Seed Size, Fruit Size, and Dispersal Systems in Angiosperms from the Early Cretaceous to the Late Tertiary.

Abstract

Fossil data from 25 angiosperm floras from the Early Cretaceous (∼124 million years ago) to the Pliocene (∼2 million years ago) were compiled to estimate sizes of seeds and fruits and the relative proportion of two different seed-dispersal systems by animals and by wind. The results suggest that, first, seed and fruit sizes were generally small during most of the Cretaceous, in agreement with previous suggestions, but the trend of increasing sizes started before the Cretaceous-Tertiary boundary; second, there was a decrease in both seed and fruit sizes during late Eocene and Oligocene, reaching a level that has continued to the Late Tertiary; third, the fraction of animal dispersal was, in contrast to previous suggestions, rather high also during the Cretaceous but increased drastically in the Early Tertiary and declined congruently with the declining seed and fruit sizes from the late Eocene; and fourth, the fraction of wind dispersal showed a bimodal pattern, being high in the Late Cretaceous and in the Oligocene-Miocene but with a drop in between. We find that the observed trends are only weakly related to the availability of animal fruit dispersers. Instead, the trends are congruent with a climate-driven change in environmental conditions for recruitment, where larger seeds are favored by closed forest vegetation. The prevalence of semiopen, dry, and probably herbivore-disturbed vegetation during the Cretaceous, the development of closed multistratal forests in the Eocene, and the later development of a more open vegetation and grasslands starting in the Oligocene-Miocene, are reflected in the distribution of angiosperm seed and fruit sizes and in the dispersal systems.