Evolution: Unearthing the hidden origins of flowering plants

Several hundred newly discovered plant specimens from 125-million-year-old peat in China provide novel insights into the origins of flowering plants, or angiosperms. Comparison with other plant fossils suggest that the close fossil relatives of angiosperms are reproductively diverse, according to a study presented in Nature.

Two hallmarks of angiosperms are the carpel — the female reproductive organ that encloses one or more ovules — and an outer tissue layer of a cup shaped structure called the cupule, which surrounds the ovule. The outer layer is a characteristic feature of angiosperms that is not seen in other seed plants. Explaining how the these features arose is a vital component for establishing the origins of angiosperms.

Gongle Shi, Peter Crane and colleagues examined the anatomy of several hundred exceptionally preserved plant specimens extracted from a newly discovered fossilized peat dating to the Early Cretaceous epoch from Inner Mongolia, China. Comparison of these now extinct seed plants with possibly related plant fossils from the Mesozoic era (around 252 to 66 million years ago) housed in international collections revealed that their reproductive structures display defining angiosperm-like features — notably the presence of a cupule that is bent backwards. Phylogenetic analyses of a large dataset of seed plants — the equivalent of piecing together a plant family tree — further highlighted these shared morphological and anatomical similarities, suggesting a very close relationship to modern flowering plants. These results indicate a distinct angiosperm ancestry among plants that the authors term angiophytes, which is traceable to before the Mesozoic era, much earlier than the emergence of flowers in the early- or mid-Cretaceous period.

Although more fossils are required to further resolve the origins of angiosperms, recognition of these fossil relatives will aid future research into the evolution of seed plants and the emergence of additional characteristic features of angiosperms, such as the carpel and the stamen.