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.

Factors that shape seed mass evolution

“…Median and maximum seed mass increased dramatically from Ϸ85 Ma to shortly after the Cretaceous-Tertiary boundary (65 Ma; ref. 6), and it appears that angiosperms radiated into a wider range of growth forms over this same time period. Fossil wood deposits are almost exclusively from gymnosperms before the Turonian (93.5 Ma), but records of dicotyledon wood increase through the later Cretaceous (25).…”

“…By the late Campanian or early Maastrichtian (around 75 Ma), there were at least some angiosperm-dominated forests in existence, including dicotyledonous trees with trunks up to 1 m in diameter (3). Most Cenozoic floras encompass a wide range of seed mass strategies and a wide range of plant growth forms (2,3,6).…”

“…The fossil record shows a particularly rapid period of change in seed size from 85 million years ago (Ma) until shortly after the Cretaceous-Tertiary boundary (65 Ma). During this period, angiosperms radiated out of the tropics, and they shifted from being predominantly small-seeded to having a much wider range of seed size strategies, with a larger mean size (2,6). Present-day species have seed masses spanning more than 11 orders of magnitude, from the dust-like seeds of orchids up to the 20-kg seeds of the double coconut (5,7).…”

“…This hypothesis is consistent with correlations between large-seededness and animal dispersal in presentday species (9). Eriksson et al (6) argue that it is more likely that a closure of canopies resulting from changes in climate around the Cretaceous-Tertiary favored species with larger seeds. The strong relationship between light environment and seed mass across present-day species (10)(11)(12) and the superior survival of large-seeded species when grown under deep shade provide some support for this hypothesis.…”

New Paleocene skeletons and the relationship of plesiadapiforms to crown-clade primates

“…The radiation of angiosperms in the late Cretaceous-early Tertiary, including a systematic increase in fruit and seed sizes correlated with an increasing proportion of animal-dispersed taxa (40,41), would have presented early euarchontans with ample opportunities to exploit fruit, flowers, gums, and seeds in the arboreal milieu (27). It is clear that stem primates (''Plesiadapiformes'') radiated explosively in this newly formed adaptive landscape, with even the earliest members being differentiated from insectivores by lower crowned molars with broad talonid basins and more bunodont cusps for increased exploitation of nonleafy plant resources (2) (see SI Text, Part 7,and Fig.…”