Exceptional preservation of tiny embryos documents seed dormancy in early angiosperms

The discovery of embryos and their associated nutrient storage tissues in exceptionally well-preserved angiosperm seeds from the Early Cretaceous period. Early angiosperms observed The earliest flowering plants bloomed during the Cretaceous, under the feet of the dinosaurs. They are generally thought to have been small, herbaceous plants, adapted for the quick colonization of disturbed or marginal habitats. This hypothesis is dramatically validated by this study of more than 200 Early Cretaceous angiosperm seeds, preserved in 11 mesofossil floras from eastern North America and Portugal. The seeds were tiny, adapted for rapid dispersal and quick germination; all were preserved during a dormant phase in their development. The rapid diversification of angiosperms through the Early Cretaceous period, between about 130–100 million years ago, initiated fundamental changes in the composition of terrestrial vegetation and is increasingly well understood on the basis of a wealth of palaeobotanical discoveries over the past four decades 1 , 2 , 3 , 4 , 5 and their integration with improved knowledge of living angiosperms 3 , 6 . Prevailing hypotheses, based on evidence both from living and from fossil plants, emphasize that the earliest angiosperms were plants of small stature 7 , 8 , 9 , 10 , 11 , 12 with rapid life cycles 7 , 8 , 12 , 13 that exploited disturbed habitats 3 , 9 , 11 , 13 , 14 in open 3 , 9 , 11 , 13 , 14 , or perhaps understorey, conditions 15 , 16 . However, direct palaeontogical data relevant to understanding the seed biology and germination ecology of Early Cretaceous angiosperms are sparse. Here we report the discovery of embryos and their associated nutrient storage tissues in exceptionally well-preserved angiosperm seeds from the Early Cretaceous. Synchrotron radiation X-ray tomographic microscopy of the fossil embryos from many taxa reveals that all were tiny at the time of dispersal. These results support hypotheses based on extant plants that tiny embryos and seed dormancy are basic for angiosperms as a whole 17 , 18 . The minute size of the fossil embryos, and the modest nutrient storage tissues dictated by the overall small seed size, is also consistent with the interpretation that many early angiosperms were opportunistic, early successional colonizers of disturbance-prone habitats 2 , 15 , 16 .