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Greatest mass extinction responsible for the making of modern mammals

Two juveniles of the Early Triassic cynodont Thrinaxodon. Photo: Roger Smith

The ancient closest relatives of mammals — the cynodont therapsids — not only survived the greatest mass extinction of all time, 252 million years ago, but thrived in the aftermath, according to new research.

Cynodonts, the ancient closest relatives of mammals, arose during the Late Permian, and then diversified steadily through the Triassic. Their fossils have been found on every continent, but they are especially well known from South Africa, Argentina, and Russia. The cynodonts, not only survived the Permo-Triassic mass extinction, which was the greatest mass extinction of all time, but thrived in the aftermath. After the extinction, the cynodonts radiated dramatically through the Triassic to form two new major groups namely the cynognathians and the probainognathians. They occupied many new ecological roles, the cynognathians being mostly herbivorous and the probainognathians being mostly carnivorous. It is from this latter group that the first true mammals arose. The first mammals evolved over 225 million years ago, and include small shrew-like animals such as Megazostrodon from South Africa, Morganucodon from England, and Bienotherium from China. They had fur, differentiated teeth (incisors, canines, molars), large brains, and were probably endothermic, all characteristics which contribute to their huge success today.

However, new research suggests that this array of unique features arose step-wise over a long time span, and that the first mammals may have arisen as a result of the Permo-Triassic mass extinction, 252 million years ago, which wiped out 90 per cent of marine organisms and 70 per cent of terrestrial species. Although mass extinctions are usually seen as entirely negative, in this case, the cynodonts, which were rare before the extinction, radiated to fill many different niches during the Triassic. The cynognathians radiated rapidly following the mass extinction and continued to do so throughout their history. However, although the probainognathians also diversified rapidly after the mass extinction, their rates of evolutionary change decreased steadily through the Triassic. The cynognathians went extinct during the Late Triassic, but the probainognathians continued to evolve and eventually gave rise to the first mammals some 25 million years after the mass extinction.

It is traditionally thought that mammals experienced a "burst" of evolutionary innovation, and that the first mammals would have had obvious features that clearly set them apart from their ancestors, the cynodonts. However, they were found to be so similar to other cynodonts that it would be difficult to discern the first mammals from the latest cynodonts. These results suggest that cynodont diversification went through two phases: an initial phase characterised by rapid evolutionary rates or "early bursts" (in the case of most cynognathians and early probainognathians) and a second, prolonged or "long fuse" phase for the more slowly evolving groups such as most probainognathians and the earliest mammals.

This research is published in Ruta M., Botha-Brink J., Mitchell S.A., Benton M.J. 2013. The radiation of cynodonts and the ground plan of mammalian morphological diversity, in the Proceedings of the Royal Society B.  It is also available online at http://dx.doi.org/10.1098/rspb.2013.1865

Image

Two juveniles of the Early Triassic cynodont Thrinaxodon. (Courtesy of Roger Smith, Iziko Museums of South Africa Social History / Natural History / Art Collections).