What’s in their head? Looking at the brains of crocodiles reveals insights into their evolution

New study uses digital reconstructions of living and extinct crocodylians to untangle the evolutionary history of long-snouted crocodiles.

Although modern crocodiles, alligators, caimans, and gharials are restricted to the tropics, their fossil record tells a very different story. Ancient crocodylians once inhabited much of the globe and exhibited a remarkable diversity of skull shapes and ecological adaptations.

In a new study, researchers have used high-resolution computed tomography (CT) scans to reconstruct the endocranial anatomy of both living crocodylians and their extinct relatives. These findings provide new insight into the evolution of one of Earth's most enduring vertebrate lineages and help resolve longstanding questions about the relationships of several extinct crocodile groups.

“Despite only being found in the tropics today, we have fossil crocodiles all across the world,” says Paul M.J. Burke, a Postdoctoral Researcher at The Swedish Museum of Natural History. “If you name a country, there will probably be a fossil crocodile waiting to be uncovered.”

Solving the puzzle of long-snouted crocodiles

One of the major challenges in crocodylian evolution is understanding the origins of species with elongated, narrow snouts. This distinctive skull shape has evolved independently multiple times throughout crocodylian history, referred to by paleontologists as convergent evolution. As a result, researchers have often struggled to determine whether certain extinct species belong to one evolutionary lineage or another.

By digitally reconstructing the internal anatomy of fossil skulls, the researchers identified a series of morphological differences in the braincase and associated sensory structures among long-snouted crocodiles from different geological periods. These features provide new characters that can be used to distinguish between superficially similar groups.

Among the most enigmatic of these are the so-called ‘thoracosaurs’, an extinct group of long-snouted crocodyliforms whose evolutionary relationships have remained uncertain for decades.

“By identifying unique features that distinguish these groups of crocodiles, we can be more confident in where these extinct crocodiles are positioned within evolutionary history,” says Paul M. J. Burke.

A remarkably conservative brain

The study also suggests that the brains of crocodylians show little change throughout their evolutionary history. Despite changes in environments and the evolution of different skull forms, many aspects of their neuroanatomy show no drastic change. “Crocodile brains have barely changed over 100 million years of evolution,” says Paul M.J. Burke. “It is as though they found what was successful then and stuck to it.

A number of endocranial features, such as the brain and the inner ears of crocodiles, appear to show a phylogenetic signal, which implies that closely related species are more likely to resemble each other than distantly related species.

Reconstructing crocodilian evolutionary history

The findings demonstrate the growing importance of non-destructive imaging techniques in palaeontology. By combining fossil evidence with modern imaging technologies, researchers can recover anatomical information that would otherwise remain hidden within fossil skulls.

The new data not only provide fresh insights into the evolutionary history of crocodiles and their relatives but also offer a framework for reassessing the placement of enigmatic extinct species and understanding how crocodiles have survived throughout the KPg mass extinction, and over 100 million years of evolution.