The discovery of assemblages of small, well-preserved plant fossils from the Early Cretaceous age has provided researchers with a wealth of new insights into the early history of angiosperms. The fossils were found in loosely compacted sedimentary rocks from Portugal and eastern North America and include flowers, fruits and seeds, often so well preserved that it is possible to compare their anatomy with those of living plants on a cellular level. The image below shows an Early Cretaceous angiosperm seed that has been virtually sectioned to reveal a surprising feature – a distinct endothelium.
The endothelium is a specialised seed tissue of maternal origin. It can be recognised in the seed above by the prominent, radially extended cells, which are rich in cytoplasm and tannins. The role of the endothelium in living angiosperms is not fully known, but it probably plays a role in regulating the development of the embryo. Its presence in the fossilised seed is surprising, because the presence of an endothelium is traditionally regarded as a relatively advanced feature of seed development. In living plants, an endothelium is present in the ovules of many asterid and some rosid species, but amongst lineages that diverged early in angiosperm evolution, an endothelium has been recorded only in the ovules and seeds of the species Lactoris. Analysis of the fossilised seeds showed that an endothelium was present in several groups of early angiosperms. This means that the presence of an endothelium in Lactoris seeds may be less of an anomaly and instead could reflect the retention of a feature that was once common among early angiosperms.
The discovery of an endothelium in early angiosperm seeds provides new perspectives on its functional significance and evolution. You can read more in the article by Friis et al.
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