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Diversity of Seed Plants and their Systematics
Gymnosperms I
Fossils
Dr. NUPUR BHOWMIK
Department of Botany
University of Allahabad
Senate Hall
Allahabad – 211002
[email protected]
Date of submission: 27/06/2006
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Fossils
Fossils are generally defined as any direct or indirect evidence of pre-historic life (excluding those fossil-like
objects that have collected on earth since the beginning of recorded history, approximately 6000 years) found in
rocks extending from Arctic to the Antarctic. The exposed strata of rock containing fossils may be found along
rivers, streams, along sea-shores, in excavations etc.
Some fossils have been used to identify stratigraphic layers of earth’s surface and have been called as “index
fossils”. A good "index fossil" is abundantly represented in the rock strata and can be easily identified. Pollen
grains and spores have been used as "index" fossils in biostratigraphy and in correlation of rock units. Some
megafossils such as leaves and seeds have aided in correlating rock units that are widely separated geographically.
A vast majority of plant fossils are preserved in sedimentary rocks. Formation of sedimentary rock occurs when
deposition of rock particles of various sizes collects in a body of water. The particles are derived from erosion of
igneous, metamorphic or other sedimentary rocks by agents of denudation like wind, water, freezing, erupting
volcanoes and movement of glaciers. Moving water carries sand grains, silt particles, pebbles, seeds, leaves, twigs
etc. and slows down when it meets a body of standing water such as pond, lake or sea, where the load begins to be
dropped or to sediment out. The heavier particles are dropped first, closer to the shore and lighter particles are
dropped farther from shore. In this way a delta is built up. This deltaic sediment may become converted into
sedimentary rock – the sand becoming sandstone and the mud becoming shale. Often plant parts carried by the
stream, sinks along with mud and sand and if not decayed, become incorporated into the sediment and finally
included in the rock. As sediments accumulate, water is squeezed out of them, so they become much more
compact and plant fragments contained within them become flattened. But plant fossils are preserved in a variety
of ways and different kinds of physical and chemical processes are involved at the time of preservation. Moreover,
plant fossils are formed under very special environmental conditions, usually a swampy environment where
deposited plant parts remains undisturbed and intact in a deep body of acidic water. Acidity being formed by
decomposition of metabolic wastes. In such an acidic environment rate of activity of decomposers becomes
restricted and anaerobic organisms contribute little to the decay process. The swamps also have sedimentary
material like soluble silicates, carbonates, iron compounds or other minerals. A lower pH (increase in acidity)
enhances deposition of insoluble compounds into the plant parts – permineralizig them.
Plant parts are preserved as compressions in sedimentary rocks if there is abundance of sedimentary material like
silt, clay or fine grained sand in the environment (see figs. 1C, 2A, 3I). As the plant parts accumulate in the body
of water, they become covered with the sediment and entombed in the subsequently formed rocks.
Sometimes parts like twigs, seeds and fruits are also found preserved in the environments other than swamps.
They may have been retrieved from sediments in the floors of desert caves, where high temperature and low
humidity of desert environment and protection of the cave eliminated decomposition.
Another excellent means of preservation was by-refrigeration when organisms become trapped in snow and icefields of Siberia like the mammoth which has been preserved for thousands of years. Even tillites deposited at the
time of glacial retreat during the Pleistocene glaciation contained unaltered branches and trunks of trees. Besides
these, preservation has also occurred in oil saturated environment where large tree trunks about 100 million years
old have been found preserved in black, gooey oil at the base of Cretaceous oil sands of northern Alberta.
Ancient environments that allowed excellent preservation of plant parts resulted in different categories of
fossilization. Schopf (1975) recognized four distinct modes of preservation (see figs 1,2 & 3) :
A. Petrifaction : Where cellular permineralization of plant parts occurred due to infiltration of soluble silicates,
carbonates, iron compounds etc. The dissolved minerals permeated all the cells and tissue systems. Later,
precipitation of dissolved minerals formed a rock matrix supporting the plant tissues and hardening of plant
fragment took place from within and outside. Cell walls consisting of organic matter becoming chemically
altered and intercellular spaces and lumens became filled with mineral material. By this process sometimes
various cellular contents got preserved which included structures like starch grains, nuclei, various types of
membranes, tapetal deposits, megagametophytes of seed plants etc. When mineral was completely solidified,
plant fragment became entombed within solid rock. (see figs. 1A,B,D; 2A-D)
Permineralized specimens revealed not only external but also internal form of plants and were most useful
when studying internal structure.
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To study petrifactions, thin sections of the rock are cut by special saws to allow passage of transmitted light.
The cut surface of the rock is opaque and to make it further transparent, cut surface is ground with a grinding
machine, so that more light can pass through the rock and the rock is thin enough to be examined with a
microscope. Usually, study of permineralizations are made by the peel technique which is a simple method for
preparing sections of petrified materials.
A few examples of silicified permineralizations (see figs. 1A,B) are the Devoinan Rhynie Cherts,
Precambrian Gunflint Cherts, Triassic woods from petrified forests of Arizona, the giant tertiary Sequoia
trunks in the Yellowstone Park and silicified specimens from Tertiary Deccan Intertrappean series of India.
Coal deposits formed by fossilization of tropical forests are sometimes associated with calcareous specimens
commonly called Coal balls. They have also been of immense help in Palaeobotanical studies. Coal balls are
found only in the Carboniferous rocks associated with seams of bituminous coal. (see fig. 1D) They are
variously shaped (often spherical or ovoid) limestone rocks containing within them, plant parts generally
preserved with cellular details in calcium carbonate. Coal balls represent peat that was infiltrated by carbonate
before there was extensive compaction of the plants within them. They provide detailed information that helps
in investigating reproductive biology of plants that lived in peat swamps or they may reveal ontogeny and
phylogeny of Carboniferous plants.
B. Compression : Coalified compression fossils are formed when plants get deposited in sedimentary
environment. Sedimentary rocks generally preserve fossil plants. (see figs. 1C, 2A, 3I) Formation of
sedimentary rocks takes place by accumulation of small rock particles derived by weathering. Plant parts are
fossilized in areas where sediment is accumulating usually in a delta, where course of the river constantly
shifts. The delta remains undisturbed for sometime and plant fragments carried to abandoned water bodies
where a high concentration of sediment buries the plant parts and fills in the water body. Compressions may
also be formed in lagoons, along the meandering rivers, ponds, swamps or other situations. As sediments
accumulate water is squeezed out of them and they become more compact and plant fragments contained
within the sediments become flattened. Cell walls of plant parts soften and then collapse becoming squashed.
In the meantime there is loss of gas, moisture and other soluble materials because of pressure exerted by
accumulated sediments and water. The residues are altered and consolidated to form a carbonaceous film that
conforms to the original outline of the plant part involved. This is one of the most common type of fossils and
is called a compression. Most well preserved compressions are found in clay or shale. (see fig. 1C)
Sometimes fossils represent compression of plants growing near an area of volcanic activity. These
compressions are found in consolidated volcanic ash. The rain water rand ash make a fine grained mud that
picks up and buries plant parts as it moves down the slope. When the mud hardens, it entombs pieces of plant
material.
Other unique forms of sedimentary rocks containing plant and animal fossils are diatomite and amber.
Diatomite, a rock formed from cell walls of a group of unicellular algae called diatoms. They occur in
abundance in fresh water and marine environments. On dying, the highly resistant silica cell walls of diatoms
become deposited on bottoms of lakes, seas and oceans, where with the passage of time and consolidation
they form a white, light weight sedimentary rock called, diatomite. Diatomite is fine-grained, may contain
well preserved remains of fossilized plants and animals. It is an unusual situation where sedimentary matrix is
composed of fossils in which fossils are embedded.
Amber is a semiprecious stone of rare beauty. It is a sedimentary material formed from fossilized plant resins
that have undergone chemical change during the process of fossilization. Because of its sticky nature when it
was produced by the plant it served as a fossilizing matrix for other organisms. Trapped within the amber may
be found insects, floral parts, wind blown pollen grains, fungal spores and other plant and animal fragments
that increase the value of amber. (see figs. 3E, F). It is another example of a fossil preserving a fossil.
The sedimentary rocks containing fossils have numerous closely spaced layers of fossils. To uncover plant
fossils the rock is split along the bedding plane with a hammer or knife revealing the coalified compression on
one surface and its impression or counterpart on the opposite face. A vast majority of megafossils found
preserved as coalified compression are remains of leaves, branches, fructifications, seeds, megaspores etc.
Microfossils include spores, pollen grains and cuticular fragments. Most compressions show only surface
detail and general morphology such as in a leaf, leaf shape, presence or absence of a petiole, lamina margin,
pattern of venation, hairs etc. (see fig. 3I). In a few instances where compressions were formed under low heat
and pressure, entire cell contents were not lost. Niklas et al (1978) have shown in an exquisitely preserved
compressed angiosperm leaf of Miocene age, not only cellulosic microfibrillar organization of cell walls but
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also cell organelles like chloroplasts with grana stacks and starch deposits, nuclei with condensed chromatin
and plasmodesmata within mesophyll cells. Compressions mostly reveal cellular details of epidermis. Aerial
parts of vascular land plants are covered with a thin film of waxy material called cuticle. It is not a layer of
cells but an amorphous material that bears impression of underlying epidermal cells such as their outlines,
structure of stomatal apparatus, distribution of stomata, presence of hairs, glands and other distinguishing
features. Cuticular features have helped in reconstruction of entire plants from fragmentary fossils because
detached plant parts such as leaves, seeds, fructifications etc. show same cuticular structure thereby
suggesting disarticulated plant parts belonged to the same plant.
Coalified compressions are studied by transferring the compression from the rock matrix to a transparent film
that can be examined under the microscope. (see figs. 3I, J) Film is made by pouring a clear finger nail polish
or liquid plastic substance on the fossil embedded in shape and separating the film on drying from rock matrix
with the cuticle adhering to it. The film is then examined under microscope to see details of venation,
epidermal patterns, hairs etc. Another method of studying compression is to submerge the fossil and its
silicified shale in hydrofluoric acid until the fossils are freed from shale. The fossils are then sieved out and
washed thoroughly in water. By this technique nearly complete fossils can be extracted out of the rock matrix.
Coal, the combustible rock can also be classified as sedimentary where fossilized plant remains form the
sediment. It is a compressed form of fossil and is a heterogenous mixture of macro-molecular organic
compounds derived from plant substance. Details of plants comprising the coal can be seen when coal is less
metamorphosed as in lignite where plant parts are not excessively crushed and easily recognizable.
Bituminous coal is more metamorphosed and plant parts more flattened. Anthracite coal is the most highly
metamorphosed type where plant parts are altered to an extent where little of original plant material is hardly
recognizable.
Fossil charcoal or fusain is carbonaceous residue that results from incomplete combustion of organic material.
Another form of coal formed in rare instances consists entirely of cuticular fragments and amorphic organic
material, is called paper coal.
C. Impression : Is a negative imprint of compression where on splitting the rock along the bedding plane, instead
of the carbonaceous film, the opposing face of rock is revealed. The “negative” shows all surface details of
compression such as leaf shape and venation but there is no actual plant material preserved.(see fig. 1E)
Impressions may be formed as imprints are formed when leaves fall and settle into the wet concrete just after
it is poured. As the concrete hardens it conforms to the contours of lower side of leaf that rests on it. Finally,
the leaf disintegrates and is blown away but a negative replica of actual plant part remains on hardened
concrete. A very good example of this type of fossilization is impressions of dinosaur footprints.
No cellular details are seen on an impression but sometimes when matrix is very fine-grained a replica of the
impression can be made with latex. The replica reproduces all surface details on the impression. Examination
of the replica under scanning electron microscope might reveal epidermal patterns, hairs, glands etc. with
great clarity.
D. Moulds and Casts : Sometimes during the process of fossilization, plant parts like stems or seeds, carried by
moving water into a standing water body which is accumulating sediment, may get buried into the sediment
three dimensionally. In some instances the sediment surrounding the three-dimensional plant parts hardens
even before the plant fragment disintegrates. When the plant material eventually disintegrates the hollow that
remains in sediment represents a mold. No organic material is preserved but the mold reveals surface feature
of the particular plant part such as characteristic leaf bases present on stem surface or surface ornamentation
of seeds and fruits. (see figs. 1G, 2B)
A cast may be formed if sediments wash into cavity of mold and solidify. A three dimensional replica of
original plant material forms. (see figs. 1F,G & 2C) Here again no actual plant part remains, but surface
features are those of original plant part. Moulds and casts show external form of plant parts in a three
dimensional plane.
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Unaltered Plant Fossils :
In some cases plant part become fossilized and yet remain in an unchanged form such as pollen grains, spores,
cuticles, amber and calcium carbonate remains of certain types of algae. Formation of such type of fossils occur
when conditions of burial are rapid under very dry or cold environmental conditions.
Chemical fossils may also represent unaltered plant material. In some instances these chemical signatures called
biomarkers are specific for certain groups of organisms as for example hydrocarbon in the form of Ordovician oil
deposits were produced by cyanobacterium Gloeocapsomorpha. (Hoffman et al., 1987; Foster et al., 1990) Other
biomarkers like pristanes and phytanes are believed to have been derived either from chlorophyll degradation or
produced by non-photosynthesizing organisms.
Of late chemical compounds such as sterols, aromatics, carboxylic acids, lignin, fatty acids etc. have been
identified in fossil plants. These chemical constituents are also in unaltered state and may be associated with the
fossils or be present in residual form in the rock matrix.(see fig. 3G,H) Besides this fossil DNA fragment of a 920base pair from the chloroplast gene of a fossil angiosperm leaf from Miocene beds has been extracted and
amplified indicating survival of DNA for a much longer period of time than thought earlier.
Mummified fossils of Eocene deposits also show little chemical or structural alteration. In this rare type of
preservation, plant tissues are rapidly dehydrated and buried and cells and tissues are studied by same techniques
as those used for extant tissues. (see fig. 2D)
Degree of preservation is dependent on a number of factors. In case of coalified compressions plant parts initially
may be transported in rapidly moving water, together with coarse sediments. Prevailing environmental factors
contribute to partial maceration of plant parts before their deposition and fossilization. The finer the sediment the
better the preservation. Besides this, fossilization also depends upon the amount of decay of plant tissues before
preservation. Some plant tissues and structures are highly resistant to biological oxidation and mechanical
breakdown. Cells of xylem and some bark tissues are more resistant to degradation than soft tissues like cells of
phloem and parenchyma cells of cortex and pith. Cuticles of leaves, stems, fructifications and exines of pollen
grains and spores are highly resistant to biological degradation. Study of plant cuticles and spores is possible by
employing the maceration technique where the shale is subjected to a treatment of strong oxidizing reagent
followed by a basic solution. By this process, cuticles, spores and pollen grains are released from the sedimentary
matrix into the acidic solution. The plant parts are sieved out and thoroughly washed in water before adding the
basic solution. The cuticles and spores are then mounted on a slide for microscopic study.
Fossil gymnosperms are believed to have been preceeded by vascular plants having gymnospermic secondary
wood but reproduced by free-sporing pteridophyticmethods. Such plants were called progymnosperms
(Progymnospermopsida) and are regarded as ancestral group from which seed plants evolved.(see fig. 4)
According to Beck (1976b) plants assigned to progymnosperms extended from Middle Devonian to Lower
Mississippian and included three orders - Archaeopteridales, Aneurophytales and Protopityales. Plants of this
class were characterized by the following features :
1.
Shrubby to arborescent habits.
2.
Branching was pseudomonopodial with no production of axillary buds.
3.
Terminal appendages were either dichotomously branched units or laminate leaves with dichotomous
venation.
4.
Vascular system ranged from a protostele to a eustele with mesarch primary xylem maturation.
5.
Secondary tissues produced by bifacial cambium.
6.
Pycnoxylic xylem consisted of tracheids with circular bordered pits and rays.
7.
The free-sporing plants produced fusiform sporangia that were borne along adaxial or lateral surface of
branches or on modified leaves.
8.
Plants were both - homosporous and heterosporous.
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The Division Gymnospermae of plant kingdom are "seed plants" characterized by being devoid of "flowering
plant" features. It is a group of vascular plants comprised of at least two divergent phyletic lines :
1.
Cyacadophytes : Plants relatively small with usually unbranched or poorly branched aerial or
subterranean trunks, large frond-like pinnate leaves, stems with well developed pith and cortex,
manoxylic wood (secondary xylem). Manoxylic xylem composed of very long tracheids with large
diameter and having several rows of circular bordered pits mostly on radial walls. The wood rays
were multiseriate and very high. Cones simple, composed of modified mega and microsporophylls.
2.
Coniferophytes : Plants usually large sized trees with profusely branched stems and simple leaves.
Stems with small pith and compact pycnoxylic wood. Tracheids of secondary xylem have smaller
diameter and narrow medullary rays. Ovulate and pollen cones interpreted or modified fertile shoot.
Usually borne in axils a bracts or scale leaves.
A third division - Clamydospermophyta includes those gymnosperms which exhibit peculiar
angiosperm-like forms.
The classification of Gymnospermae into Cyacadophytes and Coniferophytes is helpful for identification of
specimens of fossil gymnosperms but recent studies reveal that the two categories represent growth habits rather
evolutionary lineages. Presently, several distinct groups have been included within both cyacadophytes and
coniferophytes. According to Pant (1957) the division Cycadophyta consists of several Classes and Orders :
Class-I:
Pteridospermopsida (seed-ferns) It includes both Palaeozoic as well as Mesozoic forms.
Families :
1. Lyginopteridales (Carboniferous)
2. Medullosales (Carboniferous - Permian)
3. Glossopteridales (Permian - Triassic)
4. Pettaspermales
5. Corystospermales
6. Caytoniales
Cyacadopsida
Cycadales
Pentoxylopsida
Pentoxylales
Bennettitopsida (Cycadeoideopsida)
Bennettitales (Cycadeoideales)
Class-II:
Order 7:
Class-III:
Order 8:
Class-IV:
Order 9:
Pteridosperms, a group of vascular plants that was transitional between ferns and seed ferns (see figs. 5,6,7)
have been reconstructed as having following characters :
a.
Some were small trees bearing helically arranged massive fern like fronds similar to modern
tree ferns. Others were prostrate, scrambling or vine-like in habit.
b.
Stele ranged from protosteles to eusteles. Primary xylem was mesarch.
c.
Wood consisted of parenchyma and thin walled tracheids. A large number of smaller, less
branched plants have manoxylic wood of cycad-type but tall much branched trees had
pycnoxylic wood like that of conifers.
d.
Outer cortex characterized by longitudinal bands of sclerenchyma forming a supporting
system. Sclerenchyma strands sometimes anastomosed forming a fibrous network.
e.
Both pollen-bearing structures and seeds were borne on leaves. Seeds were large and solitary
in some forms while in others they were small and produced in multiovulate cupules. Pollen
organs were grouped into clusters or in some taxa were arranged into large synangiate organs.
Order : Lyginopteridales
1.
Stem vasculature composed of single vascular segment (monostelic).
2.
Branching axillary
3.
Small petioles with "V-" or "W" shaped trace formed by fusion of several smaller traces.
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4.
Fronds with bifurcate rachis.
5.
Ovules small (3 to 5 mm in length) borne on cupules.
6.
Ovules with a hydraspermen pollen chamber.
7.
Nucellus fused with integument of ovule.
8.
Pollen organs small, laminar or terminal in clusters on branches of fronds.
9.
Pollen or "prepollen" of trilete type.
Genus Lyginopteris oldhamia - known in permineralized state in coal balls. The entire plant was
reconstructed by Oliver and Scott (1904) from detached plant parts that were externally covered with
numerous multicelluar glands. Roots, however were devoid of glands.(see fig. 6)
Habit
Vine like
Stem about 4 cm in diameter. Branches arising in axils of leaves.
Leaves were large, spirally arranged, planated fronds recovered as compression-impression fossils and
placed with form-genera of Sphenopteris or Pecopteris type with bifurcate rachises.
Reproductive bodies were borne on fronds, with cupulate ovules of Lagenostoma type or synangiate
clusters of microsporangia presumably of Crossotheca - type (see fig. 6G) on modified pinnules.
Microsporangia produced trilete pre-pollen frequently found in Lagenostoma pollen chambers.
Glands large and capitate occurring on the surface of stems, leaves and cupules.(see fig. 6A,C,E & F)
Transverse sections of permineralized stems showed a eustele with scattered nests of sclerotic cells in
the parenchymatous pith. Surrounding the pith were five to ten mesarch primary xylem strands. The
eustele is surrounded by a few millimeters thick manoxylic secondary wood.(see fig. 6B) Tracheids of
secondary xylem were large with multiseriate bordered pits on radial walls. Medullary rays were
numerous, one to several cells wide. The bifacial cambium produced secondary phloem outside
secondary xylem. Outside the secondary phloem was a "pericycle" with groups of short cells forming
sclerotic nests similar to those in the pith. A periderm-like tissue was often present outside the
"pericycle". The cortex is divided into an inner and outer cortex. The parenchymatous inner cortex is
rarely preserved. Outer cortex is preserved with characteristic radially broadened fibrous strands that
formed an anastomosing network appearing similar to Roman Numerals and which continued into
petiole base.(see fig. 6B) Leaf traces arising from primary xylem strands separated into a pair of strands
in the cortex and fused in petiole base to form "V" or "W" shaped bundle. Detached petioles with above
type of anatomy have been called Lyginorachis. The epidermis of stems, petioles, cupules etc. was
covered with numerous multicellular glands about 3.0 mm long. It was the presence of these structures
present in detached plant parts that helped Oliver and Scott (1904) in assigning them to one genus
Lyginopteris and also reconstructing the entire plant.
Leaves - Fronds were larger several times compound and planated with opposite to sub-opposite
pinnae. Pinnae were borne at right angles to rachis. Pinnules were lobed and were borne alternately on
rachis and conformed to the form genus - Sphenopteris. (see fig. 6D) Detached raches were called
Lyginorachis..(see fig. 6C)
Roots - were adventitious, borne on all sides of stem in vertical rows. They were about 7.0 mm in
diameter and contained secretory cells in the cortex. Stele varied from triarch to polyarch. Detached roots
were assigned to the form genus Kaloxylon.
Reproductive parts - The ovules were produced inside tulip-shaped uniovulate cupules which were
named Calymmatotheca.(see fig. 6E) The cupules were 8-10 lobed with lobes fused at base and outer
surface wrinkled. The free lobes at upper end were each supplied by a single, terete vascular strand. The
outer surface of cupule bore the same type of capitate glands as were present on different plant parts of
Lyginopteris. The cupule contained within it a single, erect, ellipsoidal to barrel-shaped ovule about 6
mm long and 4 mm wide attached by its base to bottom of cupule. The cupules were usually found empty
due to abscission of most ovules during their period of development. The permineralized seeds of
Lyginopteris were given the name Lagenostoma lomaxii.(see fig. 6E, F) A large number of detached
seeds are found in the fossil record. Inside the pollen chamber are found "pre-pollen" of trilete type.
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Pollen producing organs of Lyginopteris have not yet been identified conclusively. The compressionimpression fossil Crossotheca is often regarded as its microsporangiate organ because it was borne on
Sphenopteris or Pecopteris-type foliage and produced microspores similar to those found in pollen
chambers of Lagenostoma ovules.
Crossotheca consisted of flattened distal pinnules with fused sporangia pendent from lower surface.(see
fig. 6G) Fertile pinnules were associated with sterile pinnules on same pinna. Other microsporangiate
organs presumably assigned to Lyginopteris are Feraxotheca, Telangium or Telangiopsis.
Order - Medullosales
This pteridosperm family extended from Lower Carboniferous into the Permian.(see fig. 7)
1.
Plants were known to be largest of pteridosperms.
2.
Stems polystelic, with several segments of vascular tissue, each surrounded by secondary xylem,
quite unlike other pteridosperms. But Quaestora, (Mapes and Rothwell, 1980) like other
pteridosperms, was monostelic.
3.
Petioles massive, containing large number of scattered vascular traces.
4.
Fronds with bifurcate rachis.
5.
Seeds large, three angled and non-cupulate.
6.
Nucellus of seed free from integument, except at base where it is attached by a stalk.
7.
Pollen chamber simple.
8.
Synangiate pollen organs bearing tubular sporangia.
9.
Pollen grains large, monolete.
Genus Medullosa noei (Stewart and Delevoryas, 1956). A reconstruction of the fossil describes the plant
as being erect, about 4 to 5 meters high, with lower portion of stem covered by periderm and
adventitious roots resembling tree ferns. Higher up, the stem showed remains of spirally arranged leaf
bases. Apex of stem supported large bipartite fronds. The fronds bore large ovules and pollen organs
but whether the plants were monoecious or dioecious is unknown.(see fig. 7A)
Stems - Transverse sections of permineralized stems indicated a polystelic nature, but later studies on
the stele revealed it as being a series of vascular segments of a single stele. Each vascular segment is
elliptical to band-shaped with a "mixed" protostele composed of clusters of large metaxylem cells
separated by abundant parenchyma. (see fig. 7B,C) Primary xylem is mesarch. Protoxylem is oriented
towards outer margin of stele. Surrounding each primary vascular segment is a cylinder of secondary
xylem of variable thickness and which is more extensively developed towards the centre of stem.
(endocentric). Secondary xylem is manoxylic, vascular rays are abundant, each ray being several cells
wide. The vascular segments were embedded in a ground tissue of thin walled parenchyma having
secretory cells or canals. A periderm was present in the inner cortical zone in many stems of
Medullosa. Outside the periderm was a massive parenchymatous cortex with peripherally placed strands
of sclerenchyma, abundant scattered elongate secretory ducts filled with amorphous contents and leaf
traces. The secretory ducts were lined with small epithelial cells.
Leaves - Fronds were massive, dichotomously branched and regularly pinnately compound.(see fig.
7D) The detached petioles of Medullosa are called as Myeloxylon. Transverse sections are circular or
ellipitical consisting of a parenchymatous ground tissue with a ring of peripheral sclerenchyma fibres
and many scattered centrally located vascular bundles. Scattered throughout the ground tissue are
numerous secretory canals.
Two of the more common Palaeozoic foliage form-genera assigned to medullosans are Neuropteris and
Alethopteris.
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Alethopteris pinnules are longer than broad with pointed apex, decurrent base and a midrib extending
to tip of pinnule. Lateral veins arising at a steep angle from midrib and curving outwards to margin.(see
fig. 7D)
Anatomy of pinnules revealed the midrib as containing one to four vascular strands. Below upper
epidermis is one to two layers of palisade parenchyma that merged into loosely arranged cells of
mesophyll. Stomata hypostomatic and were associated with large, multicellular hairs that also occurred
along rachis.
Neuropteris pinnules possessed a rounded to cordate base and were attached to pinna rachis by an
inconspicuous stalk. From the prominent midrib arose many secondary veins by repeated dichotomies
that arched towards pinnule margin.
Sectional views of neuropteroid pinnules revealed presence of parenchymatous bundle sheaths,
undulating margins of upper epidermal cells, stomata confined to lower epidermis and each pair of
guard cells surrounded by 4-6 subsidiary cells.
Roots - were adventitious seeming to appear in vertical series and arose from margins of primary
xylem strands of the stem. They turned downwards between the leaf bases and emerged near the base of
stem. The roots are protostelic, triarch to tetrarch. Lateral roots originated from thin-walled pericycle
opposite protoxylem strands. Outside pericycle was a narrow endodermis and parenchymatous cortex.
Reproductive parts : Seeds attached to medullosan pteridosperms are known from a variety of
preservation states. They are largest of any seed-fern group, being more than 10 cm long and when
found permineralized resemble extant seeds of cycads.(see fig. 7D) Seeds had three longitudinal ribs
that divided the integument into three equal valves. Seeds were present as casts exhibiting variable shapes
depending on their method of formation. Such casts were named Trigonocarpus by Brongniart (1828).
The name is also applied to compression forms. Anatomically preserved medullosan seeds are called
Pachytesta. Permineralized specimens varied in length from 1 to 11 cm, were ovoid and consisted of
three-parted integument comprising of parenchymatous outer layer (sarcotesta), a middle zone of fibres
(sclerotesta) and a uniseriate innermost endotesta. (see fig. 7E) Secretory canals identical to those of
Myeloxylon and Medullosa were present in seed coats. The sclerotesta had three prominent ribs
extending from base to near micropylar opening. (see fig. 7F)
Pollen organ - numerous forms of microsporangiate structures have been described with compression
forms being assigned to medullosans based particularly on pollen type. They are all synangiate ranging
from simple and solitary forms to those organized into compound fructifications.(see fig. 7G,H) They
are made up of elongate, tube like sporangia embedded in parenchymatous ground tissue. Dehiscence
occurred towards centre of organ. Pollen grains were large and monolete. Some of the pollen organs were
called as Halletheca, Stewartiotheca, Schopfitheca, Sullitheca, Aulacotheca, Bernaultia etc. Out of these
only Bernaultia formosa which was abundantly present corresponded to that of Pachytesta type ovule
and Medullosa noei type of stems. The bell-shaped structure (companulum) of Bernaultia replaced
pinnae of Alethopteris - Myeloxylon-type frond. The fertile frond might have had all of its pinnae
replaced by companula. Genus Bernaultia was earlier called Dolerotheca and included all these
specimens of Dolerotheca that consisted of four synangial units united together. The name Dolerotheca
is still retained for similar synangia whose external and internal organizations are not known in detail.
Some specimens of Bernaultia were 4.0 cm in diameter and consisted of radiating pairs of elongate
pollen sacs. The companulum consisted of four radial synangia that have been symmetrically folded. Like
other medullosan pollen organs dehiscence of each sporangial pair was directed inwardly through
breakdown of specialized cells. (see fig. 7G)
Order Glossopteridales
The plants included under the group are all extinct and had in the past (280 million years back) dominated the
Gondwana continent (Australia, Africa, S. America, Antarctica and Indian Peninsula) during Permian times. The
Glossoptridales represented the dominant vegetation type in the southern hemisphere (Gondwanaland) during the
Late Paleozoic which was called Glossopteris flora because of the abundantly found tongue-shaped leaves named
10
by Brongniart (1828) as Glossopteris. (see fig. 8A) The entire, linear leaves had a prominent midrib near the leaf
base and reticulate venation in lamina.
Genus Glossopteris Brongniart habit from time to time habit of Glossopteris plant had been suggested as
being similar to herb, shrub or small tree. Reconstruction of plant of Glossopteris by Pant (1977) and
Gould and Delevoryas (1977) indicate to its being a large deciduous tree (see fig. 8B) with a trunk of 40
cm diameter. The height of the tree was estimated to be 6 meters and the gymnospermous wood to be of
Araucarioxylon type. The trunk was supported by a root system of the Vertebraria type. (see fig. 8C)
Conspicuous growth rings were present in roots trunks and branches. Leaves were attached in spirals or
in whorls on long or short shoots as in modern Ginkgo and leaf fall occurred in Autumn Winter months.
Stem : Studies of permineralized stems by Pigg (1988) revealed that they bore both foliar leaves and
small scale leaves. Stems were typically gymnospermous with conspicuous growth rings. The
pycnoxylic wood was similar to the form genus. Araucarioxylon, showing multiseriate hexagonal pits on
radial walls of tracheids. Xylem rays were uniseriate. In the centre of pynoxylic wood was a pith
surrounded by a ring of primary vascular bundles.
Leaves : The leaves were tongue shaped with a prominent midrib and reticulate venation pattern. (see fig.
8A) They exhibited great variation in size and shape (small to 30 cm long in size and shape of lamina
varied from narrow to wide with pointed to rounded apex). Leaves were sessile or sometimes petiolate,
characteristically hypostomatic (stomata confined to lower surface). Epidermal cells straight-walled or
sinuous, stomata haplocheilic, placed irregularly between veins.
Anatomy of Glossopteris leaf described by Gould and Delevoryas (1977) and Pigg (1988) mentions the
presence of heavy midrib composed of several vascular bundles from which laterals diverged. The lateral
veins frequently anastomosed forming reticulations or meshes. Palisade and spongy mesophyll might or
might not have been differentiated. The hypodermis was well developed and bundle sheath enclosed the
vascular bundle of veins.
Other leaf forms are Gangamopteris, Paleovittaria, Belemnopteris, Rhabdotaenia, Rubidgea and
Euryphyllum.
Scale leaves associated with Glossopteris were rounded to lanceolate in shape, had broad to truncated
bases, indistinct midrib, reticulate venation and distinct cuticular pattern.
Root : Detached roots of Glossopteris are included under form-genus Vertebraria. Roots were shallow
and are known from anatomically preserved specimens. Even casts or compression-impression fossils are
characterized by wedge-like segments radiating from centre of axis. Transverse sections revealed a
central zone of exarch, polyarch, protostele, alternating with four to seven radiating arms of secondary
wood separated by hollow areas. (see fig. 8C) The hollow cavities between radiating arms were not filled
with tissue, indicating Vertebraria as representing root system of a plant that grew in a semiaquatic
environment. The secondary xylem was continuous near periphery of axis and contained growth rings.
Surrounding the zone of secondary xylem was a narrow band of periderm consisting of well developed
cork. Branching in roots was frequent. The secondary xylem of Vertebraria is exactly similar in
structure to pycnoxylic wood of Araucarioxylon present in stems. Vascular rays are uniseriate and 1-20
cells high.
Reproductive parts : Several glossopterid pollen organs have been described but one of the most
commonly encountered form is Glossotheca. It is a spathulate foliar organ with a midvein. According to
the reconstruction the branch bore paired (pinnate) laterals that terminated in clusters of Arberiella-type
sporangia. Pollen type presents within the fructification are not known.
Another common pollen producing fructification is Eretmonia. Reconstruction of microsporophylls
showed a unit with a more or less triangular distal lamina on a stalk. On adaxial surface of stalk, slightly
beyond midpoint, two branches arose bearing whorls of Arberiella-type sporangia. (see fig. 8G)
Some detached pollen sacs showing glossopterid affinities have been named Arberiella, Lithangium and
Polytheca. The sporangia are uniloculate but contained pollen grains of different types. While Arberiella
contained pollen grains of bisaccate type, both Lithangium and Polytheca contained monolete pollen
grains.
11
Ovulate structures assigned to glossopterids appeared to consist of a dorsiventral structure bearing
seeds and which has been variously termed a capitulum, megasporophyll, cupule, fertiliser or cladode.
(see fig. 8D,F) The variety of morphologic forms, including bisporangiate, uniovulate and multiovulate
types produced in solitary and compound structures suggested the presence of several different types of
plants within the Glossopteris group.
The earliest interpretation of Glossopteris fructification was envisaged by Plumstead (1952, 1956). She
called the detached organ as Scutum and described it as being a bivalved cupule attached to the midrib
of the Glossopteris leaf by a pedicel. Plumstead believed half of the cupule near the adaxial surface of
subtending leaf bore carpels while the other half bore microsporangia. Later, Surange and Chandra
(1972) described a species of Scutum as having bilateral receptacle bearing ovules and being covered
on one side by a protective scale leaf with Glossopteris-type venation.
Another important ovulate fructification, attached to foliage of Gangamopteris type is Ottokaria. (see fig.
8D) The fructification consisted of a fleshy, flattened capitulum (head) with marginal frills. Ovules
were borne of under surface of capitulum. (see fig. 8E,F) Some ovulate fructification like
Dictyopteridium had strobiloid receptacles borne in axil of a stalked fertile bract that covered the
receptacle like a protective spathe. Other ovulate fructification have been called as Lidgettonia,
Denkenia, Rusangea, Senotheca, Partha etc. Thus the glossopterids seem to represent a diverse group of
seed ferns on the variability of ovulate fructification although there appears to b uniformity of their
vegetative organs.
Orders Corystospermales, Peltaspermales and Caytoniales present those gymnosperms which are called as
Mesozoic seed ferns and which flourished in the Triassic are Jurassic periods. Some of their morphological
features suggest affinities with Palaeozoic seed ferns while some other members exhibit characters that suggest
affinities with angiosperms (Preangiosperms). One thing common to all groups is the unique mechanism for
protection of ovules.
The Corystospermales were a small group of plants known from Triassic localities of Gondwanaland
and the family was instituted by Thomas (1933) for helmet-shaped, seed-bearing cupules called
Umakomaasia. (see fig. 9E) The cupule bore seeds either singly or in pairs. Seeds of corystosperms had
slightly curved bifid micropylar canals that projected beyond the helmet like cupule. Detached ovule
containing cupules are called Pilophorosperma.
The pollen bearing organ of corystosperms is named Pteruchus. It consisted of alternately or helically
arranged microsporophylls attached to an axis. Each microsporophyll terminated in a flattened circular
to elliptical head that bore clusters of elongated, pendent microsporangia. (see fig. 9F) Pollen grains
inside microsporangia are bisaccate. Several foliage types have been assigned to corystosperms,
although none have been found attached to axes bearing reproductive structures. The relationship is
inferred by similarity by epidermal features. The most common of these fronds in Dicroidium which is
pinnate to tripinnate and is characterized by a basal bifurcation of the rachis. The pinnae are usually
sub-opposite and decurrent. (see fig.9D) Venation varies from sphenopterid to taeniopterid type.
Because Dicroidium is the dominant foliage type in the Triassic of Gondwana it has been used as "index
fossil" to delimit megafossil zones for biostratigraphy. Other foliage types are Xylopteris, Pachypteris,
Johnstonia etc.
The Peltaspermales has been reported from Gondwanaland as well as Greenland, Europe and Russia.
The plant parts consisting of stem fragments, leaves, ovulate discs and microsporangiate organs were
assigned to one genus Lepidopteris on account of the characteristic blister-like swellings and similar
stomatal apparatus.
The principal foliage type, Lepidopteris is a bipinnate frond with pinnules attached sub-oppositely to
alternately by a broad base on a rachis having small, irregular blisters. Venation is the open type. (see
fig. 9A)
The pollen-bearing organ of Lepidopteris is a pinnately divided axis with short branches at intervals of
1 to 2 cm. Branches subdivided into two or three ultimate branches that lay at right angles to rest of
microsporophyll. Each ultimate branch bore two rows of pendent microsporangia. The Gondwana
pollen organ is called Antersia. It produces small, monosaccate pollen grains.
12
The ovulate organs placed under form-genus Peltaspermum consisted of elliptical to radial umbrellalike discs that bore ovules in a ring on underside. (see fig. 9B,C) Ovules have curved micropylar beaks.
The Caytoniales is the best known group of Mesozoic seed ferns and had a wide distribution; it has
been described from Triassic to Cretaceous localities in most of today's Northern Hemisphere. Plants
were believed to be small trees based on association of wood axis with Sagenopteris foliage and some
members of the group grew in periodically waterlogged habitats. The plant was initially described by
Thomas (1925) from Middle Jurassic plant bearing beds along the coast of Cayton Bay in Yorkshire.
The features of the group are so striking that they were regarded as a new group of angiospermous
plants (Thomas, 1925) or plants having seed-bearing structures that appeared to be a precursor of
angiospermous carpel (Doyle, 1978).
Leaf remains of Caytoniales assigned to form genus Sagenopteris have been known from widely
separated geographic localities like Greenland, Sardinia, U.S.A., Canada, England, Siberia, Japan and the
foliage were known long before any reproductive structure had been found. Sagenopteris consisted of
4-6 palmately arranged lanceolate leaflets. Each leaflet around six centimeters long, had a prominent
midvein and reticulate venation of anastomosing lateral veins. (see fig. 10A, B)
The pollen bearing structures called Caytonanthus consisted of a slender axis bearing flattened, pinnate
lateral branches. Each branch bore one to three elongate synangia. Each synangium (see fig. 10G) was
pointed at distal end and contained three or four pollen sacs (locules) that were arranged around a central
zone of tissue. The pollen organs, sometimes called anthers zone of tissue. The pollen organs, sometimes
called anthers are radially symmetrical with dehiscence towards centre of synangia. (see fig. 10F) Pollen
grains are small and bisccate. (see fig. 10 H)
The ovule bearing structure is called Caytonia. It consisted of an axis bearing stalked, multiovulate
cupules in sub-opposite pairs. (see fig. 10C,D & E) Each cupule is nearly circular in outline and upto 4.5
mm in diameter. The cupules are borne along the axis in such a way that cupule is recurved with a liplike projection near the point of attachment. Each cupule contained 8-30 seeds. Each seed is radially
symmetrical about 2 mm long and borne on a delicate stalk in orthotropous position along the midvein
of cupule. Dispersed seeds of this type are placed in form genus Amphorispermum. Each seed inside the
cupule was associated with an elongate canal that extended form seed micropyle to outer lip of cupule
and pollen grains present inside the cupule were probably drawn in by pollination droplets that
originated at micropylar end of each seed. (see fig. 10E) Morphologically Caytonia represented a
multiovulate cupule and not an angiospermous fruit as it was earlier thought.
Class II - Cycadopsida
Order - Cycadales
The Cycadales include both living and fossil members that can be traced back to Upper Carboniferous.
Many palaeobotanists believe Cycadales have originated from Palaeozoic seed fern order,
Medullosaeles. They reached their maximum development during the Mesozoic and have gradually
declined in numbers and are today represented by eleven genera and more than 160 species (Gifford and
Foster, 1989). Fossil cycads had world wide distribution but today they are restricted to certain regions
of the world only. Most Mesozoic cycads are believed to have had slender stems with widely spaced
leaves that abscised. (see fig. 11A,B) Morphologically they were quite unlike the short, squat trunked
modern members of the order.
Foliage forms were pinnate but few were simple. Simple leaf types believed to belong to cycads are
Taeniopteris and Nilssonia. Seed-bearing Taeniopteris-type leaves reported from different ages have
been called as Spermopteris and Phasmatocycas.
Few pollen cones of cycads have been reported. Two of the better known ones are called as Lasiostrobus
and Androstrobus. (see fig. 11E,G)
Ovulate cones of the plant that bore Androstrobus are called as Beania. (see fig. 11C,D)
13
Class III - Pentoxylopsida
Order - Pentoxylales
The order constitutes a small group of Jurassic and Cretaceous gymnosperms. This interesting group of
fossil plants was first reported by Srivastava (1935, 1936, 1946) from Lower and Middle Jurassic
Rajmahal Hills in North Eastern India. Later the order was reported in detail by the renowned Indian
palaeobotanist, Birbal Sahni.
Fossils of Pentoxylales have also been reported from Jurassic and Lower Cretaceous of New Zealand and
Australia.
Genus Pentoxylon Habit : small tress or shrubs that possessed long and short shoots. (see fig. 12A)
Stem : called Pentoxylon because it frequently showed five segments of triangular vascular tissue
arranged in a ring around a central ground tissue. The primary xylem is mesarch and secondary xylem is
pycnoxylic (resembling the wood of conifers). The short and long shoot are covered by an armour of
spirally arranged leaf bases. The short shoots often terminated in a crown of spirally arranged
Nipaniophyllum-type of leaves. Besides foliage leaves some short shoots were terminated by clusters of
ovulate cones or pollen organs. (see fig. 12A,B)
Leaves : are petiolate strap-shaped with prominent midrib closely resembling form-genus Taeniopteris
in general features. However, structurally preserved leaves have been called Nipaniophyllum.
Pollen organs : is called Sahnia. It consisted of a receptacle from whose rime like collar arose
numerous microsporophylls or microsporangiophores. The stalks gave rise to secondary laterals that
terminated in several stalked pollen sacs. (see fig. 12E)
Ovulate cones are called Carnoconites. Reconstruction of seed-bearing cones exhibited a central
branching axis with each branch terminating in a ovulate head. Each head consisted of some 20,
helically arranged, tightly packed orthotropous seeds on a cone axis. (see fig. 12B-D)
Class IV - Bennettitopsida (Cycadeoideopsida)
Order - Bennettitales (Cycadeoideales)
The order is extinct but formed an enigmatic groups of Mesozoic gymnosperms extending from Triassic
to Cretaceous. Members of this order have a growth habit reminding one of genera of Cycadales.
Habit- Plants may have been short and squat to slender and highly branched. Trunk-like stems were
unbranched or sparsely branched and covered with spirally arranged, persistent leaf bases. (see fig.
13A) The stems of cycads and bennettitales are quite similar anatomically but girdling leaf traces were
absent in Bennettitales. Even the foliage was impossible to distinguish, but cuticular studies of leaf
epidermis revealed presence of syndetocheilic type of stomata in Bennettitales and haplocheileic type in
cycads.
Genus : Williamsonia sewardiana Reconstruction was given by Sahni, (1932).
Habit - Plants dioecious, small tree like, about 1.5 to 2 m high, resembling a small Cycas plant. At the
distal end of the trunk was a crown pinnate leaves of the Ptilophyllum type.
Stem : or trunk, sparsely branched consisting of an armaur of spirally arranged persistent leaf bases. The
short, lateral leafy branches of stems also bore ovulate cones. Detached and dispersed trunks are called
s Bucklandia indica. Internal structure resembled those of cycad stems. Secondary xylem was
manoxylic. Secretory ducts were present abundantly in the large pith and cortex.
14
Leaves - Fronds were of Ptilophyllu- type and were found forming a crown of spirally arranged leaves.
Pinna linear to slightly sickle shaped, characteristically attached to adaxial surface of rachis. Bases of
pinnae broad and somewhat rounded, nearly covering upper surface of rachis. (see fig. 13A-C)
Reproductive Organs - of Bennettitales include both monosporangiate and bisporangiate forms but
cones of Williamsonia are monosporangiate. Ovulate cones usually terminated fertile lateral branches,
but attachment of pollen organ, Weltrichia on the plant is unknown. They are assigned to Williamsonia
on the basis of association and their syndetocheilic stomata.
Ovulate cones - were generally rounded consisting of dome-shaped receptacle covered by a dense
layer of stalked ovules and tightly packed interseminal scales. The ovulate receptacle with it ovules and
interseminal scales was subtended by a whorl of bracts. At the distal end of cone was a ring of fused
scales called corona. The club-shaped interseminal scales surrounding individual ovules far outnumbered
the ovules. Ovules were orthotropous and shortly stalked. Dicot embryo have been reported in some
ovules.
Pollen cones - were large about 10 cm in diameter, rare and not found in actual connection with plant.
They are called Weltrichia and consisted of a cup-shaped base with as many as 20 to 30 finger-like
projections (Microsporophylls) that arose from the rim of the cup. (see fig. 13D,E) The
microsporophylls were distally tapered and bore numerous pollen sacs on inner surface (see fig. 13 D,F)
Pollen grains were monosaccate. On inner surface of cup were numerous semi-circular structures borne
on short stalks. They have been called resinous sacs and must have functioned as attractant for
pollinators.
The Division of Coniferophyte (Pinophyta) also consists of several classes and orders :
Class
I.
Orders 1.
Class
Coniferales (Pinales)
3.
Ginkgoales
II.
Ephedropsida
Ephedrales
III.
Order
Class
Order
Cordaitales
2.
Order
Class
Coniferopsida (Pinopsida)
Czekanowskiopsida
Czekanowskiales
IV.
Taxopsida
Taxales
The Class Coniferopsida are a group of vascular plants that are highly diversified. There are numerous orders
which have become extinct (e.g. Cordaitales, Czekanowskiales, Voltziales). Their characters are :
a.
Plants are highly branched woody trees or shrubs.
b.
Primary xylem of eustelic stele is endarch.
c.
Pith and cortex are relatively small compared with development of the wood.
d.
Wood is pycnoxylic with small amount of parenchyma and abundant tracheids having circular bordered
pits, mostly restricted to radial walls of tracheids.
e.
Xylem rays usually uniseriate, sometimes multiseriate. Rays have both ray parenchyma and ray tracheid
cells.
f.
Leaves are simple, generally needle-like borne in whorls opposites or spirally. Veins usually one or two
in leaves but some broad leafed forms have many veins. Stomata are haplocheilic.
g.
Plants may be monoecious or dioecious. Cones are monosporangiate.
h.
Ovulate cones are compound, except for Taxales where ovules terminate branches.
i.
Pollen cones are simple, pollen grains are usually bisaccate. Spermatozoids
j.
Polyembryony and polycotyledonary frequently prevalent.
non-flagellated.
15
Order - Cordaitales.
The Cordaitales are an extinct group of gymnosperms that can be traced from Lower Carboniferous into the
Permian. They formed predominant flora of Late Paleozoic forests of certain localities like Iowa and Kansas.
Cordaitalean leaves have been reported from Permo-Carboniferous of Siberia, China, India, Australia, South
Africa and S. America.
Genus - Cordaites
Habit : Reconstructions indicate plants were large trees well as small, scrambling shrubs. The
arborescent forms inhabited lowlands and consisted of monopodial trunks with distally produced straplike leaves. (see fig. 14A) The shrubs and mangrove-like forms inhabited peat forming swamps. (see fig.
14B)
Stem - The generic name for structurally preserved stems is Cordaixylon. Transverse section of stems
revealed a large pith cavity surrounded by endarch primary xylem. Secondary wood outside primary
xylem pycnoxylic. Secondary phloem was distinct. Cortex contained secretory sacs and a hypodermis
of vertical strands of anastomosing fibres. In older stems cortex was replaced by Periderm. Leaf and
branch traces were also present in stem. Longitudinal section of stem displayed a septate pith, formed by
breaking down of some pith cells to form lens-shaped gaps alternating with diaphragms of
parenchyma cells. Casts of septate pith are called Artisia.
Leaves - are assigned to form genus Cordaites and range in length from a few centimeters to 1 m. Larger
leaves may be a wide as 15 cm. (see fig. 14C) Shape varies from lanceolate to spathulate or obovate,
venation is parallel with occasional dichotomies. Leaves were mostly hypostomatic, with stomata
arranged in bands between veins. Stomata haplocheilic, guard-cells sunken in a pit. Palisade and
spongy mesophyll poorly differentiated but hypodermal strands of sclerotic tissue occurred in all
species.
Roots - Detached permineralized roots of Cordaiales are placed in form-genus Amyelon. Reconstruction
showed the root system as being highly branched, shallow, forming a pad of stilt roots that supported
the stem. T.S. of Amyelon showed a central exarch actinostele surrounded by a thick layer of secondary
xylem. A Periderm developed deeply within tissue of cortex showed presence of lenticels in outer
layers. Phelloderm was aerenchymatous. In older roots, protostele became replaced by siphonostele.
Such anatomical features are found in stilt roots of modern plants growing in mangrove environments.
Reproductive organs - of Cordaites are believed to have been borne among leaves on distal branches
and were monosporangiate. (see fig. 14D,F) It is unknown whether plants were monoecious or
dioecious. Both pollen and ovulate fructifications are assigned to Cordaitanthus. Morphologically they
formed compound strobili or lax inflorescences. Pollen sacs were elongate and produced monosaccate
pollen grains of Florinites type.
Seeds produced by Cordaitanthus-like reproductive structures are platyspermic with conspicuous wings
and are assigned to genus Cardiocarpus. (see fig. 14E)
Order - Coniferales
Plants included within the order represent extinct as well as extant members and are large woody trees or smaller
shrubs.
Leaves usually needle like with one or two veins or leaves wide, containing many vascular bundles. (see
fig. 15A-I) Wood, pycnoxylic characterized by circular bordered pits.
16
Reproductive organs : consisted of simple pollen cones with abaxial pollen sacs and compound
ovulate cones bearing ovules on upper surface of ovuliferous scales. Oulifecous scale subtended by
bract-scale complex. (see fig. 16 A,E)
Plants monoecious or dioecious, but cones are monosporangiate.
Work on fossil conifers is based on detailed cuticular studies of Florin (1938-1945). Later discovery of
several new and better preserved Upper Palaeozoic and Mesozoic conifer specimens by Miller (1977,
1982) and Clement-Westerhof (1984) has depicted increased diversity in the group. Besides providing
more accurate characterization of vegetative shoots, later studies have suggested creation of several
families of fossil conifers based particularly on features of ovulate cones. (see fig. 16 F-I)
Florin had recognized one family of Palaeozoic Voltzialeans, the Lebachiaceae which contained four
genera. The Voltziales were referred to as "transitional" group between conifers and cordaites. One most
of the well known genera is Lebachia. The others were recognized as form-genera. The most famous of
form-genera was called Walchia from which "walchian conifers" were derived.
Mapes and Rothwell (1984, 1991) suggested less confusing names for Palaeozoic Voltzialeans and
proposed the name Utrechtia for Lebachia and family Utrechtiaceae in place of Lebachiaceae. Other well
known Palaeozoic Voltzialeans are Ernestiodendron Ortiseia and Otovicia of Utrechtiaceae. (see figs.
15A-C; 16A-H)
Walchia is retained as a form-genus that cannot be assigned to any one of several families.
Habit, Stems and Leaves - According to Florin, those Voltziales recognized as "walchias" were trees
resembling extant genus Araucaria. The trees of walchias had spirally arranged scale-like or needle-like
leaves. Branches were arranged in whorls of five to six. Leaves, less spreading, were entire or had
bifurcate tips, decurrent bases and were supplied by a single vein. Lateral shoots in Utrechtia exhibited
the above mentioned features. (see fig. 15A-C)
In Ernestiodendron, leaves were borne at right angles to stem. (see fig. 15C)
The form-genus Walchia was used for poorly preserved vegetative shoots that lacked cuticle and could
not be placed under well known genera.
Reproductive organs - of Palaeozoic conifers were strobiloid structures borne at tips of leafy branches.
Cones were monosporangiate. Ovulate cones appeared upright and pollen cones were pendulous. (see
fig. 16A,E)
The ovulate cones played a significant role in defining Palaeozoic conifer families. The compound
ovulate cone in Utrechtiaceae consisted of a central axis bearing helically arranged bifid bracts. Arising
from axil of each bract was a short shoot consisting of an axis bearing numerous sterile scales and one
or more fertile scables. Each fertile scale bore a single inverted ovule. (see fig. 16F-I)
The pollen cones were always borne terminally on leafy branches. (see fig. 16A) Each one consisted of
a central axis surrounded by numerous helically arranged flattened microsporophylls. The lower
surface of microsporophyll contained usually two elongate pollen sacs. Pollen grains are monosaccate
but many genera have bissaccate pollen resembling Pinaceae. (see fig. 16 B-D)
Detached ovulate cones with single fertile scale per short shoot are assigned to form-genus
Walchiostrobus (see fig. 16 I) and isolated pollen cones showing similar epidermal features as those of
vegetative shoots of Utrechtiaceae have been placed under genus Walchianthus.
All the families of modern conifers had reached their maximum diversification during the Mesozoic with
the exception of Cephalotaxaceae and Taxaceae which have very poor fossil history. The remaining
families all had their earliest representatives appeared during Late Triassic.
17
Class - Czekanowskiopsida
Order - Czekanowskiales
This extinct order which extended from Jurassic into the Cretaceous is formed for Czekanowskia and
Leptrostrobus. Earlier classification indicated an affinity of Czekanowskiales with members of fossil
Ginkgoales because Czekanowskia had characteristics of a primitive ginkgophyte leaf.
The order included plants with persistent leaves borne on deciduous short shoots surrounded by scale
like leaves.
Leaves : The linear dissected foliage leaves of Czekanowskia appearing as filiform segments were borne
in clusters at the end of short shoots with scaly bases. (see fig. 17A) Each leaf was supplied with a
single vein that entered the base and dichotomized several times before reaching the leaf margin.
Leaves were amphistomatic.
Reproductive organs : Ovulate cone Leptostrobus found associated with the leaves exhibited similar
cuticbular structure as leaves and according to Harris (1951) might have belonged to the same plant.
Specimens of leptostrobus showed spirally arranged scales around the base of a long slender axis that
bore flattened, globose capsule like-ovulate structures. Each capsule consisted of two valves and inside
the concavity of each lobed valve was a row of small ovules. (see fig. 17 B,C).
Pollen cone of Czekanowskia is called Ixostrobus (Harris am,./nd Miller, 1874). However, its assignment
to the genus is doubtful.
18
A
B
D
G
F
C
E
Figure-1 : Some types of plant fossils. A, B and D. Permineralized fossils. A. Silicified logs in a petrified
forest of Patagonia, Argentina. B. Transverse section of foliage part preserved as limonite permineralization.
C. Compressed foliage of Dicroidium. An "index fossil" of Triassic Age. D. Several coal balls, with a large
one in centre which is approximately 3 feet long. Plant parts are preserved with cellular detail in calcium
carbonate. E. Impression of Glossopteris leaf. F. A cast of underground axis, Stigmaria of arborescent
lycophytes showing helically arranged, circular, lateral appendage scars. G. Cast and mold (above) of a
tracheid showing circular bordered pits.
(A,B,D - G from Taylor and Taylor, 1993, C - from Bose and Srivastava, 1971)
19
A
C
D
B
Figure-2. Some types of plant fossils.
A. Coalified compression of conifer, Metasequoia occidentalis. B. Mold of bark fragment of a Carboniferous
lycopod, Lepidodendron. C. Cast of pith of a Carboniferous calamite stem. D. Mummified fossils showing a
cone and branch of conifer, Larix and a leaf of dicot angiosperm, Cryptocarya. (A-C from Stewart and
Rothwell, 1993; D- from Taylor and Taylor, 1993).
20
A
B
D
C
G
H
E
I
F
J
Figure-3 : Some types of plant fossils. A-D. Preservation of gametophytic structures in permineralization.
A. Prepollen grain, Vesicaspora, containing three-celled microgametophyte. B. longitudinal section of apical
portion of pteridosperm ovule showing megagametophyte(m) and two archegonia (a), one containing a
protoplast. C. Longitudinal section showing neck cells (n), position of egg (e) and cells of venter (v). D.
Transverse section of archegonium showing neck canal (c) and four tiers of neck cells (n). E,F. Amber
containing plant and animal parts. E. Mimosa-like flower with numerous filamentous stamens contained in
amber. F. An insect trapped within amber. G,H. Leaves of Quercus from which unaltered plant material
like flavonoids and other compounds were extracted. G. Fossil leaf of Quercus. H. Extant leaf of Quercus. I,J.
Product of transfer technique. I. A compressed leaf of Cercidiphyllum. J. Transfer made from a
Cercidiphyllum leaf. (A-F, I,J from Stewart and Rothwell, 1993; G,H- from Taylor and Taylor, 1993).
21
A
B
H
C
E
D
F
G
I
Figure-4. Progymnosperms -Vascular plants having gymnospermic secondary wood and pteridophytic
reproduction. Archaeopteris sp. A. Reconstruction of Archaeopteris sp. A tree about 4 m high. Upper
Devonian. B-E. Stages showing evolution of leaf of Archaeopteris sp. F. Fertile appendages with micro and
megasporangia. G. Clusters of microspores and megaspores. H. Archaeopteris halliana, reconstruction of a
branch bearing spirally arranged four-ranked sterile and fertile leaves. Upper Devonian. I. Archaeopteris sp.,
transverse section of shoot bearing leaf traces (lt), leaf bases (lb) and branch traces (bt).
(All figures reproduced from Stewart and Rothwell, 1993)
22
Figure-5. Restoration of seed ferns in a Carboniferous swamp setting. Note bifurcate fronds with large ovules
and vine-like nature of specimen supported by the trunk of a lepidodendrid. (Photograph courtesy of the Field
Museum of Natural History, Chicago).
23
A
C
D
B
E
G
F
Figure-6 . Plant parts of petridospermous taxon Lyginopteris sp. A. Capitate gland on Lyginopteris oldhamia.
B. Transverse section of stem showing outer fibrous cortex (OC), sympodium (sy), secondary xylem (sx), pith
with nests of sclereids (p) and leaf traces (lt). C. Transverse section of petiole, Lyginorachis sp. showing
vascular bundle, capitate glands and fibrous outer cortex. D. Pinnules of Sphenopteris foliage. E.
Lagenostoma sp. in cupule of Calymatotheca- type. Cupule shows capitate glands. F. Lagenostoma sp.,
diagram showing longitudinal section of ovule inside cupule. Central column (cc), lagenostome (l), pollen
chamber (p), cupule (c), integument (i), capitate gland (cg), nucellus (n), megagametophyte with archegonia
(mg). G. Crossotheca sp., suspected pollen organ of Lyginopteris sp. (A-F from Stewart and Rothwell, 1993;
G- Taylor and Taylor, 1993)
24
E
B
A
D
C
H
F
G
Figure-7 . Plant and plant parts of Medullosa. A. Reconstruction of Medullosa noei, a plant approximately
3.5 m high. B. Diagram showing transverse section of Medullosa sp. stem with two vascular segments.
"Mixed" protostele (solid black), axial strands (circles), secondary xylem (hatched), ground tissue (stippled).
C. Diagram of transverse section of stem of Medullosa primaeva showing numerous vascular segments and
large leaf bases. D. Pachytesta- type ovule attached to Alethopteris foliage in the position of a pinnule.
Carboniferous . E. Transverse section of distal end of Pachytesta - type ovule showing trimerous (three part)
nature of testa. Commissured rib (cr), endotesta (e), sclerotesta (sc), sarcotesta (sa). F. Diagram of
longitudinal section of Pachytesta- type ovule. Micropyle (m), pollen chamber (pc), nucellus (n), integument
(i). G,H. Medullosan prepollen organs. G. Halletheca sp. (modified reconstruction). H. Aulacotheca sp.
(Reconstruction). (All figures reproduced from Stewart and Rothwell, 1993)
25
D
E
A
B
H
F
C
G
Figure-8 . Reconstruction of extinct Glossopteris tree and its sterile and fertile foliage. Permian .
A. Glossopteris sp., reconstruction of leaf showing conspicuous midrib and reticulate venation. B.
Reconstruction of Glossopteris tree, about 4m tall. C. Diagram showing transverse section of Vertebraria. D.
Ottokaria sp. showing capitulum and stalk adnate to subtending leaf. E. Transverse section through ovulebearing capitulum of Glossopteris. F. Longiudinal section of single ovule from Glossopteris capitulum. G.
Eretmonia sp., fertile leaf bearing two clusters of microsporangia.
(All figures reproduced from Stewart and Rothwell, 1993)
26
A
C
B
F
E
D
Figure-9 . Some Mesozoic seed ferns.
A. Portion of frond of Lepidopteris sp.
B. Ovulate cone of Lepidopteris sp. with spirally arranged cupulate discs.
C. Lepidopteris sp. ovulate disc with pendent ovules.
D. Pinna of Dicroidium.
E. Umkomasia sp. megasporophyll of Dicroidium with cupules.
F. Pteruchus sp., microsporophyll of Dicroidium.
(All figures reproduced from Stewart and Rothwell, 1993)
27
D
G
C
F
E
H
A
B
Figure-10 . Mesozoic seed-fern Order, Caytoniales and its fossilized plant parts.
A. A palmate leaf of Sagenopteris sp. B. Enlarged portion of Sagenopteris leaflet showing reticulate venation
pattern. C. Caytonia sp. showing reconstruction of megasporophyll. D. Young cupule of Caytonia showing
position of lip and opening. E. Longitudinal section through Caytonia cupule showing position of ovules.
F,G. Caytonanthus sp., pollen-bearing organ of Caytonia sp. F. Caytonanthus synangium sectioned, showing
four microsporangia. G. Part of microsporophyll of Caytonanthus sp. H. Caytonanthus, bisaccate pollen
grain. (All figures reproduced from Stewart and Rothwell, 1993)
28
A
B
F
C
G
E
D
Figure-11 . Some fossil Cycadales from Mesozoic.
A. Reconstruction of an Upper Triassic cycad based on Palaeocycas (an ovule-bearing structure) and leaf of
Bjuvia simplex type. B. A leaf of Bjuvia simplex. C. Beania gracilis, ovulate cone of fossil cycads.
D. Longitudinal section of Beania gracilis ovule. E. Androstrobus sp., pollen cone of fossil cycads.
F. A microsporophyll of Androstrobus sp. G. Monocolpate pollen grain of Androstrobus.
(All figures reproduced from Stewart and Rothwell, 1993)
29
v
E
A
H
D
B
C
Figure-12 . Permineralized plants parts of extinct Order, Pentoxylales of Mesozoic gymnosperms. JurassicCretaceous. A. Reconstruction of Pentoxylon sahnii with long and short shoots. Short shoots terminated by
Nipaniophyllum-type leaves. B. Reconstruction of short shoot bearing ovulate cones. C. Suggested
reconstruction of female cone Carnoconites sp. D. Longitudinal section of Carnoconites ovule. E. Suggested
reconstruction of microsporangiate units of Sahnia. (A,B,D - After Stewart and Rothwell, 1993; C,E - after
Taylor and Taylor, 1993)
30
D
C
E
A
B
F
Figure-13 . Some fossil plant parts of extinct Order, Bennettitales of Mesozoic gymnosperms.
A. Reconstruction of Williamsonia sewardiana by Sahni (1932), Jurassic. B. Foliage leaf, Ptilophyllum sp.,
showing abaxial surface. C. Enlarged portion of Ptilophyllum leaf showing attachment of pinnae.
D,E. Weltrichia sp., reconstruction of male fructification of Williamsonia. F. Weltrichia sp., reconstruction of
single microsporophyll. (All figures reproduced from Stewart and Rothwell, 1993)
31
B
A
C
F
E
D
Figure-14 . Fossil plants of extinct Order, Cordaitales of Coniferophytes, Lower Carboniferous - Permian. A.
Reconstruction of cordaitean plant. B. Reconstruction of cordaitean plant with stilt roots. C. A single leaf,
Cordaites sp. D. Cordaitean branch bearing leaves and fertile shoots. E. Primary axis with ovulate secondary
shoots, Cardiocarpus sp. F. Fertile shoot (fs) in axil of a leaf (l), Cordaitanthus sp.
(All figures reproduced from Stewart and Rothwell, 1993)
32
A
D
B
E
H
C
I
F
G
Figure-15. A-C. Variety of foliage shoots produced by Palaeozoic conifers of extinct Order, Voltziales.
A. Utrechtia (Lebachia) piniformis, apex of shoot showing whorled branches. B. Utrechtia (Lebachia) sp.,
branch with spirally arranged leaves. C. Ernestiodendron, leafy shoot. D-I. Principal form-genera of
Mesozoic-Cenozoic fossil conifer foliage. D. Brachyphyllum sp. E. Pagiophyllum sp. F. Cyparissidium sp.
G. Geinitzia sp. H. Elatocladus sp. I. Cupressinocladus sp. (All figures reproduced from Stewart and
Rothwell, 1993)
33
C
E
A
B
D
I
F
G
H
Figure-16 . Fructifications of extinct Palaeozoic conifers. Upper Carboniferous - Lower Permian. A-E.
Shoots of Utrechtia (Lebachia) sp. bearing pendent pollen cones and erect ovulate cones, microsporophyll
and pollen-grains. A. Shoot bearing pendent pollen cones. B,C. Microsporophylls showing position of two
microsporangia. D. Pollen grains of Lebachia. F-I. Bract-shoot complex, Lebachia piniformis,
Ernestiodendron, Walchiostrobus sp. F. Abaxial view showing bract with bifurcate tip. G. Adaxial view of
secondary shoot with spirally arranged scales and single erect ovule. H. Bract-secondary shoot complex of
Ernestiodendron sp. I. Secondary shoot of Walchiostrobus sp. Ovules are inverted.
(All figures reproduced from Stewart and Rothwell, 1993)
34
C
A
B
Figure-17 . Some fossils of extinct Order Czekanowskiales.
A. Czekanowskia sp., shoot with leaves. B. Leptostrobus longus shoot with ovule-bearing "capsules". C. L.
longus section through capsule showing position of ovules. Jurassic-Cretaceous (All figures reproduced from
Stewart and Rothwell, 1993)