Benton M.J. Introduction to paleobiology and the fossil record

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488 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Cooksonia is a member of the Rhyniop-

sida, the basal group of vascular plants, the

Tracheophyta. Rhyniopsids are known most

fully from the Early Devonian Rhynie Chert

of northeast Scotland, a deposit that has

preserved numerous plants and arthropods

exquisitely in silica (Box 18.3). Some of the

Rhynie rhyniopsids reached heights of

180 mm. They consisted of groups of vertical

stems supported on horizontal branching

structures that probably grew in the mud

around small lakes.

Several other groups of vascular land plants

arose in the Early Devonian. Zosterophyllum

(Fig. 18.7e), a zosterophyllopsid, shares many

features with the rhyniopsids, but has numer-

ous lateral sporangia, instead of a single ter-

minal one, on each vertical stem. Later in the

Devonian, some basal tracheophytes became

taller, as much as 3 m, the size of a shrub, and

these indicate the future evolution of some

vascular plants towards large size.

THE GREAT COAL FORESTS

Lycopsids, small and large

The clubmosses, Class Lycopsida, arose at the

same time as the rhyniopsids and other dichot-

omously branching plants, but they are dis-

tinguished by having their sporangia arranged

along the sides of vertical branches, instead of

at the tips, and by having numerous small

leaves attached closely around the stems.

Low herbaceous lycopsids existed through-

out the Devonian and Carboniferous, and

they showed considerable variation in leaf

and sporangium shape, and in the nature of

the spores. From the Late Devonian onwards,

most lycopsids produced two kinds of spores,

small and large (microspores and megaspores),

that developed within terminal cones. Lycop-

sids are represented today by some 1100

species, all small herbaceous forms.

During the Carboniferous, several lycopsid

groups achieved giant size, and these are the

dominant trees seen in reconstruction scenes

of the great coal swamps of that period. The

best known is Lepidodendron, a clubmoss

that reached 35 m or more in height. Fossils

of Lepidodendron have been known for 200

years because they are commonly found in

association with commercial coalﬁ elds in

North America and Europe. At ﬁ rst, the sepa-

rate parts – roots, trunk, bark, branches,

leaves, cones and spores – were given different

names, but over the years they have been

assembled to produce a clear picture of the

whole plant (Fig. 18.9).

(a)

(c)

(d)

(e)

(b)

Figure 18.7 Early vascular plants. (a–d) The

oldest land plant, Cooksonia from the Silurian

to Early Devonian. Early Devonian examples

from Wales, showing a complete sporangium at

the end of a short stalk (a), a stoma (b) and

spores (c). The sporangium is 1.6 mm wide, the

stoma is 40 μm wide and the spores are 35 μm

in diameter. (d) Reconstruction of Cooksonia

caledonica, a Late Silurian form, about 60 mm

tall. (e) Zosterophyllum, a zosterophyllopsid

from the Early Devonian of Germany, 150 mm

tall. (a–d, courtesy of Dianne Edwards; e, based

on Thomas & Spicer 1987.)

FOSSIL PLANTS 489

Box 18.3 The Rhynie Chert: a window on earliest land life

Rhynie is a remote village in northeast Scotland consisting of only 50 or so houses; the bus stops

there once a day. In 1914, Dr William Mackie, a physician, found traces of plant fossils in some

speckled black and white chert rocks. He cut thin sections and took his specimens to Glasgow, where

Robert Kidston, the foremost expert in Britain on ﬂ oras of the Carboniferous, conﬁ rmed that the

chert contained nearly perfectly preserved plants. Kidston, together with William Lang, Professor of

Botany at the University of Manchester, England, published a classic series of monographs (Kidston

& Lang 1917–1921) in which they presented superb photographs of microscopic sections through

the Rhynie Chert plants. These publications established the Rhynie Chert as one of the oldest land-

based ecosystems on Earth.

The Rhynie fossils include the remains of seven vascular land plants, as well as algae, fungi, one

species of lichen and bacteria, as well as at least six groups of terrestrial and freshwater arthropods.

What is amazing is the quality of preservation: every cell and ﬁ ne detail can be seen, as if frozen in

an instant and preserved forever (see Fig. 3.8a).

The Rhynie ecosystem was no towering forest. If you went for a stroll in Scotland in the Early

Devonian, the green rim of plants probably did not extend far from the sides of ponds and rivers,

and the tallest plants would have barely brushed your knees (Fig. 18.8). To see anything, you would

have to go down on your hands and knees, and peer at the stems through a magnifying glass. Most

of the taller plants had smooth stems, and branched simply in two, with knob-like sporangia at the

tops of their stems – just larger examples of rhyniopsid plants like the Silurian Cooksonia (see Fig.

18.7a-d). Asteroxylon, a relative of Zosterophyllum (see Fig. 18.7e), had small scale-like leaves

growing up from the stem. Microscopic cross-sections of these plants show they had simple vascular

canals, stomata and terrestrial spores. Between the plants crept spider-like trigonotarbids and insect-

like arthropods, and some of these are even found within cavities in the plant stems. There were

crustaceans in the warm pools.

These discoveries show how extraordinary the preservation is in the Rhynie Chert. The fossils

are siliciﬁ ed through having been ﬂ ooded by silica-rich waters from nearby hot springs. Recent work

(Trewin & Rice 2004) has conﬁ rmed that Scotland in the Early Devonian was an actively volcanic

zone, perhaps related to Caledonian tectonic activity associated with the closure of the Iapetus Ocean

(see pp. 45–8). Rhynie in the Early Devonian was like Yellowstone National Park today, with hot

geysers erupting and immersing vegetation in silica-rich waters at a temperature of 35˚C – an eco-

system frozen (or boiled) in time.

Read more about the Rhynie Chert in the volume by Trewin and Rice (2004), and on web links

from http://www.blackwellpublishing.com/paleobiology/.

The giant lycopsids were adapted to the

wet conditions of the coal swamps, but these

habitats receded at the time of a major arid

phase in the latest Pennsylvanian and Early

Permian. Lepidodendron and its like died out.

Medium-sized lycopsids, about 1 m high,

existed during the Mesozoic, but truly arbo-

rescent (“tree-like”) forms never evolved

again.

The horsetails

The horsetails, or equisetopsids, are familiar

to gardeners as small pernicious weeds. Their

upright green shoots, with a characteristic

jointed structure, are linked by underground

rhizome systems. The sporangia are grouped

into bunches of ﬁ ve or 10 below an umbrella-

like structure arranged along the stem to form

a sort of cone, a unique feature of the group.

The horsetails are a small group today, con-

sisting of a mere 15 species, most of them

small, but one reaching a height of 4 m or

more. The early history of the group shows

much greater diversity.

The horsetails arose during the Devonian,

and Carboniferous forms ﬂ ourished in dis-

turbed streamside settings where they could

Continued

490 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

resprout from underground rhizomes. They

grew in incredibly dense, bamboo-like thick-

ets with locally more than 10 trees per square

meter. One form, Calamites (Fig. 18.10a),

reached nearly 20 m in height, but shows the

jointed stems and whorls of leaves at the

nodes typical of modern smaller horsetails;

the trunk of Calamites generally arose from a

massive underground rhizome. Horsetail

leaves formed radiating bunches at nodes

along the side branch (Fig. 18.10b), and there

were usually two types of cones, some bearing

megaspores (Fig. 18.10c), and others bearing

microspores. The giant horsetails disappeared

at the end of the Pennsylvanian, as did the

arborescent lycopsids. Some modest tree-like

forms up to 2 m tall existed in the Permian

and Triassic, but later horsetails were mainly

small plants living in damp boggy areas.

Ferns – fronds or leaves?

Ferns are familiar plants today, typically with

long fronds, each composed of feathery side

branches that uncurl as they develop. Various

fern-like plants are known in the Devonian

and Carboniferous, and undisputed ferns are

known in abundance from the Carboniferous

onwards. As with the lycopsids and horse-

tails, some of the Carboniferous ferns, like

Psaronius (Fig. 18.11) were tree-like. The

fronds were borne on a vertical trunk, and

they show all the features of their smaller

modern tropical relatives. Other Carbonifer-

Collembolon

Trtigonotarbid

Scutigerella

Lepidocarus

Heterocrania

Protocarus

100 mm

Figure 18.8 Reconstructed scene in the Early Devonian Rhynie ecosystem showing the

commonest vascular plants Rhynia and Asteroxylon in the foreground, and a selection of small

arthropods that lived in the water and in and on the plants (scale bars, 100 μm). (Drawing by

Simon Powell, based on information from Nigel Trewin.)

FOSSIL PLANTS 491

ous and Permian ferns were smaller herba-

ceous plants.

Ferns today are generally low-growing her-

baceous plants, common in many environ-

ments. Ferns are resilient plants. After the

huge eruption of Mount St Helens in 1980,

the ﬁ rst living things to appear through the

thick layers of ash were ferns. Their fronds

had been burned to the ground, and yet

somehow they were not killed, and they

uncurled through the ash to begin the green-

ing of the Washington State landscape within

weeks of the eruption.

Is the fern frond a leaf or a branch? Techni-

cally, it is a branch, and the individual small

green structures along each part of the frond

are leaves. So, each frond is made from many

small leaves. Leaves are common in more

advanced plants, and they may have arisen by

fusion of the small leaﬂ ets of fern-like plants

to provide a typical leaf, an efﬁ cient broad

photosynthesizing structure that can turn to

face maximally toward the sun.

The ferns showed a second burst of evolu-

tionary radiation during the Jurassic and Cre-

taceous, and they are the dominant plants in

some Jurassic ﬂ oras. Again, there were tree-

like forms, as well as the more familiar low-

growing ferns seen today.

Archaeopteris: the missing link?

One of the greatest developments in the evo-

lution of plants was the seed, a key feature

of the dominant modern plant groups, the

gymnosperms and angiosperms. All the other

plant groups considered so far – bryophytes,

rhyniopsids, clubmosses, horsetails and ferns

– lack seeds. Gymnosperms and angiosperms

also show an advance in their woody tissues

that permits the growth of very large trees;

their ligniﬁ ed tracheids, vascular canals, can

develop in a secondary system. The extraor-

dinary early tree Archaeopteris from the

Mid to Late Devonian, seems to represent a

Cystosporites

Lepidocarpon

Lepidostrobophyllum

female cone

Lepidophylloides

Lepidophloios

Knorria

Stigmaria

Lepidostrobus

Lycospora

Figure 18.9 Reconstructing the arborescent

lycopsid Lepidodendron, a 50 m-tall tree from

the Carboniferous coal forests of Europe and

North America. No complete specimen has ever

been found, but complete root systems,

Stigmaria, and logs from the tree trunk are

relatively common. The details of the texture of

the bark, branches, leaves, cones, spores and

seeds are restored from isolated ﬁ nds.

(a)

(b)

(c)

Figure 18.10 Giant Carboniferous horsetails:

(a) Calamites, a 10 m-tall tree; (b) Annularia,

portion of a terminal shoot bearing 10 mm-long

leaves; and (c) Palaeostachya, diagrammatic

cross-section of a cone-like structure, 15 mm in

diameter, bearing small numbers of megaspores.

(Based on Thomas & Spicer 1987.)

492 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

half-way stage. It looked superﬁ cially like a

tree fern, but its trunk showed the develop-

ment of secondary woody tissues and growth

rings, as seen in modern conifers.

SEED-BEARING PLANTS

The origin of seeds

The ﬁ rst plants with seeds are known from

Late Devonian rocks, and seed bearers rose to

prominence during the Carboniferous. After

the end of the Carboniferous, and the extinc-

tion of arborescent lycopsids, ferns and horse-

tails, seed-bearing plants, or gymnosperms,

took an increasingly dominant role in ﬂ oras

around the world.

Seeds in gymnosperms are naked, that is,

they are not enclosed in ovaries as they are in

ﬂ owering plants (angiosperms). Seeds follow

from the fertilization of an ovule, the struc-

ture containing the egg. The gymnosperm

ovule (Fig. 18.12) consists of the megasporan-

gium, the site of the female reproductive

structures, and an outer protective layer, the

integument, with an open end through which

the pollen grains enter. The pollen grains settle

on the ovule, and may send pollen tubes into

the tissues of the ovule, through which sperm

head for the fertile female structures, the

archegonia. Upon fertilization, the ovule

becomes a seed, containing a viable embryo

that develops within the seed coat, and feeding

on the nutritive material that composed the

bulk of the ovule.

Seed-bearing plants evolved from forms

that lacked seeds, and produced spores that

were scattered freely. Spores are commonly

encountered as fossils and they form the basis

of palynological studies for research and com-

mercial purposes (Box 18.4). Free-sporing

plants, such as ferns and horsetails, expel

their microspores and megaspores, which

develop into male and female gametophytes,

respectively. The male gametes (sperm) are

motile and must swim to fertilize the female

egg. This is a risky business that requires at

least a veneer of surface water, limiting sexual

reproduction to damp conditions. Seed plants

retain the egg in a watertight capsule (seed)

that is fed by the vascular system. They

produce sperm in tiny watertight capsules

(pollen) that are blown onto the seed. After

fertilization, the beneﬁ t is that the embryo has

a ready-made water and food supply system

(when attached to parent plant) and after dis-

persal is housed in a drought-resistant shell

that only bursts open when soil conditions

are optimum in terms of wetness and

Figure 18.11 The tree fern Psaronius, a 10 m-

tall fern from the Pennsylvanian of North

America. (Based on Morgan 1959.)

integument archegonia

food store

Figure 18.12 A typical gymnosperm seed, the

ovule of Pinus, the pine, showing the archegonia

(fertile female structures) surrounded by a

substantial food store. Sperm enter through a

narrow gap in the protective integument, and

pass through pollen tubes to the archegonia.

FOSSIL PLANTS 493

Box 18.4 Palynology

Palynomorphs, fossil pollen and spores, provide evidence about ancient paleoenvironments, often

when other fossils are absent, and they are key tools in biostratigraphy (see pp. 26–32). Fossil pollen

and spores have proved to be essential in understanding the biostratigraphy of the Late Paleozoic,

Mesozoic and Cenozoic, especially in terrestrial rock sequences. Pollen analysis is also a routine part

of studies of Quaternary paleoenvironments, especially those studied by archeologists. They can

sometimes be used in correlating marine and non-marine rocks, because pollen and spores are easily

blown from land out over lakes, rivers and shallow seas.

Palynomorphs differ from most other microfossils because of their chemical composition. They

are not usually mineralized, but their polymerized, organic outer coat (exine) is extremely durable.

This coat is resistant to nearly all acids, so hydroﬂ uoric acid is used to dissolve surrounding sand

grains and leave the pollen for microscopic examination.

Some spores show bilateral symmetry (Fig. 18.13). The proximal pole is marked by the germinal

aperture, which may be a rectilinear slit (monolete condition) or it may have a triad of branches

(trilete condition). The laesurae are the contact scars with neighboring spores, commonly converging

at a point or commissure. When extracted in a dispersed form from sediments, an arbitrary size

distinction is applied that classiﬁ es spores above 200 μm as megaspores and smaller ones as

microspores.

Pollen grains are usually smaller, ranging in size from 20 to 150 μm. Inaperturate spores lack a

germinal aperture (Fig. 18.13). A single aperture at the distal pole characterizes the gymnosperms,

the monocotyledons and primitive dicotyledons. Acolpate or asulcate pollen grains lack an obvious

germinal aperture. Many pollen grains, however, such as those of pine and spruce, are saccate, with

both a body or corpus and vesicles or sacci. The terms colpus and sulcus are often used for similar

depressions or furrows; strictly speaking, the sulcus refers to a furrow not crossing the equator

of the pollen. Monosulcate pollen with a single distal sulcus that developed during a series of

meioses is typical of gymnosperms and monocotyledon angiosperms. The tricolpate pattern, seen

in dicotyledon angiosperms, has three germinal apertures or colpi arranged with triradiate

symmetry.

Virtually all fossil pollen and spores are identiﬁ ed and classiﬁ ed on the basis of the morphology

of the resistant outer wall or exine. As is the case with a number of other palynomorph groups, only

a parataxonomy is possible – that is a “form system” that does not reﬂ ect evolution. In one scheme,

the palynomorphs are grouped together into “turma” categories; thus spores belong to the Ante-

turma Sporites and pollen in the Anteturma Pollenites. However, the pollen and spores of plants are

often quite distinctive, and they can be used on their own to infer the presence of families, genera,

and even species.

Exploration geologists frequently describe the shapes of pollens and spores with a code that

describes the exine structure, germinal aperture, outline, shape, size and ornament. Spores (S) are

classiﬁ ed on laesurae (scar) type: c, trilete; a, monolete; b, dilete; 0, lacking laesurae. Pollen grains

(P) are classiﬁ ed on colpation or sulcation type: a, monocolpate; c, tricolpate; 0, lacking

colpation.

warmth. Hence seed plants can colonize drier

environments.

Gymnosperms are said to have owed their

success in the Carboniferous to the fact that

they retained their ovules, and that the devel-

oping embryo had extra protection from the

parent plant. In addition, the free-living game-

tophyte phase was eliminated, and water was

not required for the sperm to swim through,

so that gymnosperms could inhabit dry upland

habitats. Gymnosperms may have had adap-

tive advantages in certain situations as a result

of seed bearing, but it would be wrong to

assume that they always prevailed. Ferns,

Continued

494 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

ektexine

endexine

intine

sexine

nexine

polar eq.

Monocolpate

Monoporate

Trilete

(3-slit)

Syncolpate

Saccate

Dyads

Tetrads

Inaperturate

Polyads

Bulomus

Gramineae

Sphagnum

Pedicularis

Nymphoides

Eriocaulon

Pinus

Potamogeton

Scheuchzeria

Ericaceae

Typha

Mimosa

Orchidaceae

Figure 18.13 Basic morphology and terminology of spores and pollen, shown in polar and

equatorial (eq.) views.

Through time, different palynomorphs came and went (Fig. 18.14). These can generally be

matched with the broad outlines of plant evolution. The oldest spores are from the Ordovician (see

Box 18.1). Spore diversity increased through the Silurian, when some 15 sporomorphs have been

reported, including so-called cryptospores that lack monolete or trilete markings, and are commonly

found in monads, dyads and tetrads, often with an outer membranaceous envelope. Smooth-walled

forms dominated assemblages until the end of the Early Silurian. Some simple spore types with trilete

markings may come from bryophytes, but most were probably from tracheophytes. In rare cases,

spores may be found in direct association with plants, such as numerous specimens of Ambitosporites

in the sporangia of some Cooksonia (see Fig. 18.7).

FOSSIL PLANTS 495

Stratigraphy

Triletes

Monoletes

Monosulcates

Classopollis

Inaperturates

Polyplicates

Disaccates

Monosaccates

Pseudosaccates

Evolved pollens

Paleozoic Mesozoic

Ceno-

zoic

Cretaceous

Tertiary

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

Gymnosperms

Pterydophytes,

Bryophytes

Spores Pre-pollens Pollens

Botanical

classification

Morphological

groups

Appearance of

heterospory

Normapolles

Striated

Angiosperms

Figure 18.14 Stratigraphic distribution of the main pollen and spore types.

Devonian and Carboniferous palynoﬂ oras were much more diverse (Fig. 18.15). The plants

of the Lower Devonian Rhynie Chert (see Box 18.3) were all homosporous, producing a single

kind of spores, and having sculpture and spines. Monolete and trilete spores from lycopsids

appeared during the Devonian. Heterospory, the property of having microspores and megaspores,

arose independently in plants at least 11 times beginning in the Late Devonian. Lycopsid megaspores

with a variety of wall sculpture appeared at this time. The seed ferns of the Carboniferous

produced monolete pollen, and the conifers predominantly saccate pollen with a distal aperture.

Monocolpate pollen, typical of the cycads and ginkgos, was supplemented, during the Carboniferous

and Permian, by both polyplicate and saccate grains. During the Permian, spores are less

common than the more dominant saccate pollens that continued through the Triassic.

Gymnosperms continued to dominate the ﬂ oras of the Early Jurassic (Fig. 18.16), including mono-

saccates from Cordaitales, disaccates from some Coniferopsida, monosulcates from Bennettitaleans,

Cycadales and Ginkgoales, polyplicates from Gnetales, and inaperturates from other

Coniferopsida.

Palynomorphs changed dramatically in the Cretaceous with the radiation of the angiosperms.

Angiosperm pollen has a double outer wall, and the seeds also have a double protective casing.

The ﬁ rst undoubted angiosperm pollen grains are reported from the Lower Cretaceous where

morphs such as Clavatipollenites are oval and monosulcate. During the Cretaceous the

monosulcate condition was supplemented by the tricolpate, in for example Tricolpites. Monocoty-

ledon pollen is monosulcate and bilaterally symmetric, and dicotyledon pollen has both furrows and

pores.

Read more about palynology from links given at http://blackwellpublishing.com/paleobiology/.

Continued

496 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

(a) (b) (c) (d)

(e)

(h)

(i)

(j)

(f)

(g)

Figure 18.15 Some Devonian and Carboniferous spore taxa: (a) Retusotriletes, (b) Retusispora,

(h, i) Ancyrospora, and (j) Auritolagenicula. Magniﬁ cation ×400 (a–d, f, i), ×750 (e), ×90 (g),

×125 (h), ×40 (j). (Courtesy of Ken Higgs.)

FOSSIL PLANTS 497

horsetails, lycopsids and more primitive plants

such as mosses continued to diversify, espe-

cially in damp situations, and they continued

their successful evolution without the “beneﬁ t”

of seeds.

Seed ferns

The seed ferns, or “pteridosperms”, have been

regarded traditionally as a major gymnosperm

class, but they share no unique characters,

and it is clear that they are a paraphyletic or

perhaps polyphyletic assemblage of gymno-

sperms of varied afﬁ nities. Pteridosperms

were important components of Late Paleozoic

and Mesozoic ﬂ oras.

The Carboniferous and Permian seed ferns

belong to a variety of groups, such as the

Medullosales, which looked superﬁ cially like

tree ferns, but bore ovules and pollen. Another

group of Late Paleozoic seed ferns, the Glos-

sopteridales, include Glossopteris (Fig. 18.17),

a 4 m-tall tree with radiating bunches of

tongue-shaped leaves. This seed fern was the

key member of the famous Glossopteris ﬂ ora

that characterized Gondwana, the southern

hemisphere continents, from the Pennsylva-

nian to Late Permian (see p. 42). The Glos-

sopteridales existed through the Triassic, and

a number of other groups of seed ferns of

uncertain afﬁ nities radiated during the

Triassic and Jurassic.

Plant ecology of the coal measures

Early reconstructions of Carboniferous vege-

tation tended to show crowds of ferns, horse-

tails, tree ferns and clubmosses growing

in dense profusion around vegetation-ﬁ lled

lakes. However, detailed studies have shown

that the ﬂ oodplain vegetation consisted almost

exclusively of clubmosses such as Lepidoden-

dron and Sigillaria, with rare examples of

horsetails such as Calamites. Seed ferns,

(a) (b) (c)

(d) (e)

(h) (i)

(f)

(g)

Figure 18.16 Some Jurassic spore and pollen taxa: (a, b) Klukisporites, (c) Dettmanites,

(d) Dictyophyllidites, (e) Retusotriletes, (f) Callialasporites, (g) Classopolis, (h) Podocarpidites,

and (i) Protopinus. Mgniﬁ cation ×400 for all. (Courtesy of Ken Higgs.)