August 11 , 2016

by Jon Tennant

Did you know that birds and crocodiles are practically cousins? Around 230 million years ago, you wouldn’t have been able to tell the difference between the two different lineages. This is because birds and crocodilians (which includes alligators, caiman, and gharials) are part of a much larger group called Archosauria, or ruling lizards, which means they share a common ancestor far back in time. When they split from each other, they formed two major evolutionary pathways: the bird-line archosaurs, which also includes all dinosaurs, and the crocodile-line archosaurs, which includes crocodilians and their ancestors, the crocodylomorphs.

Back at around the time of this split, during a time known as the Late Triassic, the world was much more different than it is today. Small crocodylomorphs prowled the land, along with the earliest dinosaurs. There were a host of other bizarre reptiles, such as the predatory rauisuchids, which might be closely related to the first crocodylomorphs, and small, fox-like sphenosuchians.

One of these was an animal known as Carnufex. Not only is that an awesome name, but it was also an impressive beasty to withhold, coming in at around 3 metres in length. It had serrated teeth for tearing apart its prey, and a long, slender body for rapid movement. Importantly, it is from around the time when this dinosaur-crocodile split occurred, and therefore should hold important clues to the evolutionary history of these groups.

By analysing the anatomy of Carnufex along with a large range of other similar animals, they were able to work out its evolutionary relationships. What they found is that, quite like many early diverging species, Carnufex had a mosaic of features, some more crocodilian, some more dinosaurian.

This is important for several reasons. Carnufex was no tiddly croc, but a pretty large and fearsome predator. Other crocodylomorphs around at this time were usually small, nimble hunters, quite different from Carnufex. What this means is that the very first crocodylomorphs, such as Carnufex, were much larger than we previously thought, and developed their smaller body size later on, something which we can trace based on their evolutionary relationships. This also means that evolution of a smaller body size was something that occurred subsequent to the acquisition of features defining crocodylomorphs, rather than before.

In the Late Triassic, this means that Carnufex would have been one of the top predators roaming the plains of North America. This is quite exceptional, as other crocodylomorphs at the time were by no means top tier predators, with this role usually taken on by other now extinct archosaurs. Shortly after (geologically speaking..) Carnufex, this top predator tier was taken by theropod dinosaurs, which went on to dominate for around 150 million years.

What is clear is that Carnufex was a key stage in crocodylomorph evolution, and may have been critical in helping them survive the end-Triassic mass extinction, which took out almost all other archosaur groups around at the time.

Reference

Drymala SM, Zanno LE (2016) Osteology of Carnufex carolinensis (Archosauria: Psuedosuchia) from the Pekin Formation of North Carolina and Its Implications for Early Crocodylomorph Evolution. PLoS ONE 11(6): e0157528. doi:10.1371/journal.pone.0157528

http://blogs.plos.org/paleocomm/2016/08/01/the-first-crocodile-ancestors/