-Tully species are first recorded at the base of subunit X,
track their preferred environment in unison, maintain similar relative abundances throughout, and disappear in concert at the top
of the interval.
E E subunits= ecological-evolutionary "faunas" defined by Brett and Baird 1992, 1995); I X = O n o n d a g a fauna; IXA = modified
Onondaga assemblage plus unique elements of the Stony Hollow fauna; X = Hamilton-Tully fauna; X I = Genesee fauna. Adjoining
portions of subunits IX and XI are included only for comparison. Relative sealevel curve is calibrated by fossil benthic assemblages
(BA) as defined by Boucot (1975); note that only the deeper end of this scale is shown, with BA 3.5=diverse coral-rich offshore
communities; BA 4 = diverse brachiopod communities; BA 4.5 = ambocoeliid communities and BA 5 = high dominance leiorhynchid
communities typical of dark gray to black shale. Fossil species are subdivided roughly by biofacies into two groups: (1) those that
characterize deeper water, dysaerobic areas typified by small ambocoeliid, chonetid, and leiorhynchid brachiopods (BA 4-5); (2)
those that typify diverse brachiopod biofacies of oxic shallower, calcareous gray mudstones or argillaceous limestones (BA 3-4).
Abbreviations for biostratigraphic zones include P. costatus = Polygnathus costatus costatus zone; aus = P. australis zone; koc =
Tortodus kockelianus kockelianus zone; P. ensensis = P. xyleus ensensis zone; l., m., and u. varcus = lower, middle and upper P. varcus
subzones; h. c. = P. hermanni-P, cristatus zone. Abbreviations for major shallowing pulses include: (for Onondaga) U M =
Uppermost Moorehouse Member grainstones; TI="Tioga-B'" bentonite; US=upper Seneca Member; (for Hamilton-Tully) S H =
Stony Hollow (thin limestone bed in western NY); CV=Cherry Valley Limestone; H H = Halihan Hill fossil bed; S L = Solsville
horizon; PK=Pecksport horizon; M V= Mottville (Stafford) Member; C H = Cole Hill fossil bed; D S = D e l p h i Station cap bed;
S R = Slate Rock bed (cap Pompey Member); CF=Centerfield (Chenango) Member; S T = Staghorn coral bed; J O = J o s h u a coral
bed; D C = D a r i e n Center coral bed; BL=Bloomer Creek fossil bed; TT=Tichenor Limestone; MT=Menteth Lemestone; R C =
Rhipidomella-Centronella fossil bed; P = unnamed top Kashong phosphatic beds; B V = Bay View coral bed; TA = Taunton fossil beds;
BC=Bellona coral bed; (for Genesse) F T = F i r Tree fossil bed; LO=Lodi fossil bed.
Abbreviations for species: Dysaerobic Assemblage: Pac= Pacificocoelia acutiplicata; Ambo=Ambocoelia cf. A. umbonata; Emn=
Emanuella cf. E. praeumbona; Tru= Truncalosia truncata; Lon =Longispina mucronata and/or L. deflecta; Dev= "Devonochonetes"
scitulus; Eum = Eumetabolotoechia cf. E. multicosta (all brachiopods); Fully Aerobic Diverse Assemblage (all brachiopods unless otherwise
noted): Acro = Acrospirifer duodenaria; Mgk = Megakozlowskiella raricosta; Coe = Coelospira cf. C. camilla; Lep = Leptaena "rhomboidalis"; Odt = Odontocephalus sp. (trilobite); Ph c = Phacops cristata (trilobite); Sch = Schizophoria sp.; Var = Variatrypa arctica; A th =
Athyris cf. A. cora and A. spiriferoides; Eli=Elita fimbriata; M s t = Megastrophia cf. M. concava; Nuc=Nucleospira aft. N. ventricosa
(Onondaga) and N. concinna (Hamilton); Psa="Pseudoatrypa" sp. (Onondaga) and cf. P. devoniana (Hamilton-Tully); Str=
Strophodonta cf. S. demissa; Hel h = Heliophyllum halli (coral); Hel c= H. confluense (coral); Med= Mediospirifer cf. M. audaculus;
Muc = Mucrospirifer mucronatus; Mcn = "Mucrospirifer" consobrinus; Spi A = Spinocyrtia sp. A cf. S. granulosa (no medial notch on
fold); Spi B = Spinocyrtia sp. B cf. S. granulosa (deep notch on fold) - - note alternation of morphotypes between A and B: Tro =
Tropidoleptus carinatus; Spn= Spinatrypa spinosa; Hyp= Hypothyridina cf. H. cuboides; Ph r= Phacops rana (trilobite).
6
C.E. Brett et al./Palaeogeography, Palaeoclimatology, Palaeoecology 127 (1996) 1-20
stenotopic taxa as well. Not only are speciation
and extinction rates bimodal, but the magnitude
of net evolutionary change that occurs in a given
lineage within, versus between, stable blocks also
appears to be bimodal. Species within lineages
display very trivial morphological change during
a block of coordinated stasis, whereas at the
transition between blocks, those same lineages may
display marked evolutionary changes often resulting in the assignment of new families, genera or
subgenera. The combination of bimodal evolutionary change and immigration has the effect that
recognizable ecological change is also bimodal (as
noted in Boucot, 1990b). Despite wobbling or
minor directional change in morphology, species
lineages within blocks of coordinated stasis retain
their characteristic positions in biofacies, as measured by relative abundance and associated lineages, until they are disrupted by crisis events
associated with a boundary turnover.
Coordinated stasis as described in the
Appalachian Basin is not just a phenomenon
observed in a single facies, nor one observed at
different times in adjacent facies (Brett et al., 1990;
Brett and Baird, 1995). Stability in this case is
manifest concurrently across the entire spectrum
of environments present (for example, dysoxic
deep water biofacies, shallow marine shelf and reef
assemblages, and nearshore low diversity faunas
all appear to be relatively stable at the same time).
And while these different types of communities do
not display the same degree of change at interval
boundaries (stressed nearshore and dysoxic communities for example show considerably less turnover than do highly diverse shallow-shelf biotas),
all of these biofacies appear to be affected more
or less simultaneously during turnover events.
Thus, it is the entire floral-faunal gradient of a
particular environmental complex that exhibits
stability or turnover, rather than simply a single
biofacies at a time. This pervasive pattern delineates the "ecological-evolutionary sub-units" (EE
subunits) of Brett and Baird (1992, 1995); see
Boucot, 1990c), in which a generally similar array
of biofacies persists for the duration of a stable
interval, each biofacies exhibiting coordinated
stasis.
The Appalachian Basin example suggests that
biofacies in an EE subunit are consistent to within
about 20% in (a) species richness, (b) dominancediversity; (c) rank abundance of most common
species, (d) number of guilds; (e) number of species
within guilds. Moreover, guilds (or broadly defined
groups of niches; cf. Bambach, 1983, and Watkins,
1991) are staffed mainly by the same or very
closely related species throughout the interval.
Turnover between EE subunits is rapid and
synchronous, and no single unifying causal mechanism for this change can be discerned, although
some bounding crises do correlate with evidence
for environmental changes such as major sea level
fluctuation and climatic change. While several of
the EE subunit boundaries in the Appalachian
Basin also correlate with previously recognized
global bioevents, as already mentioned, it remains
to be seen whether most locally identified EE
subunits are (like EEUs) global in extent (see
Boucot, 1996), or even whether coordination
across biofacies occurs in other settings.
3. Taxonomic scale
Within-lineage changes may in some cases be
afforded species-level status; however, there are
undoubtedly inconsistencies in the degree to which
these minor variants are split into distinct species.
These inconsistencies reflect differences in taxonomic practices and biostratigraphic utility for the
group in question. In many such cases, these minor
morphological changes within lineages can be contrasted with much more substantial changes across
the fauna that are associated with interval boundaries (i.e., generic-level change or extinction of
lineages altogether). We argue, therefore, that
clades that display species-level turnover are not
exceptions to a pattern of coordinated stasis, so
long as they display a pronounced and coordinated
bimodality in the magnitude of their evolutionary
change (allowing the recognition of blocks of
relative stability). Generic level stability, despite
turnover in species assemblages, has been documented for portions of Cambrian biomeres
(Westrop, 1994, 1996) and benthic marine communities from the Ordovician (Patzkowsky, 1994)
and Neogene (Stanton and Dodd, 1994). Boucot
C.E. Brett et al./Palaeogeography, Palaeoclimatology, Palaeoecology 12 7 (1996) 1-20
( 1975, 1983) also defined his "community groups"
based on generic-level stability. In each case, relatively stable intervals are separated by rapid,
larger-scale turnover. This, in fact, may prove to
be the more general pattern, of which the pronounced species-level stability seen by Brett and
Baird in the Appalachian Basin is simply a
special case.
4. Documentation of coordinated stasis
Assessing the possibility of stasis first requires
a comprehensive sampling strategy. Bulk sampling of fossil assemblages provides data on (at
least) the abundant taxa within assemblages.
Representative collections should be obtained
throughout the interval, or at least from the stratigraphically highest and lowest occurrences of a
biofacies. Sampling also needs to be comprehensive
enough to recover rare species in the fauna, and
this is not an easily resolved issue. Cobabe and
Allmon (1994) suggest an approach to the problem
by estimating sample sizes necessary to accurately
represent abundances of different portions of the
fauna. Perhaps the most effective strategy is to
obtain data from large collections at numerous
horizons. This may or may not be possible in
certain cases, and it is very time consuming, but it
is the only realistic way to determine at least the
presence of rare species in different horizons. Such
data exist for many of the classically studied
assemblages in North America and Europe, and
these well documented faunas form a logical starting point for studies of coordinated stasis (e.g.,
Morris et al., submitted). In addition, classical
studies of biofacies provide a wealth of information
that could be tapped as initial indicators of a
coordinated stasis pattern (Schopf, 1996).
To document associations and their paleoecology, assemblages must be censused and assessed
in terms of ecological parameters, including: (a)
species composition, (b) species richness corrected
for sample size, (c) dominance diversity (ShannonWeaver Index or others), (d) rank abundance of
the more common genera and species, and, (e)
trophic/substrate guild structure, (proportion of
deposit feeders, suspension feeders, infauna versus
7
epifauna, etc.). Bennington and Bambach (1996;
Bennington, 1994; Bambach and Bennington, in
press) stress the importance of comparing sample
variation within individual sample horizons to that
between horizons from the same and (ideally)
additional outcrops in order to compare spatial
variability in assemblage properties at a given time
with temporal variability seen from one level to
the next. A variety of multivariate statistical techniques have been employed to assess these properties in a fauna (see e.g., Bennington and Bambach,
1996; Holterhoff, 1996; Morris, 1996).
The morphologies of individual taxa must also
be studied critically to test for the presence or
absence of significant morphological change; working solely from species lists may only tell us about
the dynamics of taxonomic nomenclature. The
appropriate analysis of morphological change
throughout a study interval of this magnitude is
laborious to be sure, for a number of lineages
must be studied throughout the interval in question
(unlike studies testing for punctuated equilibrium
in a single lineage or restricted group). Work on
several Hamilton Group taxa (Lieberman, 1994;
Lieberman et al., 1995) supports the claim for net
morphological stasis among co-occurring species
(Lieberman and Dudgeon, 1996). The basic guidelines for carrying out such a study, as well as
initial results, are outlined by Morris (1994, 1996).
Several earlier studies of morphological variation
through time, originally designed to test the model
of punctuated equilibrium in single lineages, also
coincidentally show simultaneous speciation and
probable linked stability in a number of lineages
(Williamson, 1981; Cheetham, 1986, 1987; Jackson
and Cheetham, 1994). Admittedly, most morphometric studies to date have involved common taxa.
Boucot (1990c) has asserted that rare taxa should
display more significant evolutionary change than
those that are common. There are obvious problems in obtaining samples of rare species large
enough to allow for statistically significant morphological analysis, hence Boucot's hypothesis
remains to be critically tested. Observations of
long-ranging rare species in the Devonian of New
York, however, do not reveal evidence for substantial change. For example, Lieberman's (1994)
study of the rare proetid trilobites Basidechenella
8
C.E. Brett et al./Palaeogeography, Palaeoclimatology, Palaeoecology 127 (1996) 1-20
rowi and Monodechenella macrocephala in the
Middle Devonian Hamilton Group indicates near
identity at all levels.
Lastly it is valuable, though not essential, that
a time series be sufficiently long as to encompass
the possibility of not only an interval of coordinated stasis, but a turnover event as well. In
situations where range data are available for a
number of coexisting taxa throughout a significant
period of time, it may be possible to test for the
likelihood of coordinated stasis by looking for
bimodality in the rate of turnover. This approach
has been attempted by Alroy (1996) and yielded
negative results (in this case, for Cenozoic mammals of North America). This method may be
compromised if not all the biofacies encompassed
by the data exhibit coordinated stasis, or if mixing
of biogeographic zones has inadvertedly occurred
in tabulating data. It would be a useful exercise to
apply Alroy's method to sections where coordinated stasis has already been independently
demonstrated to test the statistical "strength" of
the bimodal signal.
5. Artifacts and biases: Potential confounding
factors
Two sources of bias in the fossil record may
produce a false appearance of coordinated stasis
and thus should be anticipated in the interpretation
of any particular suite of fossil distribution data.
The first, explicated by Holland ( 1994, 1995, 1996),
is the truncation of fossil range data by sequence
boundaries. Specifically, Holland suggests that not
only can the effect of missing time at an unconformity create the illusion of pulsed turnover, but
that major facies shifts associated with sequence
boundaries and transgressive surfaces (regardless
of whether or not time is missing) can produce a
similar effect. This potential artifact can be ruled
out with geographically widespread data sets in
which it is possible to trace lateral shifts of a given
biofacies within precisely correlated stratigraphic
intervals, as has been done to some degree with
the Appalachian Basin faunas (e.g., papers in
Brett, 1986, and Landing and Brett, 1991; Brett
and Baird, 1995). If, however, major unconformi-
ties occur at boundaries between apparently stable
faunas, it may not be possible to determine whether
or not a pattern of simultaneous change in many
lineages is real or merely artifactual.
Some of the known intervals of coordinated
stasis appear to have their boundaries in relatively
conformable successions. The boundaries of at
least six EE subunits (blocks of gradient-wide
coordinated stasis) in the Appalachian Basin occur
not at erosional sequence boundaries, but rather
associated with highstands in conformable, if also
somewhat condensed, successions of dark gray
and black shale facies (Brett and Baird, 1995).
Moreover, a number of these blocks of stability
contain within them sequence-bounding unconformities of considerable magnitude. For example,
the Hamilton-Tully fauna in the Appalachian
Basin contains a major (second order) sequence
boundary (Taghanic unconformity of Johnson,
1974, which separates the lower and upper halves
of the Kaskaskia supersequence; Sloss, 1963).
Typical Hamilton species, as well as characteristic
biofacies, recur in the upper part of the Tully
Limestone well above this unconformity. Hence,
relatively large unconformities are commonly not
associated with EE subunit boundaries; such
boundaries are commonly present in other parts
of the sedimentary cycle, such as highstand or
transgressive systems tracts. Furthermore, even
though unconformities may impart an artificial
coincidence to the beginnings and ends of species
ranges, they can not account for the persistence of
similar assemblages of species and the paucity of
evolutionary change within bounded sequences.
A second possible pitfall in the identification of
coordinated stasis involves the notion of "taphonomic mirage", as suggested by William Miller
(1993, 1994, 1996). Miller cautions that certain
recurring fossil assemblages may merely represent
taphofacies. In other words, consistent preservational conditions associated with persistent sedimentary environments may favor preservation of
only certain members of original communities. If
those members also happen to be long-ranging
taxa, a mirage of coordinated stability could be
the result (see also Westrop, 1996). Taphofacies
are a reality and taphonomic processes may select
for robust or insoluble skeletal elements. However,
C.E. Brett et al./Palaeogeography, Palaeo¢limatology, Palaeoecology 127 (1996) 1 20
taphonomic bias alone cannot explain distribution
patterns such as those common in the Hamilton
Group wherein different associations of taphonomically equivalent species recur in distinct but sedimentologically similar facies. For example, among
similar sized calcitic brachiopods, differential
taphonomic processes could not possibly produce
the marked differences in species assemblages
observed among varied Paleozoic mudrock facies
within particular EE subunits. Taphonomic mirage
can generally be ruled out, especially if the assemblages involved are relatively diverse and wellpreserved.
Miller (1994, 1996) has also distinguished
between what he sees as coordinated stasis sensu
stricto and the concept of "structural continuity"
in which taxonomic staffing and functional categories (guilds) tend to recur predictably when
similar habitats happen to appear, without
spatiotemporal continuity of specific associations
(see also Bennington and Bambach, 1996).
Coordinated stasis, defined as a pattern of shared
morphological stasis in many lineages and of
recurring biofacies, does not specify that species
associations must persist without any change.
Indeed, species associations or biofacies may recur
either because they persist as tracking assemblages
or because similar associations of species reassemble in similar environments without actual temporal continuity. Neither pattern is specified or
precluded in the present definition of coordinated
stasis, although their ecological implications differ.
So long as the same species lineages are involved
in the recurring assemblage, Miller's structural
continuity would be encompassed (in part) within
this concept of coordinated stasis. Apparent examples of both phenomena have been reported in the
literature (Brett and Baird, 1995; Holterhoff, 1996;
Bennington and Bambach, 1996; see Schopf, 1996,
for discussion). Without good geographic and
paleoenvironmental control, it will be impossible
to distinguish between these two possibilities. A
strong indication of persistence and tracking, however, is provided by recurrent appearances of
highly stenotopic taxa in specific facies and associations. Because these species only occur in single
environments, it would seem that those particular
conditions must persist somewhere throughout the
9
entire range of the stenotope species to prevent
extinction of the species (but see Holterhoff, 1996,
for an alternative).
6. Generality of the pattern
The notions of coordinated stasis and ecological
evolutionary subunits were founded in large measure on the basis of the well preserved fossil record
of marine invertebrates from the mid Paleozoic
(Brett and Baird, 1995; Boucot, 1994; Fig. 1).
Recently, however, a number of researchers have
reported findings suggestive of coordinated stasis
from many disparate ecological settings; many of
these studies are published in this theme issue.
Below, we briefly review the spectrum of environments from which this or similar phenomena have
been reported (see also Boucot, 1996). It should
be noted that the studies chosen to illustrate
pattern in different settings by no means compose
an exhaustive account of relevant work in these
areas; rather they represent a sub-sample heavily
weighted toward work in this theme issue. These
case studies indicate that the pattern of concurrent
morphological stasis and ecological stability at the
scale of coordinated stasis is neither unique to the
middle Paleozoic, the Appalachian Basin, nor to
marine benthic biotas.
6.1. Marine benthos
A number of studies have now documented
patterns of coordinated stability and change in
various marine invertebrate ecosystems ranging
from the Cambrian to the Neogene. Two well
documented examples of stasis at the species level
are provided in papers by Bennington and
Bambach and by Holterhoff in this issue.
Bennington (1993, 1 9 9 4 ; Bennington and
Bambach, 1996) presents work on recurrent assemblages in four successive marine tongues within a
dominantly nonmarine fluvial deltaic succession in
the Pennsylvanian of Kentucky. Using cluster
analysis to characterize faunas, he is able to
demonstrate a considerable degree of similarity
amongst faunas that differ in age by as much as
10 m.y. Similar species lists and even similar rank
10
C.E. Brett et al./Palaeogeography, Palaeoclimatology, Palaeoecology 127 (1996) 1-20
abundance of common species characterize all four
major cycles. Although detailed studies reveal
small but statistically significant differences among
faunas (largely owing to variations among rare
taxa), on the whole we find that Bennington's
study provides one of the best documented cases
for general continuity of lineages and similar
recurring assemblages.
The study by Holterhoff (1994, 1996), also in
the Pennsylvanian, examines a number of crinoid
biofacies in cyclothems of the North American
midcontinent. He, too, is able to identify a number
of internally consistent assemblages that persist
through major changes in sea level. His work will
also shed light on the question of faunal tracking
versus disintegration and reassembly of faunas, for
one of his biofacies apparently occurs only in the
transgressive portions of his three sea-level cycles,
while it is absent from the corresponding regressive
portions. He implies that the biofacies, lacking a
refugium, is reassembled from the "species pool"
d e n o v o each time the environment is present,
making it all the more interesting that cyclic reassembly should repeatedly produce this "same"
biofacies (see Schopf, 1996). Some degree of uncertainty will always remain, however, about whether
this biofacies may have persisted elsewhere in a
refugium during the intervals in which it has not
been found.
Another set of studies has revealed species level
stability and rapid turnover in the marine realm,
in this case for reef corals in the late Cenozoic.
Jackson (1992) has demonstrated pronounced stability in Caribbean reef coral community zonation
and composition for the last 2 million years,
despite large climatic and sea-level changes associated with Plio-Pleistocene glaciation. Budd et al.
(1994) have extended this pattern back into the
Miocene, documenting long term fauna-wide stability punctuated by a rapid turnover event at
roughly 8 Ma. They have been able to collect
information on the geographic expression of the
turnover episode, and interestingly, while the turnover occurs in the same way at each site in the
Caribbean, each locality across the region apparently experiences turnover at a slightly different
time (Budd and Johnson, 1995). Their work will
be crucial to understanding the mechanisms of
ecosystem disintegration and the establishment of
new stable states.
A number of other studies from marine settings,
while observing continual turnover at the species
level, have been able to demonstrate patterns of
coordinated stasis at the generic level. As discussed
above, we believe such patterns may be expressions
of a phenomenon similar or identical to that seen
at the species level in other cases. Cambrian trilobite biomeres are the best example of this situation.
Biomeres, as defined by Palmer (1965, 1984), bear
numerous resemblances to the EE subunits of the
Silurian and Devonian Appalachian basin (Brett
and Baird, 1995). They display persistent and
recognizable biofacies characterized by similar
suites of morphologically consistent trilobite
genera (particularly in later phases of biomere
development; Westrop, 1996), separated by sharp
boundaries at which there is substantial extinction
of long-lasting lineages followed by rapid evolutionary and immigrational turnovers. There is,
however, considerably more turnover at a species
level within these biotas than in the stable faunas
of the middle Paleozoic (Westrop, 1994, 1996).
Westrop (1996) suggests that the more rapid
species level turnover may be a consequence of the
higher volatility (sensu Gilinsky, 1994) of
Cambrian trilobites compared with later Paleozoic
clades, as Foote (1988) has shown relative to
Ordovician and later trilobites. We maintain that
so long as this turnover is within lineages and
biofacies retain their distinctive character over the
interval, the bimodal signature of coordinated
stasis may be manifested here at the generic level.
Similar scenarios are described by Patzkowsky
(1994) and Stanton and Dodd (1994). Patzkowsky
(1994) has documented two intervals of about 15
m.y. each in the Middle and Late Ordovician of
eastern North America during which brachiopod
biofacies remain stable. Again many genera, but
not species, persist in similar associations. These
intervals are inferred to represent prolonged environmental stability terminated by abrupt paleooceanographic shifts. Stanton and Dodd (1994)
also describe generic level stability in molluscan
associations from the Neogene of California
despite considerable species-level change.
C E. Brett et al./Palaeogeography, Palaeoclimatology, Palaeoecology 127 (1996) 1 20
tl
6.2. Terrestrial plants
6.4. Microfossil assemblages
Very similar patterns of morphological stasis
and stability of associations of plant species have
also been documented for Carboniferous flora
(Wing, 1984; Raymond, 1993). In particular,
DiMichele and Phillips (1992, 1995, 1996);
DiMichele, (1993, 1994b) have documented that
persistent assemblages of coal swamp plants display similar ecomorphic guilds filled by the same
or closely related species for up to 9 million years.
These floras, like the faunas in the Appalachian
Basin, show tracking in response to environmental
change and stability across the entire "landscape",
or biofacies gradient.
Three recent studies of benthic foraminiferal
assemblages from the Palaeogene suggest that
coordinated stasis may be expressed in these communities as well. Gaskell (1991), McGowran
(1987), and McKinney and Frederick (1994) all
report intervals of persistent or recurrent stable
associations separated by brief intervals of turnover in, respectively, the U.S. Gulf Coastal Plain,
Southern Australia, and the U.S. Atlantic Coastal
Plain. McKinney and Frederick (1994; McKinney
et al., 1996), however, caution that this pattern
may only be characteristic of the more common
taxa in an assemblage. This is a point that deserves
further consideration with respect to other studies
as well (see Boucot, 1996).
Lastly, there are even suggestions of some form
of coordinated stability in the plankton. Lu and
Keller (1994, 1995) document three intervals of
relative faunal stasis in Paleocene-Eocene planktonic foraminiferan assemblages from the Tethys
region. Steady levels of species richness and low
rates of species turnover characterized the stable
intervals, and these were terminated by brief
(200 400 k.y.) intervals of rapid turnover. Stability
persisted despite fluctuations in temperature indicated by oxygen isotopic values, and turnover at
least in one instance seems to correlate with large
scale climatic change.
6.3. Terrestrial animals
Early indications that some terrestrial vertebrate
communities may display coordinated stasis came
from Olson (1952). His account of Permian vertebrate "chronofaunas" reports a pattern of long
term stability in vertebrate associations separated
by brief intervals of turnover and reorganization.
Later, Vrba (1985) described antelope faunas from
Africa that also display comparably long intervals
of morphological stasis and ecological similarity
punctuated by rapid change. These observations
provide the basis for her "Turnover-Pulse
Hypothesis". Prothero (1994; Prothero and
Heaton, 1996) has demonstrated the persistence
of many Western Interior mammal species
through climatic crises associated with the
Eocene Oligocene boundary, potentially indicating some form of resistance to disturbance events
by the fauna. Barry et al. (1995) report intervals
of relative faunal stability in mammal assemblages
from the Neogene Siwaliks in northern Pakistan.
Even the North American Land Mammal Ages
used in Cenozoic biostratigraphy display staffing
of persistent guilds by the same or related species
for periods of up to 10 million years (Woodburne,
1987; Prothero and Berggren, 1992). Some invertebrate assemblages on land may also exhibit a
pattern of coordinated stasis and change, as Morris
(1994, 1996) suggests based on preliminary studies
of gastropods from Pliocene rift basin faunas
in Zaire.
6.5. Counterexamples
Hence, a wide array of studies from diverse
ecological settings and varied taxonomic groups
seem to demonstrate comparable patterns that
might be consistent with our definition of "coordinated stasis". There are a number of studies,
however, that appear to show contradictory patterns for long-term community behavior. Most
commonly cited are the many studies from the
Pleistocene and Holocene, where high resolution
work has been able to decipher biogeographic
changes for individual taxa on the scale of
103-104 yr (e.g., Huntley and Webb, 1988, 1989)
for terrestrial plant communities based on pollen
records, and Valentine and Jablonski (1993) and
Roy et al. (1995) for western North American
12
C.E. Brett et al./Palaeogeography, Palaeoclimatology, Palaeoecology 127 (1996) 1-20
marine mollusk assemblages). Such studies report
continual change in community composition governed by individualistic responses and reaction lag
times of taxa to environmental change. Boucot
(1983, 1990b) reviewed this apparent departure
from the apparent stability of more ancient marine
communities and termed it the "Pleistocene
Paradox". While these studies are certainly telling
us something about the last 125,000 yr, we question
the comparability of short-term, high resolution
studies during a period of glaciation and rapid
climate fluctuation to the longer term studies of
coordinated stasis in the fossil record (see also the
review in Schopf and Ivany, in press). Time averaging in the fossil record may serve to filter out such
high frequency variation and reveal an equally
meaningful longer-term pattern. Boucot (1990b)
also makes this point, and adds that marine benthic
communities were likely more "buffered" against
the severe short-term climate fluctuations that so
affected the terrestrial systems from which most of
these inferences are drawn. It should also be
remembered that intervals of turnover in the midPaleozoic Appalachian Basin persisted on the
order of tens- to hundreds-of-thousands of years,
comparable to the entire duration of intervals
encompassed in these more recent studies.
Another recent paper that could be cited as a
counter to coordinated stasis is the Buzas and
Culver (1994) study of benthic foraminiferan
assemblages from the Tertiary Coastal Plain of
eastern North America (see, e.g., Jackson, 1994).
These authors tallied benthic foraminiferal assemblage composition for each of six transgressive
intervals and examined immigration/emigration,
speciation/extinction, and persistence of foraminiferan species between each interval. From their
results they were able to conclude that assemblages
in each successive interval were very different from
one another and therefore did not support migration of a coherent ecological association in
response to sea-level change. The analogy to
studies of coordinated stasis, however, breaks
down when one considers the scale at which this
study was done (see Schopf and Ivany, in press).
Foraminiferal assemblages for a given trangressive
interval in this case represent a single list of taxa
present during any part of that interval, each of
which encompasses 2-8 m.y. There are no data
for times between successive transgressions, hence
there are gaps of up to 24 m.y. for which no
information is recorded. Were coordinated stasis
to be present in this setting, it would unquestionably be missed by the sampling strategy employed
by Buzas and Culver. In fact, were the Appalachian
Basin faunas to be sampled in a similar way, the
results would be the same as those seen in the
foraminiferid study because only a single sample
from each of the intervals of coordinated stasis
(EE subunits) would be available for comparison.
The Buzas and Culver study is valuable for examining the restaffing of assemblages from the overall
species pool following large scale, long-term sealevel change. Using this paper as a counter-argument to coordinated stasis, however, would be
unjustified unless the authors are able to resolve
changes in assemblage composition within each
successive transgressive interval.
6.5.1. Macroevolutionary implications
Coordinated stasis appears to be a common
phenomenon that characterizes fossil faunas in
widely different paleoecological settings. A number
of constraints on the modes of large scale evolution
would seem to be imposed by the pervasiveness of
this pattern. We contend that coordinated stasis
runs counter to expectations of many traditional
evolutionary models, including those of the
Modern Synthesis (Dobzhansky, 1937). The
pattern indicates that stability is the norm throughout life's history and that evolutionary change is
rare and discontinuous. Moreover, and echoing
the claim of Boucot (1990b), the pattern indicates
that both significant morphological change within
organisms and major ecological restructuring
occur in a very small proportion of earth's history,
perhaps less than one percent of geologic time.
Many long-standing ideas about the mechanisms
of large scale evolutionary change must be reconsidered in light of this widespread pattern of stasis.
6.5.2. Diversity trends
A trend of progressively increasing taxonomic
richness through the later Mesozoic and Cenozoic
has long been recognized within the marine fossil
record (Valentine, 1973; Sepkoski, 1978, 1981).
C.E. Brett et al./Palaeogeography, Palaeoclimatology, Palaeoecology 127 (1996) 1-20
Similarly, an increasing number of guilds of benthic organisms was documented by Bambach
(1975). Nonetheless, empirical studies of Mesozoic
and Cenozoic marine communities indicate that
species number, guild structure, and generic, if not
actual species identity, may be maintained through
long blocks of time (see Dockery, 1986; Gaskell,
1991; Stanton and Dodd, 1994; Tang and Bottjer,
1995). Thus, such long-term trends must not
always arise as a result of continuously ongoing
processes, but may also be the net result of a series
of stepwise changes.
Species packing models might predict that
following a turnover event communities would
begin with a small numbers of species and then
gradually accumulate a number of additional
species over an interval of several million years.
Aside from the Cambrian biomeres, there is little
conclusive evidence that communities display this
predicted buildup in diversity throughout an ecological evolutionary subunit or EEU (Boucot,
1983, 1990b). Observations suggest that within an
interval of stability a fixed number of guilds as
well as a nearly constant number of species within
guilds is maintained for the duration of the
interval. The observation of nearly identical species
diversities within communities at the beginnings
and ends of blocks of stasis provides a strong
challenge to the notion of the "ecological wedge",
originally formulated by Charles Darwin and later
recast in the form of the Red Queen hypothesis
(Van Valen, 1973). There is little or no fossil
evidence to justify the notion of species packing
during long intervals of geologic time. Rather,
particular guilds appear to be filled early during
restructuring events and to remain filled by the
same species or at least by the same lineages with
only very minor species level changes (see Sheehan,
1992, 1994, 1996).
6.5.3. Escalation
Notions of progressive change in marine communities, such as the Escalation Hypothesis proposed by Vermeij (1987), typically imply a pattern
of continuous competitive struggle and intensification of interactions through geologic time.
However, as with the Red Queen hypothesis, the
scale at which most previous studies have been
13
done (e.g., Vermeij, 1977, 1987; Signor and Brett,
1984) makes it impossible to test for continuity
versus episodicity of process. Authors have documented general trends in increasing stereotypy or
increasing frequency of interactions that demonstrate a type of escalating process. The question,
however, remains as to whether or not this type
of intensification is ongoing during most of geologic time. The direct fossil record of organismal
interactions, including predation, documented in
the numerous case studies compiled by Boucot
(1990b) suggests quite the opposite. Organism
interactions appear to arise abruptly and then to
persist for long periods of time within coevolving
lineages, even across times of major crisis.
However, few attempts have been made to test on
a fine scale whether escalation, when it occurs,
takes place in a quantum, step-like pattern or
through a gradually intensifying process (but see
Kelley and Hansen, 1993; Hansen and Kelley,
1995).
Very preliminary data on interactions within
blocks of stability in the Appalachian Basin suggest
that there is very little escalational change in
organismal interactions throughout intervals of
several million years (see also Morris, 1996). For
example, drill holes of a predatory gastropod-like
organism appear in Middle Devonian brachiopods
near the beginning of the Hamilton fauna; comparison of brachiopod samples through the upper two
thirds of the Hamilton Group indicates similar
frequencies of attack and similar prey selection
throughout the group (see Smith et al., 1985). The
evolution of new organism interactions must
involve some type of restructuring of local community dynamics; however, once established these
types of relationships appear to become stabilized
and to remain constant through extended intervals
(Boucot, 1990b). Thus, we suspect that the "escalation" of interactions, such as that observed in
the "Mesozoic Marine Revolution" and/or the
"Paleozoic Precursor" to that revolution (Signor
and Brett, 1984) actually involves a series of
discrete steps timed with the restructuring events
that bound intervals of stability. During the rest
of geological time, organism interactions may be
in a state of near equilibrium.
Is there a constant struggle for survival and an
14
C.E. Brett et al./Palaeogeography, Palaeoclimatology, Palaeoecology 127 (1996) 1 20
ever-escalating arms race that marches continuously through time? The data of coordinated stasis
strongly suggest that the answer is no. This raises
a key issue with regard to the role of interspecific
competition in large-scale evolution. If competition
is strong and constant among species in a community, the expected result would be continual
escalational change; this is demonstrably not the
case. One possibility is that competition is minimal
during most of geologic time, being only intensified
during times of crisis; thus, there is normally no
selection pressure for escalation or increased diversity. Alternatively, interspecific competition is
intense during most of geologic time, but only
between resident members of a community and
potential newcomers; this would prevent successful
immigration of exotic taxa as well as the establishment of newly formed species. Both of these
hypotheses are consistent with a pattern of nearly
complete ecological and evolutionary stasis within
biofacies. However, the resilience of established
communities to introduction of new species seems
to favor the second alternative (see Ivany, 1996).
Extinction brought on by environmental disruption may relax this competition, permitting the
evolution of new and more complex ecosystem
dynamics.
6.6. Ecological implications and possible sources of
stability
6.6.1. Non-invasability
The implications of coordinated stasis for ecology are striking. Many neo-ecologists have favored
a pattern of more or less continuous change and
contingency within communities (Gleason, 1926;
Bush et al., 1992). In other words, local communities represent little more than temporary collections of species that happen to be able to live
together at a particular time and place. A corollary
of this individualistic view is that membership in
such loosely structured communities should not be
highly constrained. Yet, coordinated stasis in the
fossil record demonstrates that species membership
in individual biofacies may be nearly constant for
vast periods of time; there is very little immigration, emigration or extinction except at the ecological evolutionary subunit boundaries. Where careful
search has been made for rare species within the
Appalachian Basin EE subunits, fewer than 10%
of lineages were found to terminate within a block
of stability (Brett and Baird, 1995). Likewise,
relatively few new immigrants are noted within an
interval, and even fewer of these "late" arrivals
become established as permanent parts of the
tracking fauna. The observation of incursion epiboles is relevant here. These are thin (centimeters
to a few meters), but geographically widespread,
stratigraphic intervals in which a species normally
absent from a particular region is present or even
transiently dominant (Brett et al., 1990). In some
instances the source of the immigrant species can
be identified in another biogeographic province or
region (Brett and Baird, 1995). It is evident,
therefore, that species are capable of invading
communities under special circumstances, yet
despite their widespread distribution during such
episodes, these taxa disappear from the record as
abruptly they appear, being replaced/displaced by
members of the original community (Fig. 1). This
evidence demonstrates that resident communities
were generally refractory to permanent new introductions. Hence, epiboles provide a natural experiment indicating that locally established biotas,
consisting of widespread abundant species, are
quite stable on time scales of hundreds-of-thousands to millions of years even in the face of biotic
change. Only during times of major crisis or turnover do large numbers of immigrant species
become successfully (i.e., permanently) established
within a particular region.
Non-invasability of communities in local ecosystems is also suggested by the fact that contemporary biogeographic provinces may coexist even
in the absence of distinct geographic barriers. For
example, in the Devonian of the Eastern Americas
Realm, two distinctive biotas exist side by side,
the Michigan Basin fauna, and the typical
Appalachian Basin fauna (Imbrie, 1959; Oliver,
1977; Eldredge, 1972, 1974; Brett, in prep.).
Despite similarity in family and generic level composition in comparable facies, these two areas
maintained distinct suites of species (and many
genera) throughout more than 10 m.y. of Middle
Devonian time. These two regions were in close
proximity throughout this time and do not seem
C.E. Brett et al./Palaeogeography, Palaeoclimatology, Palaeoecology 127 (1996) 1 20
to have had geographic barriers separating them
during much of the interval, although subtle temperature differences might have helped to maintain
the boundary. Presumably, distinctive suites of
species were established during times of isolation,
perhaps associated with sea-level lowstands. The
discreteness of their faunas was maintained even
during times of highstand when the free exchange
of larvae would seemingly have been possible
between these two adjacent regions. Similarly,
Hallam (1978) documents that among ammonoids
and belemnoids the Jurassic Boreal and Tethyan
Realms remained distinctly separated along a
rather sharply defined "border", despite the apparent absence of a physical barrier.
6.6.2. Role of interspecific interactions in
maintaining stasis
These observations do not imply that communities are superorganisms. Indeed, the fact that
species show differing ranges along particular environment gradients, the occurrences of epiboles and
outages (absenteeism of normally common taxa)
(see Fig. 1), and slightly shifting dominance positions within members of biofacies, all argue that
communities as rigid semi-permanent entities do
not exist. Despite this, there do appear to be rules
of membership that apply to biofacies during vast
intervals of geologic time.
We suggest that the degree of exclusivity and
non-invasability exhibited in biofacies arranged
along environmental gradients implies an important role for biotic interactions in the maintenance
of faunal stability. Some form of internal coherency appears to be necessary to maintain the
stability of assemblages (derived from either traditional notions of incumbency, or ecosystem organization e.g., ecological locking of Morris et al.
(1992, 1995) or structural hubs of Miller (1996);
see discussion in Ivany, 1996). Only during times
of disruption and turnover is this constraint
relaxed, the general structure of biofacies broken
down, and a wide array of restructuring processes
allowed to take place. Individualist dynamics such
as those documented frequently by neo-ecologists
likely prevail at these times. Niches or guilds
become temporarily vacated as a result of extinction or possibly emigration. These can be filled
15
with locally derived new species, by extensions of
ranges of surviving taxa, or by new immigrants.
Such a situation may exist for geologically brief
intervals (104-105 yr; see Lu and Keller, 1994,
1995) or for somewhat longer intervals following major mass extinctions (see Westrop, 1994,
1996; also Sheehan, 1996). The late PleistoceneHolocene interval may represent one such time of
crisis and restructuring, rather than the norm of
stability that prevailed during most of geologic
history (see also DiMichele, 1994a; Schopf and
Ivany, in press).
Depending upon the availability of species for
restaffing of guilds, a few species may come to
occupy broadly defined niches or a large number
of species may partition available resources.
Regardless, this partitioning appears to occur relatively rapidly following an environmental crisis,
and there is little opportunity for reorganization
of biofacies after the initial phase of reestablishment. Expansions of species' niche breadths to
encompass new environments appears to occur
only at these times, lending further support to the
hypothesis of biological influence on stability. For
example, the orthid brachiopod Tropidoleptus carinatus occurs exclusively in nearshore sandy environments during the Early Devonian, but extends
to offshore mudrock facies by Middle Devonian
time (Boucot, 1975). This observation suggests
that taxa may be physiologically capable of living
in environments other than those in which they
are typically found, but over long intervals of time
they are constrained to particular biofacies.
Moreover, at certain times entire guilds appear to
remain unfilled following an ecological crisis. For
example, following a major late Middle Devonian
Givetian crisis in the Appalachian basin phacopid
trilobites, which had been abundant and widespread in a number of biofacies during the preceding interval, virtually disappear and do not
re-emerge in the appropriate biofacies or environments. Were the guilds filled by soft bodied, nonpreservable organisms or did they merely remain
unfilled for lack of suitable replacements?
6. 7. Boundary events
Given the evident stability of species and species
assemblages over profound periods of geologic
16
C E. Brett et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 127 (1996) 1-20
time, an obvious question is what are the processes
in earth history that serve to break down stable
faunas and permit invasion, not only by immigrant
species from other areas or provinces, but also by
new species produced in peripherally isolated populations? A number of factors must be considered
in any attempt at explanation of the boundary
intervals. First, although certain ecological crises
may be local, a number of ecological evolutionary
subunit boundaries appear to correlate approximately in time in widely separated parts of the
world. For example, the majority of EE subunit
boundaries recognized in the middle Paleozoic of
the Appalachian Basin region are also recognizable
in western Europe, Bohemia, Morocco and the
Gondwana continents (Boucot, 1990a, 1996). In
addition, one of the three "saltational" episodes
(turnovers between stable intervals) recognized by
Lu and Keller (1994, 1995) in late Paleocene
foraminiferans also appears to be a globally recognized climatic change event. Hence, at least some
ecological evolutionary subunit boundaries correspond to global bioevents (see also Boucot, 1983,
1990b, 1994; papers in Walliser, 1996). Strictly
local physical factors, such as basin tectonics and
orogenesis, or biotic disturbances such as disease
epidemics therefore appear less likely as ultimate
causal factors than do global perturbations such
as climatic warming and cooling or major changes
in circulation patterns or eustatic sea level. Marine
and terrestrial ecosystems were not invariably
affected simultaneously, although recently Algeo
et al. (1995) have argued that major Devonian
anoxic events and marine biotic crises might be
genetically linked to episodes in the evolution of
land plants. Moreover, it is apparent from the
Appalachian Basin studies that varied marine environments may all display a response to the same
disturbance.
No single factor seems to coincide with all EE
subunit boundaries in the Appalachian Basin.
Major sea level drops producing sequence-bounding erosion surfaces appear less tightly correlated
with EE subunit boundaries than do major highstands; widespread anoxia associated with major
transgressions appears to correlate with several
boundaries. However, the sea level rise and/or
anoxia may themselves be secondary effects associated with other factors of greater importance such
as global climatic warming (see also Patzkowsky
and Holland, 1993). Circumstantial evidence from
the pathways of immigrant faunas suggests that
changes in local temperature conditions accompanied some of the boundaries. Similarly, for terrestrial communities, episodes of abrupt climatic
change such as warming, cooling, or the development of more arid conditions seem to correlate
with radical changes in local community structure
and accelerated rates of extinction and evolution
(Vrba, 1985; Kerr, 1992).
Unfortunately, at present most EE subunit (and
coordinated stasis) boundaries appear sufficiently
abrupt and incompletely recorded that the details
of environmental change associated with these
ecological and evolutionary restructuring events
will remain obscure. Still, in the most continuous
sedimentary sections associated with local depocenters, it may be possible in some instances to
resolve the fluctuations that are associated with
these drastic turnovers (Budd et al., 1994; Morris,
1994, 1996). Furthermore, for the larger extinction
events such as the biomere boundaries of the
Cambrian, the restructuring interval may be more
prolonged, thus permitting a more detailed observation of processes (see Westrop, 1994, 1996;
Sheehan, 1994, 1996).
The recent emphasis on recovery from extinctions (Harries, 1995) might also produce valuable
insights into these processes. EEU boundaries generally correlate with mass extinctions (Sheehan,
1996) and thus these events must also truncate
constituent EE subunits. The expansion of mass
extinction studies (see Allmon and Morris, 1995)
to include recovery periods and the establishment
of new stable faunas may thus be the most promising avenue for insights into the development of
coordinated stasis in faunas (see Sheehan, 1996).
6.8. Prospectus
The observation of periods of earth history
characterized by coordinated stasis challenges a
number of assumptions underlying traditional evolutionary and ecological models, but research into
C E. Brett et al./Palaeogeography, Palaeoclimatology, Palaeoecology 127 (1996) 1-20
this pattern is still in its infancy. Many questions
are raised by the preliminary data of those studies
which have been carried out: How pervasive is
coordinated stasis? Is it more characteristic of
certain times in Earth's history than others? Has
there been a tendency for coordinated stasis to be
damped through time? (Or the reverse? - - see
Westrop, 1996.) Is it, for example, more prevalent
in the mid Paleozoic than in the Mesozoic or
Cenozoic times? Is it more typical of some ecological settings than others? What are the causal
factors of prolonged intervals of shared stability,
not only in species compositions and morphologies
but in organismal associations? What causal
factors result in the breakdown of intervals of
coordinated stasis? What is the specific pattern of
redevelopment of biofacies and species following
these events?
These and other questions provide a critical
agenda for evolutionary paleoecologists. Above
all, many more case studies must be carefully
documented before we can attempt to fully model
the circumstances involved in this possibly widespread evolutionary-ecological pattern. This theme
issue is the beginning of such an effort. Ultimately,
the documentation of coordinated stasis and the
need to account for the mechanisms that generate
this pattern may lead to a new synthetic view of
the evolutionary process which integrates the processes of species evolution, ecology, and mass
extinction. Therein lies the most significant goal
and challenge of evolutionary paleoecology.
Acknowledgements
Thanks to Wendy Taylor for help in drafting
Fig. 1, and to Paul Morris for helpful comments
and discussion. The manuscript was much
improved by comments from William Miller III,
Stephen Jay Gould, and Arthur J. Boucot.
Research by Brett has been supported by NSF
EAR92-9807. Ivany is supported by a dissertation
grant from the American Association of
University Women.
17
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