Seed size reflects a compromise among several factors, including amount of nutritive tissue supporting the plant’s initial development, amount of parental investment in individual offspring, and dispersal mechanism. The late Paleozoic is a key interval to investigate evolutionary patterns in seed size because it records a major transition in the structure and composition of terrestrial ecosystems with the origin of seed plants and increasing prevalence of herbivory. Although previous works have focused on overall seed size, megaspore volume is a more accurate estimate of nutritive resources in propagules and provides a useful alternative to compare with overall size. Results from a compilation of 650+ Paleozoic seed species indicate that mean overall size increased steadily through the Carboniferous and early Permian, reflecting an increase in maximum size while the smallest sizes were maintained. Because previous analyses of Cenozoic seed floras indicated a potential latitudinal gradient to seed size, sampling was restricted to a paleolatitudinal band (30oN to 30oS). A steep increase in variance was driven by diversification of genera with large seeds in several groups of seed plants. This suggests that the earliest were near a structural or functional boundary at the lower limit and that open “evolutionary space” lay only toward larger sizes. Although the overall size distribution moves from positively skewed to more symmetrical through the late Paleozoic, higher groups (e.g., medullosans, cordaites) do not show parallel changes in skewness. Preliminary analysis of megaspore data indicates that there is also an increase in mean megaspore volume during the Carboniferous, but that patterns within groups are not consistent with overall size. The contrasting large-scale evolutionary trends indicated by these results suggest that there may have been different mechanisms affecting overall seed size and megaspore volume during the radiation of seed plants.

Key words: megaspore, paleoecology, Paleozoic, seed size, seeds, trends