Before the Word for World was Forest

...though to be perfectly honest, I've never read The Word for World is Forest, I just thought that it'd supply a catchy name for this post.


The environment of the Devonian Rhynie Chert, as illustrated by Zdenek Burian (via Palaeos.com.

If you want to find out about the evolution of terrestrial life for vertebrates, there are countless sources out there for you to turn to. But if you want to find out about the evolution of terrestrial life for plants, then your options are probably much thinner. Which is just one more example of how screwed up our priorities as humans are, because there's no doubt which is the greater achievement. When the first terrestrial vertebrates emerged, they found a world already made lush by a covering of vegetation. But the first terrestrial plants would have found nothing waiting for them but bare, hostile rock*. It's amazing that they ever managed at all.

*To be honest, I lie slightly. In places where there was available moisture, I'm sure that a film of bacteria would have grown. Ditto for unicellular algae and other such organisms. If lichen-type associations were around at the time (and a cyanobacteria-zygomycete association is preserved in the Rhynie Chert - Selosse & Le Tacon, 1998), the world would have been their mollusc. Sadly, with little potential for their fossilisation and discovery, we may never really know about the contributions of these first unicellular pioneers.


Reconstruction of the trimerophyte Psilophyton, from here.

But manage they did, and by the early Devonian the world was home to a small but respectable diversity of land plants. Most of the vascular plants of the time have been divided between the rhyniophytes, lycophytes, trimerophytes and cladoxylopsids (doubtless there were also moss- and liverwort-like plants around too, if not actual mosses and liverworts, but the spotty fossil record of bryophyte-grade plants doesn't quite reach that far back). Almost all of them, admittedly, would have been fairly similar to the non-expert eye - small, shrubby affairs with simple branching systems and no true leaves or roots. Examination of their fine structure (particularly of their vascular systems) is necessary to recognise their true affinities - rhyniophytes in the stem lineage for all vascular plants; lycophytes including the ancestors of modern Lycopodium, Selaginella and Isoetes; trimerophytes on the stem leading to modern ferns and seed plants; and cladoxylopsids on the stem of modern ferns*. Each of these groups quite possibly represents a grade rather than a clade, but in most cases it is not possible to actually demonstrate this one way or another.

*It is worth noting (especially in relation to a question asked in the comments to a previous post) that the vascular cells of rhyniophytes, lycophytes and trimerophytes each have distinct morphologies from each other** (Friedman & Cook, 2000), and this has led some authors to suggest that the vascular system may have developed independently in each of the three lineages. For now, though, it seems more parsimonious to assume a common origin followed by evolutionary divergence.

**It is also worth noting that when Friedman & Cook (2000) wrote their review, we actually knew more about the structure of the vascular cells in Devonian lycophytes and trimerophytes than in living lycophytes and ferns. Previous studies of vascular cell structure in living plants had almost exclusively looked at seed plants alone.

Trimerophytes differed from the more basal rhyniophytes in their mode of branching - whereas the basalmost land plants had branched dichotomously (dividing into two branches with each branch growing equivalently), trimerophytes branched anisotomously (one branch growing more than the other), effectively giving the trimerophytes some degree of a central stem (this process is called overtopping). Secondary branches from the central stem still branched dichotomously. Sporangia were borne on the tips of the branches, and at least some trimerophytes grew elongate sporangia in pairs that twisted around each other (Gerrienne, 1997).

One particular trimerophyte, Psilophyton princeps, holds a particular significance for palaeobotany as the first Devonian plant to be reconstructed, by William Dawson in 1859 (Taylor & Krings, 2008), with a large creeping rhizome extending successive upright shoots. But perhaps even more significant was the size reached by some trimerophytes. While most Devonian vascular plants would have been struggling to reach half a metre in height, the trimerophyte Pertica dalhousii has been estimated to have reached up to three metres (Mauseth, 2008) - about the height of the ceiling of an average house (the related but smaller species Pertica quadrifaria is shown to the left, in a reconstruction from the Maine Geological Survey). Together with the similarly-sized cladoxylopsid Pseudosporochnus, these were effectively the first trees - not much compared to their modern successors, perhaps, but very impressive compared to anything that came before them (with the exception, of course, of the primordial oddity Prototaxites). It is interesting to imagine what the environment of these early "forests" would have been like. How did they handle the weather, for a start? In the absence of a strong root system to anchor them down, were they prone to collapsing in the wind? If this was so, did they grow rapidly to compensate for their short lives, or did the rhizome readily send up new shoots to replace lost ones? (Remember, with no leaves either, the entire stem would have probably been photosynthetic.) How did this affect life for the early terrestrial animals taking advantage of their presence? There may have been the beginnings of a forest, but a world recognisably our own was still a long way off.

REFERENCES

Friedman, W. E., & M. E. Cook. 2000. The origin and early evolution of tracheids in vascular plants: integration of palaeobotanical and neobotanical data. Philosophical Transactions of the Royal Society of London Series B 355: 857-868.

Gerrienne, P. 1997. The fossil plants from the Lower Devonian of Marchin (northern margin of Dinant Synclinorium, Belgium): V. Psilophyton genseliae sp. nov., with hypotheses on the origin of Trimerophytina. Review of Palaeobotany and Palynology 98: 303-324.

Mauseth, J. D. 2008. Botany, 4th ed. Jones & Bartlett Publishers.

Selosse, M.-A., & F. Le Tacon. 1998. The land flora: a phototroph-fungus partnership? Trends in Ecology and Evolution 13 (1): 15-20.

Taylor, E. L., & M. Krings. 2008. Paleobotany, 2nd ed. Academic Press.