Molecules to Morphology: Developing an Understanding of Plant Evolution Through Time
Gensel, Patricia G. , Kenrick, Paul .
Fossils and the origins of leaves.
Improved knowledge of the morphology and phylogeny of early fossils, particularly lycophytes and euphyllophytes, suggests independent origins of leaves. Lycophyte leaves (microphylls) appeared in the Late Silurian or earliest Devonian (Pragian). Based on phylogenetic and morphometric analyses, leaves (megaphylls) arose several times in euphyllophytes, possibly by a very limited number of developmental routes. Key stages include the evolution of determinacy and dorsiventrality producing the basic leaf architectural framework, and the development of marginal meristems giving rise to a distinct lamina. With the possible exception of some early Chinese fossils, the origin of lamination significantly post-dates determinacy and dorsiventrality. Lamination appears to be related to falling levels of atmospheric CO2 through the Devonian Period. Within this broad evolutionary framework, much remains to be learned about the precise trajectory of leaf evolution within specific lineages.
Early Devonian trimerophytes are basal stem-group euphyllophytes, and as such their leaf-like appendages are key to understanding the basic homologies among leaves in ferns and seed plants. New data on permineralized fossils from Canada provide evidence of morphological and anatomical modification within branches to produce leaf equivalents. Lateral branches are arranged in a helical pattern along stems, and lateral branch vasculature is bilaterally symmetrical, whereas stem vasculature is radial. This differs from stem-group seed plants of the Middle Devonian (i.e., aneurophytaleans) in that the anatomical change to a “leaf equivalent” occurs between the main axis and first order branch. In aneurophytes this appears to be a later ontogenetic change that first appears in the pinnately arranged ultimate appendages. These data indicate that both determinancy and dorsiventrality share a similar structural origin in the early leaf-like appendages of euphyllophyte clade plants, which is consistent with the pattern of KNOX-ARP interactions observed in the developmental regulation of the leaves and leaf-producing meristems of modern ferns and seed plants.
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1 - University of North Carolina, Department of Biology, Cb#3280, Chapel Hill, North Carolina, 27599-3280, USA
2 - Natural History Museum, Paleontology Department, Cromwell Road, London, SW7 5BD, United Kingdom
Presentation Type: Symposium or Colloquium Presentation
Location: Stevens 5/Hilton
Date: Wednesday, July 11th, 2007
Time: 1:45 PM