Diversity, turnover, and seed size evolution in the late Paleozoic radiation of seed plants

During the late Paleozoic, there was a major transition in the taxonomic composition of terrestrial floras from pteridophyte-dominated Paleozoic to the gymnosperm-dominated Mesozoic. Seed plants originated in the Late Devonian and diversified until today they dominate the floras of nearly all modern terrestrial environments. Here I quantify aspects of the taxonomic and morphological evolution of seed plants during the earliest stages of their radiation, using a new database of Paleozoic fossil seed occurrences. I first assess species and genus diversity patterns, exploring the effect of different methods on richness estimates and evolutionary rates. Although the species-level completeness is low (35%) relative to many groups of marine invertebrates, the genus-level preservation is fairly good (56%). In the second section, I explore the potential correlation of species duration with seed size (traditionally interpreted as an evolutionary trade off between amount of maternal investment and number of propagules). Although larger-seeded lineages have higher local extinction rates on ecological time scales, according to the Paleozoic data there is no correlation of seed size with taxon duration on geological time scales. The temporal increase in mean, median, range, and maximum seed size during the Early Carboniferous reflects a trend driven by the sequential radiations of higher groups of seed plants: lyginopterids, medullosans, and cordaites. The three superfamilies assessed here have significantly different seed size distributions and remarkably consistent means throughout the Carboniferous. However, all patterns drawn from the fossil seed database must be interpreted cautiously with respect to taphonomic filters; large-seeded lineages have significantly higher preservation rates than do smaller-seeded taxa. Even so, once these biases are understood, the fossil seed record provides a rich system for exploration of macroevolutionary and macroecological pattern and process during the Paleozoic radiation of seed plants.