Cynodont Therapsids Thrive After Late Permian Mass Extinction

Cynodont therapsids – ancient closest relatives of modern mammals – the 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.  They included small shrew-like animals such as
Morganucodon from England, Megazostrodon from South Africa, and Bienotherium from China.

thrixodont
Figure 1. Two juvenile skeletons of the cynodont Thrinaxodon liorhinus. (Image by Photo credit Roger Smith, Iziko Museums of South Africa Social History / Natural History / Art Collections)

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 (Figure 2).

cynodont
Figure 2. Cynodont tree plotted onto a stratigraphic scale; rectangular bars or dots show the known observed ranges of taxa. (From Ruta et al. 2013)

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 were 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."

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.

Source: Modified from materials provided by the University of Bristol.

Resources

Marcello Ruta1, Jennifer Botha-Brink2,3, Stephen A. Mitchell4, & and Michael J. Benton (2013). The radiation of cynodonts and the ground plan of mammalian morphological diversity Proceedings of the Royal Society B DOI: 10.1098/rspb.2013.1865