Character diversification and patterns of evolution in early vascular plants

Abstract

Available data on the stratigraphic ranges of latest Silurian and Devonian vascular plant macro-fossils (sporophytes) and spores provide insights into the tempo and mode of early tracheophyte evolution. Patterns of diversification, origination, and extinction conform in general to the predictions of Sepkoski's kinetic model of diversification. Rates of generic origination and extinction vary not only through time but also between organ systems for a single time interval. This fact, coupled with data on longevity and turnover and comparative morphological observations, can be used to document mosaic evolution in early vascular plant history. Mosaic evolution is an important theme in plant evolution; indeed, what we recognize as macroevolutionary events often correlate with brief periods of pronounced mosaicism. Such evolutionary patterns reflect the developmental biology of tracheophytes in which individual organs often have life spans that are considerably shorter than the life of the whole plant. Under these conditions, individual organs or organ systems can respond to different sets of evolutionary pressures.The major period of early vascular plant diversification occurred during the late Early and early Middle Devonian Period, 30 Myr or more after the origin of the group. Such lags in diversification are not uncommon in the fossil record. Sometimes they reflect extrinsic controls on diversification, but in other cases they appear to be a consequence of intrinsic rates of origination and extinction.

Evolution of leaf-form in land plants linked to atmospheric CO2 decline in the Late Palaeozoic era

“…2) would have been the selective pressure driving improved water and nutrient transport, crucial for plants as their heights rapidly increased from the Middle Devonian onwards 10 . However, by the Middle to Late Devonian, fossilized plant assemblages reveal that vascular systems were probably more ef®cient 20 , and water uptake from the soil improved by the evolution of deep rooting systems 21 .…”

EARLY EVOLUTION OF LAND PLANTS: Phylogeny, Physiology, and Ecology of the Primary Terrestrial Radiation

“…Studies of the developmental controls on Devonian plants (127) similarly suggest to some of us that initial increases in the complexity of development created many opportunities for morphological diversification with relatively few constraints. Maximum plant stature increased in many clades, along with the average diversity of organ types borne by any single species and the frequency of compound reproductive structures (70). Yet DiMichele et al (26) argued that the rate of increase in diversity of body plans slowed during the Late Devonian, as those architectures that characterize modern taxonomic classes and orders became clearly recognizable.…”

“…Within paleobotany, there are few more popular review topics than the origin and initial radiation of vascular land plants in the Silurian (438^10 mya) and Devonian (410-355 mya) periods (20,21,23,25,34,37,39,40,51,56,57,66,67,70,95,97,132). Fortunately, each crop of reviews is separated by remarkable empirical and conceptual advances in a wide range of fields that amply justify frequent reappraisals.…”

Origins of Heterospory and the Seed Habit: The Role of Heterochrony

“…Thus, although ovular form may have had little significance in the early radiation of seed plants, its relationship to potential life-history complexity played a much greater role as competitive interactions increased. Although total species diversity continued to increase (Niklas et al, 1980;Knoll et al, 1984), new morphologies evolving during the later phases of this process were minor modifications on existing forms, particularly those with complete or nearly complete integuments.…”