Although fossil plants are well documented from the Silurian Period of Earth history, spores from land plants are known from the preceding Ordovician and Cambrian Periods. However, it is not until the Mid-Devonian that fossil forests appear in the fossil record. The Gilboa Forest from New York State was first described in the 1920s and it became known as the earliest fossil forest. It has the same status today. Only one plant was known from this forest - the Eospermatopteris, or "ancient seed fern" - thought to grow up to 10 metres above the ground. They were not woody, but they had characters that that suggest affinities with tree ferns. The original analysis reinforced the evolutionary assumption that the earliest terrestrial systems were essentially simple. Recent research has changed all this.
"Palaeoecological studies of other Devonian-period sites describe early vegetated terrestrial landscapes partitioned into a 'two-dimensional' suite of patches growing side by side, each composed of closely related plants with similar morphologies and life traits, and adapted to the same environmental conditions. This structure of Devonian landscapes has almost become a dogma in palaeobotany, but Stein and colleagues' report provides the first direct evidence that some early forests contained widely divergent groups of plants." (Meyer-Berthaud & Decombeix, 2012, 41)
New interpretation of the Gilboa fossil forest (source here)
The new findings have been made possible by a dam maintenance project that involved the removal of infill from the original quarry where the fossil forest was found. A 1200 square metre palaeosol surface has been exposed and the locations of stumps of Eospermatopteris trees have been mapped. Two other significant components of the forest have been identified. These are:
"a large rhizomatous plant (one with underground stems growing horizontally) belonging to the extinct aneurophytalean progymnosperms; and a tree with bark similar to that of the lycopsid trees that inhabited [Carboniferous] coal swamps." (Meyer-Berthaud & Decombeix, 2012, 41)
The rhizomatous plant is of great interest because secondary xylem was detected in the rhizomes, but not in the shoots. The researchers considered that the shoots were not bio-mechanically capable of self-support. Also,
"they show that the aneurophytalean subterranean system - consisting mainly of rhizomes up to 15 cm in diameter - comprised a large amount of wood and had significantly more mass than previously estimated." (Meyer-Berthaud & Decombeix, 2012, 41)
The lycopsid-like trees were clearly present, but the fossils were not well preserved. They provide, however, a faunal link with the forests of the Carboniferous Period. Diversity is clearly present in the Devonian forest ecosystems, which were "much more complex than previously thought".
The most significant element of this complexity is the "bifacial vascular cambium" that is found in so-called 'modern' trees today. The term refers to the way the central cambium divides to give off water conducting wood towards the inside and food conducting wood towards the outside (the inner layers of the bark). Although Aneurophylates are already known from other Devonian deposits, this is the time they have been shown to have secondary wood typical of both hardwood and softwood trees. Therefore two important features of 'modern' trees - bifacial cambium and secondary thickening - were present in the Devonian Period.
The evolutionary paradigm is so deeply entrenched that every pointer to complexity steers questions as to how it evolved along Darwinian lines. So, Meyer-Berthaud & Decombeix write: the finding of woody rhizomes "adds credibility to the hypothesis that, in early land plants, wood did not evolve as an adaptation for mechanical support" (p.41). One senses that a just-so story is being developed: one that is based on adaptation and the presumption that Darwinian mechanisms are necessary to establish plausibility. However, it should be asked whether an ecological perspective could provide a way forward, given that the Devonian world was very different from today.
Another finding with a bearing on ecology relates to the youthfulness of the forest: it sprang up rapidly in an environment that was tectonically unstable. Gone is the old idea of a tranquil swamp. The researchers relocate the forest close to a shoreline, and recognise "brutal episodes of seal-level rise" that repeatedly flooded the forest and covered plant life with sediment. They suggest that the sizes of the large trees were constrained by environmental disturbances and that they were potentially fast-growing. The inference is that there was little time for the accumulation of materials to form coal.
"Low-angle cross-bedded sandstones in the Gilboa region containing several Eospermatopteris levels suggest that the site was formed by recurrent, marine-influenced and possibly catastrophic, processes with a relatively high frequency of disturbance." (Stein et al. 2012. 81)
These are fascinating discoveries for anyone interested in the history of life on Earth. Wood provides evidences of modernity in the aneurophytaleans, there is unexpected complexity and Eospermatopteris trees indicate transient environments for growing followed by abrupt destruction of the forest. Further research is needed to establish whether an evolutionary framework is justified in order to understand the findings or whether an ecological framework is sufficient. At very least, we should note that the evolutionary approach has resulted in numerous surprises and significant contradictions to the expectations of researchers.
Surprisingly complex community discovered in the mid-Devonian fossil forest at Gilboa
William E. Stein, Christopher M. Berry, Linda VanAller Hernick & Frank Mannolini
Nature, 483, 78-81 (01 March 2012) | doi:10.1038/nature10819
The origin of trees by the mid-Devonian epoch (398-385 million years ago) signals a major change in terrestrial ecosystems with potential long-term consequences including increased weathering, drop in atmospheric CO2, modified climate, changes in sedimentation patterns and mass extinction. However, little is known about the ecology of early forests or how changes in early terrestrial ecosystems influenced global processes. One of the most famous palaeontological records for this time is the 'oldest fossil forest' at Riverside Quarry, Gilboa, New York, USA, discovered in the 1920s. Hundreds of large Eospermatopteris sandstone casts, now thought to represent the bases of standing cladoxylopsid trees, were recovered from a horizon that was originally interpreted as a muddy swamp. [. . .] Here we describe a 1,200 m2 map showing numerous Eospermatopteris root systems in life position within a mixed-age stand of trees. Unexpectedly, large woody rhizomes with adventitious roots and aerial branch systems identified as aneurophytalean progymnosperms run between, and probably climb into, Eospermatopteris trees. We describe the overall habit for these surprisingly large aneurophytaleans, the earliest fossil group having wood produced by a bifacial vascular cambium. The site also provides evidence for arborescence within lycopsids, extending the North American range for trees in this ecologically critical group. The rooting horizon is a dark grey sandy mudstone showing limited root penetration. Although clearly belonging to a wetland coastal plain environment, the forest was probably limited in duration and subject to periodic disturbance. These observations provide fundamental clarification of the palaeoecology of this mixed-group early forest, with important implications for interpreting coeval assemblage data worldwide.
In the shade of the oldest forest
Brigitte Meyer-Berthaud & Anne-Laure Decombeix
Nature, 483, 41-42 (01 March 2012) | doi:10.1038/483041a
The uncovering of a large soil surface preserved under sediment for 390 million years has exposed plant remains which show that the world's earliest forests were much more complex than previously thought.
World's Oldest Fossilised Forest Unearthed in NY, William Stein (Binghampton University, 28 February 2012)
Floor of Oldest Fossilized Forest Discovered: 385 Million Years Old, ScienceDaily (Mar. 1, 2012)
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