Flowering plants, formally known as angiosperms, are now among the most populous across terrestrial ecosystems, providing other organisms with food through their fruits. However, their exact origins have remained a mystery for a long time - and ancient plant fossils could help unravel their secrets.

These angiosperms, also known as Magnoliophyta, comprise what is known as the largest and most diverse group of terrestrial plants, covering 64 taxonomic orders, more than 400 families, with more than 300,000 identified species. To understand how flowers came to be and how they relate to other kinds of plants, it is important to understand how different parts of the flower evolved. This particularly includes the angiosperm seeds and the fruits themselves that encase and nurture the seeds before pollination.

Angiosperms
(Photo: Chaithrashree K via Wikimedia Commons)

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A team of Chinese researchers, led by Prof. Shi Gongle of the Nanjing Institute of Geology and Paleontology from the Chinese Academy of Sciences, discovered ancient plant fossils. These preserved remains contained seed-bearing structures and were found in silicified peat in Inner Mongolia in China, dated to be from the Early Cretaceous period.

Researchers presented their findings in the latest journal Nature, May 26, in an article titled "Mesozoic cupules and the origin of the angiosperm second integument."

Ancient Plant Fossils and the Origin of Flowering Plants

The plant fossils, dated to be from about 126 million years ago, support the existing theory that the outer covering of angiosperm seeds, known as the second integument, is structurally similar to some extinct non-angiosperm seed plants from the so-called Age of Dinosaurs.

For example, seeds of cycads (ancient yet extant seed plants), ginkgo, and conifers (cone-bearing seed plants), are enclosed by a single integument that keeps the seed safe from the elements. This structure is believed to correspond to the inner integument present in angiosperms (flowering plants). However, a second integument or the outer covering is a structure unique to these flowering plants. Furthermore, its development is related to its curious recurved form whose development is controlled by a separate set of genes than those that dictate the development of the inner integument.

The same structure has been found in the ancient plant fossils, exceptionally abundant and well preserved, containing two seeds inside a cupule, the cup-shaped structure found in plants and animals. 

Supporting Previous Theories About the Second Integument

Similar cupules have been recorded from now-extinct species of plants from the Mesozoic era. It has been previously proposed, such as in a 2009 study, that the cupules in these ancient plant fossils might be precursors to the second integument of extant flowering plants. However, the lack of information from evidence hampered the theory.

With the new fossils from Inner Mongolia, comparatively studied with previously discovered samples, support the notions that the cupules might actually be the evolutionary ancestor of modern second integument found in angiosperms.

A related news release from the Chinese Academy of Sciences summarizes the similarities as the recurved (cup-like) structure found in young angiosperm seeds is a "holdover from an earlier pre-angiosperm phase of evolution." Additionally, the difference observed in various fossils from extinct Mesozoic plants might be indicative of the difference in environments such as pollination, dispersal, and the output of seeds.

 

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