Investigations of well-preserved flower, fruits and seeds discovered in clays and sands from the Cretaceous have revealed exquisite details of structure and organization in early angiosperms.

Internal structures have been documented at the cellular level using synchrotron X-ray tomographic microscopy and the studies have provided invaluable information on relationships and reproductive biology in early angiosperms. A new study of more than 100 million years old seeds from extinct angiosperms recovered in Portugal and USA documents the presence of a particular layer of strongly developed radiating cells in the innermost part of the seed coat, a so-called endothelium (see figure).

This finding was unexpected as the endothelium has traditionally been regarded as a relatively advanced feature because of its widespread occurrence among higher angiosperms and the endothelium in extant angiosperms are typically less pronounced than in the fossil seeds. The endothelium is of maternal origin and is often in close contact with the endosperm and the embryo that are of biparental origin as a result of double fertilization that characterizes angiosperms. The function of the endothelium is not fully understood, but in living angiosperms it is intimately involved in seed development, and especially the development of the endosperm and embryo.

It has been suggested that the endothelium has a role in the provision of nutrition to the developing endosperm and embryo. The strongly developed endothelium in the extinct angiosperms indicates that the endothelium we known from living angiosperms is relictual from a much more widespread and distinct structure in the past, and the study suggests that the endothelium may have originated as a means of maternal control over resource allocation to the offspring that would otherwise have been controlled by tissues of biparental origin following the evolution of double fertilization and endosperm development.

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