Fast-running vegetarian and ‘dolphin-like’ sea crocodiles were result of rapid evolution, scientists say

While modern crocodiles predominantly live in rivers, lakes and wetlands, ancient crocodile species once flourished across land and oceans, and scientists now believe this was due to rapid evolution.

Crocodiles as we know them have been around for longer than the Himalayas, with a fossil record stretching back about 55 million years, but their extinct relatives go back even further, to around 230 million years.

In the time of the dinosaurs - more than 60 million years ago - crocodiles were much more varied, and the fossil record details how evolution “experimented” with dolphin-like adaptations to living in seas, while others lived on land as fast-moving plant eaters.

Researchers led by scientists at the University of Bristol examined over 200 skulls and jaws from the known fossil record of crocodiles and their extinct relatives, exploring how shape variation reveals differences between species, and how quickly the crocodiles changed over time.

The researchers said the study revealed how some of the extinct crocodile groups, including the dolphin-like thalattosuchians and the smaller land-dwelling notosuchians, evolved very fast over many millions of years.

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During these adaptations, great changes occurred to crocodiles’ skulls and jaws as they expanded into niches today occupied by other animal groups, particularly mammals.

The research also revealed that crocodiles, alligators and gharials, the only remaining crocodilians, are much more conservative versions of many of the animals found in these extinct fossil groups, and evolved steadily for the last 80 million years.

They also found no evidence for a slow-down in their evolution and said they are not “living fossils”, as once thought.

A living fossil is an organism that has retained the same cosmetic form over millions of years, resembles species only known from the fossil record, and has few or no living relatives.

Lead author Dr Tom Stubbs, a senior research associate at the University of Bristol’s School of Earth Sciences, said: “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.”

Dr Armin Elsler, a research associate in the Bristol School of Earth Sciences and co-author, said: “New state-of-the-art methods now mean we can test for differences in the speed of evolution through time and across groups.”

It has long been suggested that dramatic shifts in habitat and diet can trigger rapid evolution, but these patterns are usually only reported in groups with great variety today, like birds, mammals and fish. This is the first time this trend has been shown in crocodiles, a group with a rich fossil history, but low modern diversity.

Dr Stephanie Pierce, associate professor of organismic and evolution biology at Harvard University, said: “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.”

Professor Michael Benton from Bristol said: “It’s not clear why modern crocodiles are so limited in their adaptations. 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.”

The research is published in the journal Proceedings of the Royal Society B.