10. Rising fast: the first trees

This is the last in a 10-part series arguing that fossils record the Earth’s recolonisation following a mass extinction near the beginning of history. Species that survived multiplied and diversified, the land was progressively reconstituted, and its repeatedly disturbed and buried surfaces were revegetated. Appearing in the order of modern ecological successions, the first plants grew in swampy settings. Here we recount how low-lying plants were soon joined by soaring forests.


The rise of plants in the Devonian was a step change, for in addition to the mosses and marsh vegetation of the previous period, now every major plant type was present. Extraordinarily complex organs and tissue systems appeared as if from nowhere – a ‘novelty radiation’ rather than a slow accumulation of infinitesimally small changes – stems with an intricate fluid transport mechanism, structural tissues, pores for controlled gas exchange (stomata), leaves and roots of various kinds, diverse spore-bearing organs (sporangia), seeds.

Almost from the first, the plants were members of small, mixed communities, not isolated pioneers. Springtails, spiders, insects, centipedes and millipedes also played a part in the fledgling ecosystems. Although some were carnivores, animals were not competing with each other in overcrowded environments where only the fittest survived, but living in balance with each other, interdependently. Species numbers were initially low.

The oldest known fossil forest is that of the Gilboa Formation, New York State, dating to the late Middle Devonian. It presents a dramatic contrast to the ground-hugging plants of earlier times. Although the new forms still grew in a water-logged soil, they were full-grown trees, their tapered pith-filled trunks rising to a height of 9 metres or more. The tallest was the fern-like Eospermatopteris. Their bulbous bases gave them a low centre of gravity,Eospermatopteris stumps near Gilboa Dam (Photo: J B Benington) so that they did not need deep roots to give them stability, though they must have had some. There were also smaller trees and shrubs.

The tree habit appeared independently in several other lineages – lycopsids, progymnosperms, horsetails – and all but the horsetails achieved this in the interval from Middle to Late Devonian. This puzzling phenomenon where Nature seems to invent the same feature more than once (termed ‘convergence’ or parallel evolution) is pervasive in large-scale phylogenies, occurs amongst all types of organism and characterises innovations of every degree of complexity. Another example is photosynthesis: a highly engineered process which occurs not only in plants but also in bacteria, algae and protists, entirely separate biological kingdoms. Within plants, major innovations such as stomata, sporangia, leaves, roots and wood tissues also arose more than once. Leaves arose at least five times, perhaps many more. As Michael Donoghue noted, ‘recent phylogenetic findings are making it increasingly difficult to sustain the traditional view’ whereby the identification of ‘key innovations’ is the way to distinguish new branches in the tree of life. Convergence is evidence that the history of evolution cannot be reduced to a single phyogenetic tree. The structures in question were products of design, not accident.

Long, foreign-sounding names and complex biology can make the plant world seem alien. Here we offer a brief guide that may help to open up the jungle.

Archaeopteris

Progymnosperms

Progymnosperms, the earliest-known trees, first appeared in the Middle Devonian and became extinct in the Early Carboniferous. The innovation of a bifacial cambium enabled them to produce wood (secondary xylem) and thereby grow outwards as well as upwards, in the same way as trees do today. Unlike the later-appearing gymnosperms, to which they were probably ancestral, they reproduced by means of spores rather than seeds. The relationships between the three progymnosperm orders are unclear.

Archaeopteris, the best known example, was the first modern tree. Being only slightly younger than Eospermatopteris, it was virtually the first tree of any kind. It had an extensive root system, produced leaves, grew to a height of 30 metres or more, and had laterally growing branches. A swelling at the base of the branches formed a supporting collar, while layers of wood dovetailed into the trunk to give additional strength. In one respect Archaeopteris was not modern, for large woody trunks today are produced only by seed plants. Its spore-system of reproduction meant that it needed to be close to bodies of ponded or flowing water, allowing sperm to swim to the egg.

Diversification led to several species of Archaeopteris. Occurring in large numbers on every continent, they colonised floodplains and coastal areas on a global scale. Along with the much less widespread Eospermatopteris they created the first forests. Forests ceased on this scale in the wake of events causing mass extinctions at the end of the Devonian and did not recur until well into the Carboniferous, when several new woody-tree genera of uncertain affinity appeared.

Lycopsids

lycopodium clavatumLycopsid is the name for clubmosses (lycopods) and related forms. The informal name refers to their moss-like appearance, though they are not true mosses, and to the club-shaped sporophyll clusters at the tips of their tiny branches. (A sporophyll is a leaf which bears the capsule in which spores develop, called a sporangium.) Only a few lycopod genera exist today, and they look diminutive compared to their largest forebears.

Lycopsids are distinguishable by their sporophylls and their spirally arranged microphylls (spine-like leaves with just one vein). The oldest known lycopsid is Baragwanathia, which we have already come across in Colonising wetlands. It first appeared in the Late Silurian and survived into the Early Devonian. Its leaves were separate from the sporangia and not fully veined. Another Early Devonian lycopod, Drepanophycus, is almost indistinguishable from the still extant shining clubmoss, Huperzia lucidula, a rare case of stasis through the group’s entire record.

Within the lycopsids were the Protolepidodendrales, a widespread order whose leaf-tips were forked. Some grew to a height of 50 cm or more. They may have been ancestral to the arborescent (tree-forming) lycopods of the Late Devonian and Carboniferous.

Arborescent lycopods differed most notably from that group in having a bipolar shoot system (with branches growing upward and roots downward) and growing erect rather than along the ground. In these respects the Middle Devonian Longostachys has something of an intermediate character. Although only 1.5 metres tall, it had a slender trunk that branched at the top to form a crown of narrow branches and terminal cones. At the base it had downward-branching roots. However, arborescent lycopods also had rootlets that grew perpendicularly and radially off the rooting organs, like the hairs of a bottlebrush. How they evolved remains a mystery, for their appearance in the record is abrupt (Gensel & Berry 2001).

Arborescent lycopsids

Arborescent lycopods such as Sigillaria and Lepido- dendron were the major components of the vast Carboniferous swamp forests that ended up as coal seams. These trees produced almost no secondary xylem, wood tissue growing out from the centre so as to increase girth. Instead the trunk was filled with pith and strengthened by a thick cortex which extended from the cambium outwards to a bark-like, decay-resistant periderm. Long grassy leaves grew from the trunk, and following leaf-fall the scars on the leaf cushions composed a distinctive geometric pattern. In contrast to the brown, non-photosynthetic stems of modern trees, photosynthesising tissue covered the whole trunk, giving it a green colour, and this super-efficient energy factory enabled the trees to reach full height in just a few years. Only towards the end of their lives did they form lateral branches and put out a leafy crown. Rising like pillars, they were able to grow in great density, up to 800 trees per acre.

pocked stigmarian root and appendages (Ibbenbären, Germany, Westphalian)Their rooting systems, called stigmaria, were designed to float. They supported the lightly-built trees by splaying out horizontally, and the collapsed state of the fossils (or sediment infill where not collapsed) shows they were hollow and immersed in water. Spiky appendages radiated around they were the stigmaria in every direction. From time to time shed and left scars, just like the leaves. Not having an anchoring function, they probably photosynthesised like the leaves. The stigmaria cannot, as usually assumed, have been rooted in “seat earths” beneath the coals. The clays, sandstones and even limestones of the underlying sediments accumulated before the floating forests sank.

Most of the arborescent lycopods went extinct before the end of the Carboniferous as their watery habitats dried out. Growing alongside them was a related group of smaller mostly unbranched forms, called the isoetaleans. These appeared about the same time as the arborescent forms, in the Late Devonian, but were also affected by extinctions. Today the order Isoetales is represented by only a single genus, the short, semi-aquatic Isoetes.

Ferns

Six major groups are distinguished under this very broad term: whisk ferns, ophioglossoid ferns, cladoxylopsids, marattioid ferns, leptosporangiate ferns and horsetails. Although the first two groups may be related, relationships among the others remain elusive (Pryer et al. 2004). The current state of play is that “relationships among major groups of vascular plants are far less completely understood than claimed by some” and ferns in the broad sense probably do not constitute a single-ancestor group (Rothwell & Nixon 2006).

Whisk ferns have no fossil record. Because of their simple body plans (for example, no roots, and highly reduced leaves), they were long thought to be related to some of the earliest vascular plants, such as Rhynia. However, most experts now believe they derive from more complex ferns that did have roots. They are an example of evolution proceeding in the reverse direction, being ‘advanced’ in time but comparatively simple in form. The similarity with the earliest vascular plants, while thought-provoking, is fortuitous. Ophioglossoid ferns go back no further than the earliest Cenozoic.

The earliest fern-like plants to appear were the cladoxylopsids, in the Middle Devonian, some of which grew to tree height. After most arborescent lycopods went extinct, tree ferns became the dominant plant group. Some North American coals from the Late Carboniferous consist up to 75% of tree fern remains.

Although only recently identified as such, the most famous of the cladoxylopsids is the tree whose stumps thronged the Gilboa forest, Eospermatopteris. It photosynthesised through fronds that spread out from the crown like a feather-duster. As the trunk grew upwards, the branches dropped off and the outer tissues of the trunk remodelled themselves into longitudinal strands, so that the branch scars gradually disappeared. Present-day cycads and palms have a similar structure, but are seed plants (see below).

The two basic types of cambium design. Figure after Donoghue 2005.Marattioid ferns first appeared in the Middle Carboniferous with later species reaching heights of up to 8 metres. Unlike most arborescent plants, they did not produce secondary xylem. Instead, the trunk was encased by a dense mantle of intertwining roots. Extant representatives of this group look much the same as their fossil ancestors.

Leptosporangiates are the most diverse of the fern groups. Of the dozens of families classified, one, the polypods, accounts for about 60% of the 11,000 present-day species, testifying to an extraordinary burst of diversification from the Cretaceous onwards (this is less anomalous in relation to the proposed timescale, which lengthens towards the present). Leptosporangiate ferns appeared in the Early Carboniferous and soon after diversified into six or more lineages, none of which are easily traceable to present-day ferns.

Horsetails are characterised by their grooved, jointed, hollow stems, whorls of highly ‘reduced’ leaves at the joints, extensive underground rhizomes, and sporangium-bearing stalks that cluster into cone-like ‘strobili’. The oldest horsetails go back to the Late Devonian. Represented today by just one genus, the group climaxed in abundance and diversity in the Late Carboniferous, when they grew as tall as trees. Like the arborescent lycopods, they produced secondary tissue only on the inside of the cambium. Modern horsetails are diminutive by comparison with their forebears.

As with the other ‘ferns’, there are earlier plants (such as Sphenophyllum, Hyenia) that although hinting at a relationship with the horsetails have so far resisted attempts to classify them within an orderly evolutionary scheme. The Late Devonian Pseudobornia reached heights of up to 20 metres.

Seed plants (gymnosperms)

Pitus withamii (Lower Carboniferous), from Craigleith Quarry near EdinburghSeed plants include both the gymnosperms – plants with naked seeds – and the angiosperms, the ‘flowering plants’, whose seeds are enclosed within a fruit. Angiosperm fossils date back no further than the Cretaceous, long after the gymnosperms first appeared, and probably descended from a gymnosperm lineage. However, the path from one to the other remains unclear. In this respect, the explosive innovation of coloured petals, stamens, carpels, scent and nectar is still the ‘abominable mystery’ that it was for Darwin.

Seeds consist of an embryonic plant and a store of food, surrounded by a protective coat. The advantage of this mode of propagation was that fertilisation took place through wind pollination rather than via water, and less surface water was required after seed dispersal for the plant to germinate. The food store enabled the embryo to establish itself in its new environment unaided, before having to draw nutrients and water from the soil.

Seed plants could thus colonise a wider range of environments than the vascular sporing plants. While gymno- sperms in the Palaeozoic may have mostly preferred the wetter parts of the landscape, like other types of plant, they also spread into drier though still fairly low-altitude locations. The photograph shows one of the giant trees recovered from Lower Carboniferous sandstones near Edinburgh, on display outside the Natural History Museum, London. These trees had been plucked from their inland environment, transported by strong currents and buried where the sediments, now settling, fanned into the sea.

Gymnosperms are no longer thought to correspond to a group traceable to a common ancestor (the first seed plant) from which all such organisms arose. The evolutionary pattern has proved altogether more complicated.

The oldest known seed plant is Runcaria, from the Middle Devonian. It displayed ‘a highly derived morphology compared to that of sporangia in progymnosperms’ and was ‘already complex’ (Gerrienne et al. 2004), indicating that seed plants must have originated earlier than the time the progymnosperms first appeared. This is also the implication of the Late Devonian seed plants at Elkins, West Virginia, and Taffs Well, near Cardiff, which reveal greater complexity and variation than expected at this stage in seed-plant evolution.

Medullosa noei – a Carboniferous seed-fern treeYet another group that has not conformed with expectations is the ‘seed ferns’. These used to be considered transitional between ferns and seed plants, but are now recognised to be a very heterogeneous group, with relationships among themselves no clearer than their overall relationship to non-seed-fern plants. The earliest seed ferns were mostly liana-like plants and vines from the Late Devonian. Examples from the Carboniferous and Permian include erect trees. Seed ferns finally became extinct in the Eocene.

Other distinct seed-plant groups without obvious ancestry include the cordaites, cycads and conifers. These emerged in the Late Carboniferous. Some cordaites were shrubs; others reached tree height, with narrow parallel-veined leaves up to 1 metre long. They died out at the end of the Palaeozoic. The cycads and conifers are of course more familiar, having descendants in the modern world.

Novelties galore

Why is it, then, that we talk about the ‘Cambrian Explosion’ but not about a ‘Devonian Explosion’? The main reason is that the Cambrian was when nearly all animal phyla first appeared and there are many more animal than plant phyla. Most animal phyla consist of marine organisms, a few of which subsequently colonised the land and/or fresh water (e.g. snails). Heterogeneous phyla such as arthropods and chordates (vertebrates), both of which first appeared in the Cambrian, really consist of several non-related groups, and some of them are exclusively terrestrial. These did not appear until the Devonian, but because they are lumped with the phyla that appeared in the Cambrian, they do not appear quite so revolutionary.

What constitutes a basic body plan is a question that has to be inferred empirically, case by case. As is clear from the arthropod and vertebrate phyla, it cannot be determined simply by agreeing that the basic plan is a certain hierarchic level in a scheme that begins with species and continues by grouping species into higher and higher levels indefinitely, on the assumption that all species are related. In the context of Darwinian evolution there can be no ‘basic body plans’: degrees of difference should increase gradually over time, not be so great, so early, that already at the beginning of the macrofossil record they were at phylum level. Darwinian evolution should be a story of continual improvisation, not of variations on fundamental designs that were presented to natural selection at the outset.

Some basic body plans are defined by gaps at a lower level of classification than phylum: for example, the gaps between mosses and trees, or between fishes and the earliest tetrapod. Plants and animals appeared on land just as suddenly in the Devonian as they did in nearshore marine environments in the Cambrian. It was a ‘novelty radiation’, where new biological types appeared, not by evolution from organisms visible earlier in the fossil record, but from off-stage, where they were previously too few and too sparse to be fossilised.

The order in which plants made themselves known in the fossil record was the order of ecological succession:

 Organism type  First appearance
 Mosses  Cambro-Ordovician
 Fungi  Ordovician
 Small herbaceous plants  Late Silurian
 Shrubs, bushes  Early/Middle Devonian
 Trees  Middle/Late Devonian

It was a reflection of the fact that, little by little, plants were being given more time to establish themselves in the same spot and develop vertically tiered communities. It was not mega-evolution Darwin-style.

New habitats

The increasing diversity of plants reflected the increasing availability of diverse habitats. Owing to the declining rate of radioactivity (the main driver of plate tectonics), geological processes such as sea-level change, volcanism and mountain-building were slowing down and environments gradually becoming more stable, allowing a variety of pre-existing plant types to begin exploiting them. Diversification within those types partitioned the landscape into ever smaller ecological niches. Channel-dissected mudflats began to fill with lycopsids and fern-like plants. Floodplains further inland became sufficiently stable for trees to proliferate and reach maturity. Opportunist species of seed ferns spread from these wetlands to exploit areas of greater stress and variability.

Adapted from Gregory Retallack Science 276:585Plants created new habitats as well as simply colonising them. Increasingly penetrative root systems helped to break up rocks and develop soils, which in turn promoted the growth of a greater range of plants. Vegetation helped stabilise land surfaces by reducing erosion. Forest canopies moderated humidity and temperature, allowing smaller plants to grow beneath them and providing shade for a variety of invertebrates. They also produced copious amounts of spores and leaf litter. Whatever was not eaten by invertebrates was decomposed by microbes and fungi. Soils enriched by herbivores and decomposers enabled the growth of other plant types, and other invertebrates preyed on the herbivores. In time a complex detritus-based food web began to develop, supporting in the drier parts of the forests insects, mites, spiders, myriapods and the like, in the wetter parts tetrapods, molluscs, eurypterids and shallow-water fish.

See also Recolonisation and plant fossil order