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 evolution 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 (flowering plants) radiated dramatically during the Cretaceous, and owed their success to fully enclosed and protected seeds, flowers 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

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 photosynthesis. 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 landscape, rates of erosion were 10 times, or higher, than they are today: a shower of rain would wash sand and debris from the landscape 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 specifically at biostratigraphic 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, flowers, 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 dinosaurs, and the origin of flowering plants. Many fossil localities produce exquisitely preserved 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. 1901). However, they are included in this chapter because of botanical (and culinary!) tradition. Fungi are classified into a number of phyla on the basis of reproductive patterns. In some cases, there are specialized reproductive structures that may be identified in well-preserved fossils.

Until recently, the first 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 symbiotic 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 finds. 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, infesting 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 reproductive structures (Fig. 18.1a, b), similar to those of modern oomycete Fungi. Signifi-

Figure 18.1 Examples of fossil fungi: (a, b) Palaeomyces, a possible oomycete fungus from the Early Devonian Rhyme 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.)

Figure 18.1 Examples of fossil fungi: (a, b) Palaeomyces, a possible oomycete fungus from the Early Devonian Rhyme 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.)

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 structure 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 sporadic records of fungi of different groups. Particularly abundant finds come after the radiation of flowering 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 breakdown into separate grains that mix with organic debris, that in turn nourish further organic growth. The first 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 flattened branching structures, some of which show differentiation into upright stems and leaves. Mosses are upright plants with slender stems and, typically, spirally 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 conducting system. Some bryophytes have the unusual ability of being able to dry up completely, 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 difficult 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 flattened 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 reproductive stages are the opposite of those in vascular plants. A possible Cambrian relative, Parafunaria, has been reported from China.

Box 18.1 The first 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 first 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 Sheffield, 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/.

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.)

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 Bryophytes
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.)

Relationships of green plants_

Paleobotanists have long sought the origins of land plants among the Chlorophyta, the green algae, and molecular evidence confirms their close relationship (see pp. 197-8). Broader cladistic studies (Kenrick & Crane 1997) have shown that many forms traditionally classified 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 chlorophyll b and similarities of their flagellate 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 evolution is hard to track in detail in these early stages because the fossils are, on the whole, microscopic and their diagnostic features sub-cellular and therefore rarely preserved. Finds improve with the diversification 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 benefits of large size such as longevity and the ability to produce many more reproductive and dispersal units per individual. Large size in vascular 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 vascular 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 horsetails (equisetopsids) and ferns (filicopsids).

The spermatopsids (seed-bearers) are traditionally divided into gymnosperms and angio-sperms (flowering plants). Flowering plants are so successful today, and they seem so different from other plants, that Charles Darwin famously referred to their origin as an "abominable 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 findings may require further study.

A phylogeny of tracheophytes (Fig. 18.5) shows the broad stratigraphic range and sss y s y y y s s y y

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