Diverse crocodiles underwent rapid evolution

Ancient crocodile species were far more diverse than their modern counterparts, occupying niches held today by animals that range from dolphins to small, herbivorous land mammals. This extraordinary diversity was made possible by particularly rapid evolution, according to a new study from the University of Bristol, UK.

The research, published in the journal Proceedings of the Royal Society B, shows that ancient crocodile groups evolved very fast over millions of years, undergoing significant changes to their skulls and jaws as they moved to occupy new ecological niches. It’s a far cry from the narrow diversity in modern crocodile species.

Lead author Dr Tom Stubbs, a senior research associate at Bristol’s School of Earth Sciences, says, “crocodiles and their ancestors are an incredible group for understanding the rise and fall of biodiversity.

“There are only 26 crocodile species around today, most of which look very similar. However, there are hundreds of fossil species with spectacular variation, particularly in their feeding apparatus.”

The team studied over 200 skulls and jaws spanning the 230-million-year evolutionary history of crocodilian species. They examined variations in shape to find clues about what the animals ate and, by extension, where they might have lived.

The skulls included extinct species like the dolphin-like thalattosuchians, and the small land-dwelling notosuchians, species that evolved drastically and rapidly. By contrast, today’s crocodilian species – the crocodiles, alligators, and gharials – are more evolutionarily conservative, though they have been evolving continuously since their emergence and are not “living fossils” as commonly thought.

It’s thought that dramatic shifts in habitat and diet can trigger rapid evolution, but this mechanism is usually only studied in groups that have significant diversity today, such as birds and mammals. This study marks the first time the trend has been observed and investigated in crocodiles.

Dr Stephanie Pierce, associate professor of organismic and evolution biology at Harvard University, says: “Ancient crocodiles came in a dizzying array of forms. They were adapted to running on land, swimming in the water, snapping fish, and even chewing plants. 

“Our study shows that these very different ways of living evolved incredibly fast, allowing extinct crocodiles to rapidly thrive and dominate novel ecological niches over many millions of years.”

“It’s not clear why modern crocodiles are so limited in their adaptations,” says Michael Benton, a professor at Bristol. “If we only had the living species, we might argue they are limited in their modes of life by being cold-blooded or because of their anatomy. 

“However, the fossil record shows their amazing capabilities, including large numbers of species in the oceans and on land. Perhaps they only did well when world climates were warmer than today.”

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