The Urbaum

Reconstruction of Archaeopteris, from Beck (1962).

It appears that it's been over a month now since I last posted anything at this site. I'm not going to go back and check, but I think this may be the longest hiatus that Catalogue of Organisms has been through since I first launched it nearly eight years ago. I have my excuses all prepared: it's been a busy period, what with trips back home to New Zealand, general job-hunting type stuff, and construction work around the house*. Nevertheless, I have had subjects lined up to present here all that time (nothing to do with construction, I promise you), and so I've found myself looking up material on Archaeopteris.

*An enterprise absolutely guaranteed to transform you into mind-breakingly tedious company for everyone else.

Archaeopteris, I hasten to explain, is nothing to do with Archaeopteryx, though certain parallels could be drawn (albeit with a long bow). Archaeopteryx, of course, is the Jurassic fossil genus that has become renowned as the Urvogel, the original bird. Archaeopteris is a much older fossil, coming from the Late Devonian. And if Archaeopteryx is to be known as the Urvogel, then Archaeopteris can claim to be the Urbaum, the original tree. It was not the earliest arborescent plant: the slightly earlier cladoxylopsid (a distant relative of modern ferns) Wattieza reached a height of at least eight metres (Stein et al. 2007). But Wattieza, with a single central trunk bearing a crown of fronds, would have been more similar to a modern tree fern or palm. Archaeopteris, with substantial side branches arising from its trunk, would have been more similar to the classic image of a modern tree.

Section of Archaeopteris branch, from Beck (1962). The globular structures are sporangia.

When it was first described, from its foliage alone, Archaeopteris was also believed to be an early fern. It wasn't until the early 1960s that fossils were described associating the fern-like foliage to large conifer-like logs that had been described from the same period. The entire tree was estimated to reach heights of at least sixty feet (about 18 metres) (Beck 1962). Archaeopteris was not a fern, but a member of the lineage leading to modern seed plants. As well as its overall habit, Archaeopteris resembled a modern tree in the presence of secondary thickening: a layer of cambium (generative cells) around the outer part of the trunk produced new phloem (nutrient-conducting cells) outside itself and new xylem (water-conducting cells) on the inside, thus allowing the trunk of the tree to expand as it grew (compare that to a tree fern, which gets no broader as it gets taller). However, as well as its fern-like foliage, Archaeopteris still resembled a more primitive plant in one very important regard: rather than producing seeds like a modern tree, it still reproduced through spores. Modified fronds produced clusters of sporangia, with at least some Archaeopteris species showing signs of the production of distinct male and female spore types. Whether these spores produced independent gametophytes in the manner of modern ferns is unknown, and likely to remain so: not only would such gametophytes probably be small and unlikely to be preserved, but they would have few if any features to associate them with the lofty trees.

Archaeopteris also exhibited a few other noteworthy differences from a modern tree. Most recent trees are more or less monopodial: they have a central main shoot from which branches arise laterally as adventitious primordia. Archaeopteris' main mode of growth was pseudomonopodial: instead of lateral branches arising de novo, they developed from the uneven division of the central shoot, with one part continuing upwards and the other part turning outwards. Though the end result would have looked broadly similar, there are some different functional implications. Archaeopteris' growth form may have been more constrained than most modern trees. Because branches were produced in the same spiral as leaves, there could have been a certain fractal-ness to Archaeopteris' appearance, with each major branch being something of a miniature of the tree as a whole (albeit a somewhat lopsided one, as at least some species produced larger leaves on the upper side of branches than on the lower side). Also, a purely pseudomonopodial mode of growth would not allow for the replacement of lost branches or other appendages: Trivett (1993) compared this model of the growth of Archaeopteris to "an inflating balloon or an opening umbrella with its increasingly empty interior". At the same time, she presented evidence that Archaeopteris could have produced a certain degree of adventitious growth, though it may still have been less resilient to damage than recent analogues. There is some circumstantial evidence that Archaeopteris may have sometimes shed leaves or minor branches en masse, though whether this was a seasonal occurrence or a response to stress is unknown.

Despite being potentially more vulnerable to damage than a modern tree, Archaeopteris was undeniably successful. Various species of the genus were found pretty much around the world, and were the dominant large plant wherever they were found until their extinction around the beginning of the Carboniferous. Perhaps resilience was simply less of an issue for Archaeopteris than for modern trees. After all, it lived in a world where there would have probably still been no major herbivores, and the main causes of appendage loss would have been the weather or disease. Also, long-term resilience may have simply not been so important for a tree that probably produced spores by the millions every year. Who knows how many Archaeopteris sporelings or gametophytes there may have been at a time, simply waiting their opportunity to provide a replacement for a fallen senior?

REFERENCES

Beck, C. B. 1962. Reconstructions of Archaeopteris, and further consideration of its phylogenetic position. American Journal of Botany 49 (4): 373-382.

Stein, W. E., F. Mannolini, L. V. Hernick, E. Landing & C. M. Berry. 2007. Giant cladoxylopsid trees resolve the enigma of the Earth's earliest forest stumps at Gilboa. Nature 446: 904-907.

Trivett, M. L. 1993. An architectural analysis of Archaeopteris, a fossil tree with pseudomonopodial and opportunistic adventitious growth. Botanical Journal of the Linnean Society 111: 301-329.