FROM A PEBBLE TO A GIANT: CONIFER SEEDS OVER GEOLOGICAL TIME

Seeds are tremendously variable in size, with living plants ranging over 11 orders of magnitude in mass. This variability reflects the many different dispersal mechanisms, environmental factors (habitat, climate, etc.), growth habits (trees, shrubs, herbs, etc.), and life history strategies that contribute to the ecological diversity of seed plants. But a substantial amount of this variation is due to angiosperms and is therefore a relatively recent phenomenon; all other extant seed plants appear to exhibit a more restricted range of variation. We explored these patterns by examining seed size variation in the fossil record of gymnosperms, focusing particularly on conifers, a group with an extensive fossil record, and Mesozoic gymnosperms generally, which have not received as much study as Paleozoic taxa with regards to seed size trends. From published literature, we compiled a database of fossil seeds identified as belonging to particular conifer families or Mesozoic gymnosperm orders based on seed connection with identifiable reproductive structures. We also examined trends in overall seed size distributions, including incertae sedis specimens, among well-known floras spanning the Mesozoic. We observed an increase in the number of both conifer and non-conifer gymnosperms with small seeds starting in the Triassic and continuing through the Cretaceous, although the overall range of seed size variation has changed little. Average and maximum seed sizes then increased going into the Cenozoic, a pattern which mirrors the well-known Paleogene increase in angiosperm seed size. In conifers, this pattern is clade specific; extant clades that are primarily tropical (Araucariaceae, Podocarpaceae) show evidence of a Late Cretaceous to Cenozoic seed size increase that may reflect their interaction with angiosperm dominated tropical forests. Temperate families, in contrast, show little change in average seed size over the Cenozoic, although a Neogene increase in maximum size occurs within some lineages. Although conifer seed sizes are more restricted than those of angiosperms, they nonetheless show important changes over time: first an increase in smaller sizes in the early Mesozoic and then a shift towards larger sizes in the Cenozoic within tropical families.