The teeth and bite of small, agile Pakasuchus kapilimai were similar to those of mammals. Credit: Mark Witton, University of Portsmouth

Remains of teeth from an ancient crocodile uncovered in East Africa suggest it had more in common with a modern cat than with the creature's own living relatives.

All species of crocodile surviving today look and behave similarly — they live mostly in water, ambush their prey and swallow it either whole or in sizeable chunks. But around 100 million years ago in the southern supercontinent of Gondwana, members of the crocodile family occupied a much wider range of ecological niches.

Pakasuchus kapilimai is the most extreme form found so far of an extinct group called the notosuchians. Parts of five specimens have been unearthed by a team of palaeontologists led by Patrick O'Connor of Ohio University in Athens.

At about 50 centimetres long from nose to tail, these reptiles would have been active, agile hunters of insects and other small prey. It is likely that they lived primarily on land because their nasal openings are on the front of their skulls. By contrast, surviving crocodilians all have nares on the tops of their heads to allow them to breathe while partially submerged.

Mammalian molars

O'Connor and his colleagues analysed the anatomy of the teeth and jaw of the best-preserved fossil found so far, in the Rukwa Rift Basin in Tanzania1. The specimen had a short, broad, cat-like skull — hence the genus name, as paka is the local Kiswahili language word for 'cat' — but the skull bone arrangement is otherwise distinctively crocodilian.

They used microcomputed tomography to construct three-dimensional images of the skull and to show how the jaws would have moved in the living animal. This revealed that the teeth had developed into forms strikingly similar to those seen in modern mammals — and very different from the simple conical pegs found in all crocodiles and alligators today. In particular, there were molar-like teeth that met together, providing two parallel shearing edges to slice food, just like those of a modern carnivore.

"O'Connor is correct in saying that this is a croc trying hard to be a mammal," comments palaeontologist Greg Buckley of Roosevelt University in Schaumburg, Illinois. "If only isolated teeth had been discovered, without the skull, it is very likely that some of the molariform teeth would have been mistaken for a mammal's. This is truly an exciting discovery and helps to fill in more of the missing pieces of our understanding of these often overlooked creatures."

But there is one question that current knowledge can't answer. Pakasuchus was a slender, long-limbed, mobile animal that had shed the thick scales, known as osteoderms, that protect the bodies of modern crocodilians. Oddly, however, it retained some of this heavyweight armour on its tail — a combination that O'Connor says "is unique among crocodyliforms".

The reason for this remains mysterious. "It is quite difficult to postulate a working functional model for such an arrangement," O'Connor admits.

Several specimens of notosuchian reptiles have been discovered in recent years in Africa, including Madagascar, and South America. None is quite as mammal-like as Pakasuchus, but they do show variation in their dentition, indicating wide differences in diet and lifestyle. This was a very successful group and for about 80 million years probably filled ecological niches that, on the northern supercontinent of Laurasia, were occupied by mammals.

So why did they disappear? O'Connor suspects that it was for the same still-unexplained reasons as the dinosaurs. "But the ancestors of modern crocodiles were able to weather the storm and ultimately led to the 'crocodile' body plans that we see today," he says.