End-Permian Mass Extinction Paved Way For Modern Mammals
Peter Suciu for redOrbit.com – Your Universe Online
Mass extinction certainly sounds like it would be the end of the line, and perhaps even evokes images of the end of the world. However, new research conducted by the University of Lincoln, the National Museum in Bloemfontein, South Africa, and the University of Bristol suggests that the end can also be the beginning.
This research, which was published this month in the Proceedings of the Royal Society B, noted that 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 its aftermath.
The earliest mammals, which included shrew-like animals such as Morganucodon from England, Megazostrodon from South Africa, and Bienotherium from China, arose in the Triassic period – some 225 million years ago.
These small early mammals had the characteristics that make them stand apart from their reptile ancestors, which included differentiated teeth such as incisors, canines and molars; as well as large brains. As well, these creatures were likely warm-blooded and covered in fur. These characteristics not only made the animals stand apart but may have contributed to their success at surviving.
The latest research suggests that this array of unique features arose step-wise over a very long span of time. The first mammals may have arisen as a result of the Permian-Triassic extinction event, which is believed to have wiped out 90 percent of marine organisms and more than 70 percent of terrestrial species.
The researchers’ findings also cast new light on the impact of mass extinctions. The researchers have examined how species and ecosystems have managed to recover in the aftermath of such an event.
“Mass extinctions are seen as entirely negative,” said Dr. Marcello Ruta of the University of Lincoln and lead author of the study.
“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.”
During the Permian Period a large number of anomodont species existed, and these creatures displayed a wide range of body sizes as well as ecological adaptations. Cynodonts arose during the Late Permian, and diversified steadily through the Triassic Period.
Cynodont fossils have been found on every continent, but are notable for their appearance in Argentina, Russia and South Africa. And while they may have survived the end-Permian mass extinction, they likely had not been diverse before the event.
It was in its aftermath that the number of cynodont species increased through the Triassic Period and diversified into many new ecological roles. It is also likely that this is when the two major cynodont groups arose.
“During the Triassic, the cynodonts split into two groups, the cynognathians and the probainognathians,” said Dr. Jennifer Botha-Brink of the National Museum in Bloemfontein, South Africa, and co-author of the research. “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.”
From their findings the researchers concluded that cynodont diversity rose steadily during the recovery of life following the end-Permian mass extinction, with their range of form rising rapidly at first before hitting an eventual plateau.
According to the researchers this suggests that there is no particular difference in morphological diversity between the very first mammals and their immediate cynodont predecessors.
“We saw that when a major group, such as cynodonts, diversifies it is the body shape or range of adaptations that expands first,” said Professor Michael Benton of the University of Bristol, and co-author of the research. “The diversity, or number of species, rises after all the morphologies available to the group have been tried out.”