Abstract Detail

Paleobotany

Tomescu, Alexandru [1].

Dissecting the Devonian Explosion: an Emsian leap in anatomical diversity suggests biphasic rise in euphyllophyte disparity.

Over broad time scales, evolution within clades follows a few fundamental patterns: increasing diversity, increasing organismal complexity and maximal size, and early high disparity. While the first three macroevolutionary patterns have been documented in vascular plants or are readily apparent from their fossil record, the forth has yet to be tested for this clade. Disparity, referring to morphological diversity independent of taxonomy, was shown to reach highest levels early in the history of a clade, i.e. early high disparity. Tracheophytes span 425+ Ma of evolutionary history and by the early Carboniferous, in less than one fifth of the clade’s history, had diversified taxonomically and had also reached high morphological disparity, thus qualifying for early high disparity. The same is true of a major clade within tracheophytes, the euphyllophytes. Early tracheophytes and euphyllophytes have frustrated efforts aimed at phylogenetic resolution and the taxonomy of their Devonian representatives is fluctuating, at best. These shortcomings are thought to reflect rapid diversification, termed the Devonian Explosion, which likely entailed mosaic patterns of morphological evolution. Taxonomy notwithstanding, euphyllophytes show a conspicuous, previously unrecognized rise in anatomical diversity ca. 400 Ma ago, in the Emsian. Whereas older euphyllophytes feature exclusively by terete centrarch xylem strands, Emsian euphyllophytes, particularly those recognized in the Battery Point Formation (Gaspé, Canada), mark a sudden leap to diverse xylem anatomies. Foreshadowing the anatomical diversity seen in younger Devonian lineages (iridopterids, pseudosporochnaleans, stenokolealeans, aneurophytaleans, seed plants), the Gaspé euphyllophytes feature diverse actinosteles and different architectures of lateral trace divergence; extraxylary tissues mirror this diversity, exhibiting anatomical differentiation. This sudden and substantial increase in anatomical diversity is consistent with rapid evolutionary radiation and corroborates hypotheses of early high disparity. Furthermore, this contributes an element of detail on the tempo of euphyllophyte evolution, by dating the beginning of their rise in disparity and, more generally, in structural complexity, to the mid/late Emsian. Interestingly, the rise in anatomical disparity was happening in plants that otherwise shared the simple body plan of older tracheophytes, characterized by systems of branching axes exhibiting minimal morphological differentiation: Emsian euphyllophytes show high anatomical disparity but low disparity in external morphology. Organographic differentiation and complex body plans evolved later in this clade, during the Middle and Late Devonian. Thus, during the Devonian Explosion, euphyllophyte disparity seems to have begun exploring the morphospace of body plans and organography only after a first phase of exploration of the internal anatomy morphospace.

1 - Humboldt State University, Department Of Biological Sciences, 1 Harpst Street, Arcata, CA, 95521, USA

Keywords:
tracheophyte
vascular plant
disparity
Devonian
macroevolution
anatomy
Morphology
xylem anatomy
xylem
euphyllophyte
Emsian.

Presentation Type: Oral Paper
Session: 16, Cookson/Moseley and Paleozoic paleobotany
Location: Sundance 4/Omni Hotel
Date: Monday, June 26th, 2017
Time: 5:15 PM
Number: 16007
Abstract ID:239
Candidate for Awards:None