Newly discovered plant fossils from China, dating back more than 125 million years, may help to establish which seed plants evolved into modern flowering plants.
Flowering plants, known as angiosperms, dominate most land ecosystems, providing food and habitats for a variety of animal species. The hallmarks of angiosperms are the carpel, the female reproductive organ that encloses the ovules, and a second outer layer of tissue covering the seeds. This second outer layer – known as the angiosperm second integument – is a characteristic feature of angiosperms that isn’t seen in other modern seed plants.
However, Peter Crane at Oak Spring Garden Foundation in Virginia and his colleagues think some ancient seed plants did possess a similar and related structure. They excavated several hundred well-preserved, now-extinct, seed plant fossils from a fossilised peat deposit in Inner Mongolia, China, that formed about 125.6 million years ago. The team then examined the fossil plants’ anatomy.
The fossil specimens had seeds enclosed in a structure known as a cupule. What’s more, these cupules were similar to the angiosperm second integument we see in flowering plants today – most notably they curved back on themselves to look a little like the scroll at the top of the neck of a violin, which is a distinctive feature of the angiosperm second integument.
“We think of the cupules as comparable to the centre of angiosperms,” says team member Gongle Shi at Nanjing Institute of Geology and Palaeontology, China. “So, this group of extinct seed plants are closely related to angiosperms.”
Although previous plant fossils have suggested that cupules were the precursors of structures seen in flowering plants, poor preservation of the fossils meant that the evidence was weak. The researchers think their newly discovered fossils provide stronger evidence for the idea.
The team also compared the newly discovered fossil specimens to plant fossils collected in the past, some dating as far back as 252 million years ago. Many of them displayed a similar cupule structure, although angiosperms themselves don’t appear to have evolved so long ago.
“[Angiosperms] show up rather late, appearing around 135 million years ago based on the fossil records alone,” says Crane.
Although it has long been known that flowering plants arose from a now-extinct group of seed plants, the identity of this extinct group hasn’t been clear. The new findings suggest we may now have found evidence of the group that gave rise to angiosperms, which the team suggest should be called the “angiophytes”.
Journal reference: Nature, DOI: 10.1038/s41586-021-03598-w
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