The discovery of exceptionally well-preserved, tiny fossil seeds dating back to the Early Cretaceous corroborates that flowering plants were small opportunistic colonizers at that time, according to a new study.

Angiosperms, or flowering plants, diversified during the Early Cretaceous, about 100 to 130 million years ago. Based on evidence from living and fossil plants, the earliest angiosperms are usually thought to have had small stature. New data from the fossil record presented here strongly support this notion, but also indicates key differences from modern flowering plants.

The small seed embryos -- less than 0.3 millimeters in size -- and their surrounding nutrient storage tissues in well-preserved seeds were found in eastern North America and Portugal. 

Using a visualization technique known as synchrotron radiation X-ray tomographic microscopy -- which allows researchers to examine the internal features of delicate fossils in a non-destructive way -- the researchers analyzed more than 250 mature seeds encompassing roughly 75 angiosperm taxa, some of which had the seed embryo fully preserved. Their findings show that the embryos are tiny (one-fourth to one-third of a millimeter), with excellent preservation of cell structure.

The minute size of the fossil embryos is consistent with the interpretation that seed dormancy allowed the earliest flowering plants to survive through harsh environmental conditions and colonize disturbance-prone habitats.

The discoveries support the concept that small embryos and seed dormancy are basic for flowering plants as a whole. However, the embryo to seed ratio in the fossil seeds is much smaller than in seeds of most living angiosperms and an order of magnitude smaller than has been hypothesized for the ancestral angiosperm embryo based on studies of living plants.

Seed dormancy would have ensured that the seeds of early angiosperms could survive until conditions for germination and seedling establishment were favorable, Friis said. However, the tiny embryo size and modest nutrient reserves would also have been a constraint on the rapidity with which these early angiosperms could have germinated in response to short-lived moisture availability. 

 Published in Nature