The evolutionary relationships of a bizarre fossil mammal from South America that has baffled scientists for over 120 years have finally been solved, according to a paper published today in the journal Proceedings of the National Academy of Sciences.
Research by a joint American, Argentinean and Australian team has shown that a mysterious 16-million year old mole-like burrowing mammal from Patagonia in southern Argentina is a member of an entirely extinct South American mammal group.
The strange animal, Necrolestes patagonensis – which literally translates as “grave robber”, thanks to its subterranean lifestyle - is only very distantly related to modern mammals.
Earlier studies had concluded that Necrolestes is closely related to either placental mammals (which make up more than 90% of living mammal species, including whales, bats, dogs and humans) or to marsupials (pouched mammals, such as kangaroos and wombats).
But the new study concludes that it was not related to either. Rather, it was a late surviving member of a recently recognized mammal group known only from South America.
The key to resolving the mystery was the recent discovery of a 100-million-year old South American mammal, Cronopio, by a team led by Professor Guillermo Rougier, of the University of Louisville.
Rougier initially noted close similarities between the teeth of Cronopio and Necrolestes. Now, a detailed anatomical study by Rougier, Dr John Wible of the Carnegie Museum of Natural History, Sebastián Apesteguía of the Argentinian government research agency CONICET, and Dr Robin Beck of the UNSW School of Biological, Earth and Environmental Sciences, has confirmed that the two are indeed closely related.
“This project was a little daunting, because we had to contradict 100 years of interpretation,” says Rougier.
Necrolestes and Cronopio both belong to a branch of the mammal evolutionary tree that is distant from living mammals, within a South American group called Meridiolestida that is thought to have gone extinct 60 million years ago. Necrolestes is the youngest known meridiolestidan, and shows that the group survived far longer, until at least 16 million years ago. Necrolestes may have been able to survive longer than its relatives because of its extreme adaptations to burrowing.
“There’s no other mammal in the Tertiary of South America that even approaches its ability to dig, tunnel, and live in the ground,” says Wible. “It must have been on the edges, in an ecological niche that allowed it to survive.”
Its survival indicates that the evolution of the South American mammal fauna over the last 65 million years has been a complex process, with survivors of ancient endemic lineages coexisting alongside more recent arrivals, such as rodents and monkeys.
“It makes you wonder what else is waiting to be found in the fossil record,” says Beck. “Clearly we still long way from understanding mammal evolution in the Southern Hemisphere, and there may be plenty more surprises in store.”