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

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478 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

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Chapter 18

Fossil plants

Key points

• Fungi have a long fossil record, perhaps dating back to the end of the Precambrian, but

they are not true plants.

• Green algae, and their relatives, are close to the origin of green plants.

• Plants moved onto land in the Ordovician and Silurian, a move enabled by the evolu-

tion of vascular tissues, waterproof cuticles and stomata, and durable spores.

• Various non-seed-bearing plants arose during the Devonian, but tree-like lycopsids,

equisetopsids and groups such as ferns became established by the Carboniferous. These

formed the great “coal forests”.

• Palynology, the study of fossil pollen and spores, gives remarkable insights into paleoen-

vironments and biostratigraphy.

• The gymnosperms (seed-bearing plants) radiated in several phases: during the

Carboniferous-Permian (medullosans, cordaites, cycads) and Mesozoic (conifers,

ginkgos, bennettitaleans, gnetales).

• The angiosperms (ﬂ owering plants) radiated dramatically during the Cretaceous, and

owed their success to fully enclosed and protected seeds, ﬂ owers and double

fertilization.

What’s in a name? That which we call a rose by any other name would smell as

sweet.

William Shakespeare (1597) Romeo and Juliet

480 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

It is easy to take plants for granted, but just

imagine a world without them! Not only

would there be no forests and no grass, we

would be unable to survive. Plants provide

food for us, both directly (grains, vegetables,

fruit, beans) and indirectly (through feeding

farm animals that we eat), and – together with

several other groups of eukaryotes and pro-

karyotes – they also provide the oxygen in the

air we breathe by the process of photosynthe-

sis. Without plants the landscape would not

only be empty, there would be no soil either:

soil is made of weathered rock and organic

components deriving from plant remains. So,

in the time before plants clothed the land-

scape, rates of erosion were 10 times, or

higher, than they are today: a shower of rain

would wash sand and debris from the land-

scape in a catastrophic way, as in desert wadis

today. Speak to plants: let them know they are

appreciated.

The study of fossil plants falls into two

disciplines: paleobotany, which concentrates

on macroscopic (visible with the naked eye)

plant remains, and palynology, which is

mainly the study of pollen and spores.

Palynology is usually treated as a branch of

micropaleontology (see Chapter 9) because

palynologists use microscopes and much of

the work is aimed speciﬁ cally at biostrati-

graphic correlation, often for commercial

purposes. We touch on palynology in this

chapter, but concentrate on the history of

whole plants, based on the study of leaves,

roots, wood, ﬂ owers, fruits and seeds.

The fossil record of plants is rich, and a

great deal of information is available about

the main stages in plant evolution, from their

move onto land, plants in the age of the dino-

saurs, and the origin of ﬂ owering plants.

Many fossil localities produce exquisitely pre-

served plant fossils (see p. 69), and this has

allowed very detailed microscopic study of

the cellular structure of ancient leaves, seeds

and wood. True plants, or metaphytes, are

considered in this chapter, together with their

closest algal relatives, and the Fungi, even

though modern molecular studies have shown

that these groups are not particularly closely

related (see pp. 190–1).

TERRESTRIALIZATION OF PLANTS

Fungi

The Fungi, represented by familiar molds and

mushrooms, are not true plants. They form a

separate kingdom that is more closely related

to multicelled animals (Metazoa) than to

multicelled plants (Metaphyta) (see pp. 190–

1). However, they are included in this chapter

because of botanical (and culinary!) tradition.

Fungi are classiﬁ ed into a number of phyla on

the basis of reproductive patterns. In some

cases, there are specialized reproductive struc-

tures that may be identiﬁ ed in well-preserved

fossils.

Until recently, the ﬁ rst good fossils of fungi

were from the Devonian and Carboniferous.

Now, some possible lichens have been reported

by Yuan and colleagues (2005) from the

Doushantuo Formation of China, a Late

Neoproterozoic deposit dated at 600 Ma (see

pp. 237–8). Lichens are commonly seen as

scaly, grayish growths on the bark of trees,

but what most people do not realize is that

they are composed of two organisms, a sym-

biotic partnership between a fungus and a

cyanobacterium and/or alga. Each partner

contributes to the wellbeing of the lichen: the

cyanobacteria or algae photosynthesize and

produce glucose from carbon dioxide, while

the fungus provides moisture, nutrients and

protection for the consortium. The Doushan-

tuo specimens are so well preserved, even to

cellular level, that most paleobotanists are

convinced by the new ﬁ nds. It had long been

suspected that cyanobacteria formed thin

crusts on land, as they do in desert regions

(e.g. Utah) today, photosynthesizing and

forming thin “soils” in the Neoproterozoic.

The Doushantuo lichens prove that the

surface of the land, at least close to water, was

already green at the end of the Precambrian,

long before plants really conquered the

land.

Devonian and Carboniferous fungi appear

to have acted as decomposers, feeding on

decaying plant material, or as parasites, infest-

ing the tissues of living plants. In the Early

Devonian Rhynie Chert, for example, fungal

remains include mats of hyphae, branching

tissue strands, some of them bearing repro-

ductive structures (Fig. 18.1a, b), similar to

those of modern oomycete Fungi. Signiﬁ -

FOSSIL PLANTS 481

cantly, the Rhynie Chert also provides the

earliest evidence of symbiosis between a land

plant and a fungus. Highly branched networks

of thin-walled hyphae within the rhizomes of

the Chert plants strongly resemble living

arbuscular mycorrhizae, an extraordinarily

widespread association of plant roots with

fungal hyphae, which play a key role in the

uptake of solutes in the roots of modern

plants. This association between plants and

fungi then goes back to the dawn of plant life

on land.

Coal balls have yielded information on

other fungal groups. A Carboniferous fungus,

Palaeancistrus (Fig. 18.1c), shows extensive

developments of hyphae in a mat-like struc-

ture or mycelium, with specialized hooked

terminations on marginal hyphae called clamp

connections. These are characteristic of

another living fungal group, the Basidiomyco-

tina. After the Carboniferous, there are spo-

radic records of fungi of different groups.

Particularly abundant ﬁ nds come after the

radiation of ﬂ owering plants, when fungi

adapted to parasitize the roots, stems and

leaves of the new plant group, especially in

humid tropical conditions.

The greening of the land: mosses, liverworts

and hornworts

Fungi, algae and cyanobacteria may have

formed crusts and thin soils from the Late

Precambrian, but the land did not become

green until the Ordovician at least. Algae,

fungi and plants make soil by growing on the

surface of rocks, and assisting their break-

down into separate grains that mix with

organic debris, that in turn nourish further

organic growth. The ﬁ rst land plants seem to

have been the bryophytes. There are some

25,000 species of bryophytes today, divided

into three distinctive groups. Liverworts and

hornworts are ﬂ attened branching structures,

some of which show differentiation into

upright stems and leaves. Mosses are upright

plants with slender stems and, typically, spi-

rally arranged leaves.

Bryophytes show some special adaptations

to life on land, such as a waterproof cuticle

over their leaves and stems. Many hornworts

and mosses have stomata, used for controlling

water loss (see below) but they are absent in

liverworts. A few of the larger mosses and

liverworts have a very simple vascular con-

ducting system. Some bryophytes have the

unusual ability of being able to dry up com-

pletely, then rehydrating when rain falls, and

continuing as normal.

The fossil record of the bryophytes is

patchy. This is often attributed to low

preservation potential, but fossil specimens

are also difﬁ cult to distinguish from other

simple land plants. The oldest recorded fossil

bryophytes are Ordovician (Box 18.1) to

Devonian in age, although interpretations

are uncertain. For example, Sporogonites

(Fig. 18.3) from the Lower Devonian of

Belgium has been interpreted as a part of the

ﬂ attened portion of a liverwort with, growing

from it, the slender-stemmed spore-bearing

phases of the plant. This specimen shows the

unique feature of bryophytes, that their repro-

ductive stages are the opposite of those

in vascular plants. A possible Cambrian rela-

tive, Parafunaria, has been reported from

China.

(a) (b)

(c)

0.5 mm

0.1 mm

40 ␮m

Figure 18.1 Examples of fossil fungi:

(a, b) Palaeomyces, a possible oomycete fungus

from the Early Devonian Rhynie Chert of

Scotland, showing branching non-septate hyphae

terminated by enlarged vesicles (a) and a resting

spore (b); and (c) Palaeancistrus, with

basidiomycete-like clamp connections, from the

Pennsylvanian of North America. (a, b, courtesy

of Thomas N. Taylor; c, based on Stewart &

Rothwell 1993.)

482 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 18.1 The ﬁ rst plants on land

For years, paleontologists assumed that plants and animals moved onto land in the Silurian and

Devonian: some excellent fossil examples of earliest plants and arthropods date from the Mid to

Late Silurian, and these show small waterside vascular plants together with spiders, scorpions and

precursors of insects and millipedes. Hints of older land plants were reported from the Ordovician.

For example, Ordovician soils with root-like structures, or burrows, prove that plants were already

on land: you do not get soils without plants. There are even fossil soils as old as 1.2 Ga from the

Precambrian that were presumably generated by microbial or algal activity.

But something happened in the Mid Ordovician, some 470 Ma. The character of microfossil

assemblages changed dramatically, with the ﬁ rst appearance of spores (Fig. 18.2). Spores are airborne

microscopic cells that are characteristic of land plants. So, although these earliest land plants have

not yet themselves found as fossils, they must have been there because they were producing spores

already. But were these early spores produced by land plants or by their immediate green algal

antecedents on their route to the land?

These Ordovician spores had been hard to understand until Charlie Wellman from the University of

Shefﬁ eld, England and colleagues showed in 2003 that the spores were probably produced by small

bryophytes, perhaps like liverworts. Their study of the spore walls showed some detailed similarities to

those of modern liverworts, and they also found clusters of spores packaged in a type of cuticle that

looked overall like a liverwort sporangium. It seems that non-vascular bryophyte-like plants invaded

the land in the Mid Ordovician, and later true vascular plants evolved from within this complex.

Find web links at http://www.blackwellpublishing.com/paleobiology/.

(a)

(c)

(b)

Figure 18.2 The oldest evidence of vascular plants on land? Spores from the Mid Ordovician

(470 Ma) of Oman, scanning electron microscope images of a mass of spores (a) and close-up of one

spore tetrad (b), and light microscope view of a spore tetrad (c). (Courtesy of Charlie Wellman.)

FOSSIL PLANTS 483

Relationships of green plants

Paleobotanists have long sought the origins of

land plants among the Chlorophyta, the green

algae, and molecular evidence conﬁ rms their

close relationship (see pp. 197–8). Broader

cladistic studies (Kenrick & Crane 1997) have

shown that many forms traditionally classiﬁ ed

as “algae” are close outgroups of land plants,

the closest being the Charophyceae (including

charophytes, see pp. 197–8), with the Chlo-

rophyta a little more distantly related.

These algal groups, together with land

plants, form a larger clade termed the Chlo-

robionta, or green plants (Fig. 18.4), that are

all characterized by the possession of chloro-

phyll b and similarities of their ﬂ agellate cells

and chloroplasts (Kenrick & Crane 1997).

This clade dates back to at least 1200 Ma,

the age of the red alga Bangiomorpha from

Canada (see pp. 200–1). Chlorobiont evolu-

tion is hard to track in detail in these early

Figure 18.3 Sporogonites, an Early Devonian

bryophyte, seemingly showing numerous slender

sporophytes (20 mm tall) growing from a basal

gametophyte portion. (Based on Andrews 1960.)

stages because the fossils are, on the whole,

microscopic and their diagnostic features sub-

cellular and therefore rarely preserved. Finds

improve with the diversiﬁ cation of land plants

in the Silurian and Devonian.

The Chlorobionta are divided into various

“algal” groups and the major clade Em -

bryophyta. This latter clade is divided into

two basic grades, the non-vascular plants

(bryophytes) such as mosses, liverworts and

hornworts that evolved in the Ordovician,

and vascular plants (tracheophytes) that arose

in the Mid Silurian. Bryophytes are mostly

small, whereas tracheophytes have evolved

into very large organisms. This might be

explained by a number of reasons, including

competition for light, as well as other beneﬁ ts

of large size such as longevity and the ability

to produce many more reproductive and dis-

persal units per individual. Large size in vas-

cular plants came with the evolution of the

cambium (a lateral tissue that allows increase

in girth). Hence the “race for the skies” among

the early land plants that resulted in large

trees by the Middle Devonian.

The tracheophytes (Box 18.2) are the vas-

cular plants, characterized by vascular canals

with secondary thickening, and include the

rhyniopsids and lycophytes (lycopsids and

zosterophyllopsids) as basal groups. Next up

the main axis of the cladogram are the horse-

tails (equisetopsids) and ferns (ﬁ licopsids).

The spermatopsids (seed-bearers) are tradi-

tionally divided into gymnosperms and angio-

sperms (ﬂ owering plants). Flowering plants

are so successful today, and they seem so dif-

ferent from other plants, that Charles Darwin

famously referred to their origin as an “abom-

inable mystery”. Fossils suggest that gnetale-

ans (a group with a small number of living

members) or one of several extinct Mesozoic

groups, such as the Bennettitales or Caytonia-

les, are the sister group of angiosperms.

Successively more distant outgroups are the

cycads, the conifers + ginkgos, and the medul-

losans. Molecular studies (e.g. Bowe et al.

2000) suggest, on the other hand, that the

gymnosperms form a distinct clade consisting

of cycads, ginkgos, conifers and gnetaleans,

none of which then is any closer to the origin

of angiosperms than the others. These ﬁ nd-

ings may require further study.

A phylogeny of tracheophytes (Fig. 18.5)

shows the broad stratigraphic range and

Chlorophyta

Charophyceae

Bryophyta

Rhyniopsida

Lycophyta

Equisetopsida

Filicopsida

Medullosales

Bennettitales

Cycadales

Ginkgoales

Coniferopsida

Gnetales

ANGIOSPERMAE

GYMNOSPERMAE

CHLOROBIONTA

EMBRYOPHYTA

TRACHEOPHYTA

SPERMATOPSIDA

Figure 18.4 Cladogram showing the postulated relationships among the major groups of vascular land

plants. Some synapomorphies that deﬁ ne particular nodes are: Chlorobionta (chlorophyll b),

Charophyceae + Embryophyta (cell structure), Embryophyta (alternation of generations), Tracheophyta

(vascular canals (tracheids) and secondary thickening) and Spermatopsida (seeds). Read more about the

“deep green” project to establish a complete phylogeny of green plants at http://ucjeps.berkeley.

edu/bryolab/GPphylo/.

Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

CenozoicMesozoicPaleozoic

Filicopsida

Ginkgoales

Pinales

Cycadales

Angiospermae

Z = Zosterophyllopsida

R = Rhyniopsida

Lycophyta

Equisetopsida

Bennettitales

Gnetales

Cordaitales

Progymnospermopsida

Medullosales

PSILOPHYTOPSIDA

TRACHEOPHYTA

Figure 18.5 Phylogenetic tree of the main groups of vascular land plants. The pattern of postulated

relationships is based on the cladogram (Fig. 18.4), and details of known stratigraphic range and

species diversity are added.

FOSSIL PLANTS 485

Box 18.2 Classiﬁ cation of the tracheophytes

Tracheophytes are the vascular land plants, and include modern ferns, horsetails, conifers and ﬂ ow-

ering plants, as well as numerous extinct groups. The basal groups are distinguished in terms of

branching patterns and sporangial morphology.

Division Tracheophyta

Class RHYNIOPSIDA

• Simple vascular plants with dichotomously branching stems and terminal sporangia

• Mid Silurian to Early Devonian

Class Lycophyta

• Small to large plants with lateral sporangia and (usually) small leaves

• Late Silurian to Recent

Class EQUISETOPSIDA

• Horsetails; vertical stems with jointed structure and a whorl of fused leaves at the nodes; spo-

rangia grouped in cones

• Late Devonian to Recent

Class FILICOPSIDA

• Ferns; dichotomously-branching ﬂ at leaves which uncurl as they develop; sporangia are grouped

in clusters usually on the underside of leaves

• Mid Devonian to Recent

Class PROGYMNOSPERMOPSIDA

• Plants with gymnosperm-like wood but free sporing (fern-like) reproduction. Larger members

include early trees such as Archaeopteris from the Late Devonian

Class SPERMATOPSIDA

Subclass GYMNOSPERMAE

Order MEDULLOSALES

• Primitive seed plants with large pollen grains and unusual stem anatomy

• Mississippian to Permian

Order BENNETTITALES

• Bushy to tree-like plants with sterile scales between the seeds; frond-like leaves; ﬂ ower-like

cones with enclosing structures that surround ovules and pollen sacs

• Late Triassic to Late Cretaceous

Order CYCADALES

• Bushy to tree-like plants with leaf traces that girdle the stem; frond-like leaves; seeds attach

to a megasporophyll stalk below a leaf-like structure

• Mississippian-Recent

Order GINKGOALES

• Trees with seed-bearing shoots and with fan-shaped or more divided leaves

• Late Triassic to Recent

Order CONIFERALES

• Conifers; trees with resin canals, and needle- or scale-like leaves

• Mississippian to Recent

Order GNETALES

• Leaves opposite each other, and vessels in the wood; male and female cones are ﬂ ower-like

• Late Triassic to Recent

Subclass ANGIOSPERMAE

• Ovules are enclosed in carpels, within a ﬂ ower, and fertilization is double (involving two

sperm nuclei)

• Early Cretaceous to Recent

486 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

relative abundance of each group at different

points in plant history. The phylogeny high-

lights the three major bursts of land plant

evolution, the ﬁ rst in the Devonian (rhyniop-

sids, zosterophylls and other basal vascular

plants), in the Carboniferous and Permian

(lycopsids, ferns, horsetails, seed ferns) and in

the Cretaceous (angiosperms).

Adapting to life on land

The following are the key adaptations of vas-

cular plants for life on land:

1 Spores or seeds with durable walls to resist

desiccation.

2 Surface cuticle over leaves and stems to

prevent desiccation.

3 Stomata (singular, stoma), or controllable

openings, to allow gas exchange through

the low-permeability cuticle.

4 A vascular conducting system to pass

ﬂ uids through the plant.

5 The ligniﬁ cation of tracheids to resist col-

lapse. The cellulose cell walls of the con-

ducting tubes, or tracheids, of vascular

plants are invested with lignin, the tough

polymer that makes up all woody tissues,

providing strength and waterprooﬁ ng.

These key adaptations relate to the prob-

lems a water plant must overcome when

moving onto land. In water, a plant may

absorb nutrients and water all over its surface,

but on land all such materials must be drawn

from the ground, and passed round the tissues

internally. Land plants typically have special-

ized roots that draw moisture and nutrient

ions from the soil, which are passed through

water-conducting systems that connect all

cells. The system is driven by transpiration, a

process powered by the evaporation of water

from leaves and stems. As water passes out of

aerial parts of the plant, ﬂ uids are drawn

up into the water-conducting system

hydrostatically.

Water loss is a second key problem for

plants on land. Whereas in water ﬂ uids may

pass freely in and out of a plant, a land plant

must be covered with an impermeable cover-

ing – the waxy cuticle. Gaseous exchange and

water transport are then facilitated in many

land plants by specialized openings, the

stomata (singular, stoma), often located on

the underside of leaves. Typically, stomata

open and close depending on carbon dioxide

concentration, light intensity and water

stress.

The third problem of life on land is support.

Water plants simply ﬂ oat, and the water

renders them neutrally buoyant. Most land

plants, even small ones, stand erect in order

to maximize their uptake of sunlight for pho-

tosynthesis, and this requires some form of

skeletal supporting structure. All land plants

rely on a hydrostatic skeleton, a stiff frame-

work supported by water in tubes, and some

groups have evolved additional structural

support through ligniﬁ cation of certain tissues

in the wood and cortex, the process whereby

lignin encrusts cellulose ﬁ bers.

Plant reproductive cycles

Plants may reproduce vegetatively and sexu-

ally. Vegetative reproduction, or budding, is

an asexual reproductive process that involves

no exchange of material from different indi-

viduals, no male and female cells. It is a prop-

erty of many plants that they may multiply

in this way, either naturally or by human

intervention.

Algae show all kinds of reproduction, veg-

etative, asexual and sexual. Sexual reproduc-

tion (Fig. 18.6a) involves the combination

of cellular material from two organisms of

the same species. The reproductive cells, or

gametes (sperms from the “male” and eggs

from the “female”), contain a single set of n

chromosomes, the haploid condition. When

the gametes combine, forming a zygote, the

chromosome number doubles to the diploid

condition, 2n. The diploid plant stage, the

sporophyte, produces haploid (n) spores, each

of which develops into a haploid gametophyte

plant stage. It is the gametophyte that pro-

duces the haploid sperms and eggs.

In typical vascular plants, the green plant

that we see is the sporophyte, while the game-

tophyte is very small (Fig. 18.6b). The oppo-

site is the case in bryophytes, where the visible

mosses and liverworts are haploid gameto-

phytes, and the sporophyte is a small plant

that depends for nourishment on the larger

gametophyte (Fig. 18.6c). Hence, in Sporogo-

nites (see Fig. 18.3), numerous sporophytes

appear to be growing from a portion of the

larger ﬂ attened gametophyte phase. Translat-

FOSSIL PLANTS 487

ing to the human case, this would be like

having the haploid sperm or egg dominant,

and the diploid body (sporophyte) repressed!

Vascular plants in the Silurian and Devonian

As we have seen, non-vascular land plants

had arisen at least by the Mid Ordovician,

and vascular plants by the Mid to Late

Silurian, some 425 Ma. Vascular plants are

characterized by the possession of tracheids,

true vascular conducting systems. Lignin and

stomata are typical of vascular plants, but

may not have been present in the earliest

forms.

The oldest vascular plant is Cooksonia

from the Mid Silurian of southern Ireland, a

genus that survived until the end of the Early

Devonian. Cooksonia (Fig. 18.7a–d) is com-

posed of cylindrical stems that branch in two

at various points and are terminated by cap-

shaped sporangia, or spore-bearing structures,

at the tip of each branch. The specimens of

Cooksonia range from tiny Silurian examples,

only a few millimeters long, to larger Devo-

nian forms up to 65 mm long. Extraordinary

anatomic detail has been revealed by studies

of specimens of these tiny plants that have

been freed from the rock by acid digestion,

and then mounted in resin. The sporangia

have been dissected to reveal that they were

packed with spores, the vascular conducting

tissues of Early Devonian examples have

thickened walls, and there are stomata on the

outer surfaces of the stems (Edwards et al.

1992).

spores

sporophyte

meiosis

gametophyte

zygote

2n2n

spermatozoid

egg

(a)

(b)

(c)

Figure 18.6 The origin of vascular land plant (tracheophyte) life cycles: (a) simpliﬁ ed plant life cycle

showing alternation of phases; (b) life cycle of a hypothetical tracheophyte, with a dominant

sporophyte phase and reduced gametophyte, in comparison with the life cycle of a hypothetical

bryophyte (c), where the dominant phase is the gametophyte, and the sporophyte is a reduced

dependent structure. (Based on various sources.)