Why the Greatest Mass Extinction Made Modern Mammals

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Skeleton of the cynodont Galesaurus planiceps. Credit: Image by Roger Smith, Iziko Museums of South Africa Social History / Natural History / Art Collections

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 published today in Proceedings of the Royal Society B.

The first mammals arose in the Triassic period, over 225 million years ago. These early furballs include small shrew-like animals such as Morganucodon from England, Megazostrodon from South Africa, and Bienotherium from China.

They had differentiated teeth (incisors, canines, molars) and large brains and were probably warm-blooded and covered in fur – all characteristics that make them stand apart from their reptile ancestors, and which contribute to their huge success today.

However, new research from the University of Lincoln, the National Museum in Bloemfontein, South Africa, and the University of Bristol suggests that this array of unique features arose step-wise over a long span of time, and that the first mammals may have arisen as a result of the end-Permian mass extinction which wiped out 90 per cent of marine organisms and 70 per cent of terrestrial species.

Dr Marcello Ruta of the University of Lincoln, lead author of the study, said: "Mass extinctions are seen as entirely negative. However, in this case, cynodont therapsids, which included a very small number of species before the extinction, really took off afterwards and was able to adapt to fill many very different niches in the Triassic – from carnivores to herbivores.”

Co-author Dr Jennifer Botha-Brink of the National Museum in Bloemfontein, South Africa said: "During the Triassic, the cynodonts split into two groups, the cynognathians and the probainognathians. The first were mainly plant-eaters, the second mainly flesh-eaters, and the two groups seemed to rise and fall at random, first one expanding, and then the other. In the end, the probainognathians became the most diverse and most varied in adaptations, and they gave rise to the first mammals some 25 million years after the mass extinction."

Co-author Professor Michael Benton of the University of Bristol said: "We saw that when a major group, such as cynodonts, diversifies it is the body shape or range of adaptations that expands first. The diversity, or number of species, rises after all the morphologies available to the group have been tried out."

Two juvenile skeletons of the cynodont Thrinaxodon liorhinus. Credit: Image by Photo credit Roger Smith, Iziko Museums of South Africa Social History / Natural History / Art Collections

The researchers concluded that cynodont diversity rose steadily during the recovery of life following the mass extinction with their range of form rising rapidly at first before hitting a plateau. This suggests there is no particular difference in morphological diversity between the very first mammals and their immediate cynodont predecessors.

The paper, ‘The radiation of cynodonts and the ground plan of mammalian morphological diversity’ was published in Proceedings of the Royal Society B under authors Marcello Ruta, Jennifer Botha-Brink, Steve Mitchell and Michael J. Benton.

About Cynodonts

Cynodonts arose in the Late Permian, and then diversified steadily through the Triassic. Cynodont fossils have been found on every continent, but they are especially well known from South Africa, Argentina, and Russia. Cynodonts, whose name means ‘dog teeth’, all showed differentiated teeth, in which there are incisors at the front for nipping off food, canines for piercing flesh or plant food, and cheek teeth (premolars and molars) for chewing.

Cynodonts survived the end-Permian mass extinction, but they had not been diverse before the event. After the extinction, the number of cynodont species increased steadily through the Triassic as they diversified into many new ecological roles.

There were two major cynodont groups in the Triassic, the cynognathians and the probainognathians, and mammals arose from among the probainognathians. The origin of mammals is traditionally marked by a major shift in the jaw joint from a junction between the quadrate and articular bones to the squamosal-dentary joint; the reptilian jaw joint is reduced in mammals to a linkage within the set of three middle ear bones.

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