Boyce, C. Kevin .
How green was Cooksonia?
Because of their fragmentary preservation, the earliest cooksonioid terrestrial plant macrofossils largely have served as markers for the accumulation of vascular plant characteristics--stomata, sterome, vasculature--while younger Devonian fossils with complete anatomical preservation and documented gametophytes have received greater attention concerning the early evolution of vascular plants and the alternation of generations. Despite preservational deficits, however, possible physiologies of cooksonioid fossils can be constrained by considering the (preserved) overall axis size in conjunction with the potential range of (unpreserved) cell types and sizes, because their lack of organ differentiation requires all plant functions be performed by the same axis. Once desiccation resistance, support, and transport functions are taken into account, smaller fossils do not have volume enough left over for an aerated photosynthetic tissue, argument for physiological dependence on an unpreserved gametophyte. Various bryophyte innovations that facilitate small size would have been unavailable for these plants, furthering the size discrepancy. Also, the presence of a sterome--a link to vascular plants, but strictly unnecessary in such small organisms--would actively limit photosynthetic potential. As in many mosses, axial anatomy more likely ensured continued spore dispersal despite desiccation of the sporophyte than provided photosynthetic independence. Suppositions concerning size constraints are supported by comparisons to the size range of Early Devonian plants for which physiological independence is demonstrated by rhizomatous growth or preserved rooting structures. Size is rarely considered in a taxonomic or systematic context, so that several key genera and even individual species lump together fossils that can range over an order of magnitude in axis diameter--spanning sizes from necessary physiological dependence to potential photosynthetic independence. Either current phylogenetic understanding must be revised or the possibility must be considered that an independent sporophyte evolved more than once among vascular plants.
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1 - University of Chicago, Department of Geophysical Sciences, 5734 S. Ellis Avenue, Chicago, Illinois, 60637, USA
alternation of generations.
Presentation Type: Oral Paper:Papers for Sections
Location: Williford A/Hilton
Date: Monday, July 9th, 2007
Time: 2:00 PM