THE ROLE OF SIZE FOR THE POTENTIAL PHYSIOLOGY OF EARLY COOKSONIA-LIKE VASCULAR PLANT RELATIVES

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, given that the 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, there is simply not volume enough left over for photosynthesis in the smaller fossils, 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 and death of the sporophyte rather than photosynthetic independence. Suppositions concerning size constraints are supported by comparisons to the size range of Early Devonian plants with physiological independence demonstrable through rhizomatous growth or preserved rooting structures. Size is rarely considered in a taxonomic or systematic context, so that several genera and even individual species which are key to our understanding of the radiation of vascular plants actually lump together fossils that can range over an order of magnitude in axis diameter--spanning sizes from necessary physiological dependence to potential photosynthetic competence. Either current phylogenetic understanding must be revised or the possibility must be considered that an independent sporophyte evolved more than once among vascular plants.