Tomescu, Alexandru MF , Rothwell, Gar W. .
Early land plants: the fossil record as independent test for hypotheses of morphological evolution.
FOSSILS are frequently incorporated in evolutionary hypotheses either as contributors of novel character combinations, or as calibration points for dating clade origins. A more rarely explored, yet equally powerful, application of the fossil record is to provide independent tests for evolutionary hypotheses derived from phylogenetic studies or biophysical modeling. Among these are hypotheses for morphological evolution during the colonization of land by eukaryotic autotrophs, and in basal embryophyte phylogeny. Biophysical modeling individuates cylinders (axial morphologies) and oblate spheroids (thalloid morphologies) as optimal geometries of land-dwelling photoautotrophs, and oblate spheroids as optimal geometries of semiaquatic photoautotrophs. The prevailing hypothesis for streptophyte phylogeny proposes a succession of archetypal morphologies for basal embryophytes from thalloid gametophytes bearing sessile sporophytes to axial gametophytes bearing axial sporophytes. The macrofossil record of Lower Paleozoic terrestrial life in North America and Britain shows a transition from biotas consisting exclusively of thalloid forms (Ordovician), to thalloid biotas including axial forms (Late Silurian), to biotas dominated by axial morphologies (Early Devonian and thereafter). This suggests that the biophysical potential of axial morphologies was not exploited by the earliest terrestrial or photoautotrophs, whether they were algae, lichens, or embryophytes. These fossil biotas might also record a more subtle transition from semiaquatic, ephemeral habitats of variable unfavorableness, to fully terrestrial habitats. The macrofossil record supports the morphological evolution hypothesis derived from embryophyte phylogeny, and suggests that the rise of axial morphology as dominant body plan may have been linked to the evolution of biosynthetic pathways for lignin as a compound providing significant structural support. In an ecological perspective, the terrestrial macrofossil record suggests that associations of thalloid and crustose organisms similar to biological soil crusts and ubiquitous in the earliest terrestrial biotas, were gradually displaced by axial photosynthesizers into habitats inaccessible to the latter, where they are primarily found today.
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1 - Humboldt State University, Department of Biological Sciences, Arcata, California, 95521, USA
2 - Ohio University, Department of Environmental & Plant Biology, Porter Hall, Richland Avenue, Athens, Ohio, 45701-2979, USA
Presentation Type: Oral Paper:Papers for Sections
Date: Monday, July 31st, 2006
Time: 10:15 AM