Modern crocodiles are predators living in rivers, lakes and wetlands – grabbing fish, reptiles, birds and mammals with their long snouts and powerful jaws.
However ancient crocodiles were once much more varied because of rapid evolution, according to a new study.
Researchers from the University of Bristol found that in the time of the dinosaurs, some crocodiles experimented with dolphin-like adaptations to living in the oceans, and others lived on land as fast-moving plant-eaters.
They studied more than 200 skulls and jaws, including fossils from the entire 230 million year history of crocodiles and their extinct relatives.
The experts explored shape variation to reveal differences between species and analysed how fast crocodile groups changed through time.
They found that some extinct crocodile groups, including dolphin-like thalattosuchians and small land-dwelling notosuchians, evolved very fast over many millions of years.
The creatures also underwent great changes to their skulls and jaws as they expanded into niches today occupied by other animal groups, particularly mammals.
According to the study published in Proceedings of the Royal Society B, the research shows that crocodiles, alligators and gharials, the only living crocodilians, are more conservative than these extinct fossil groups, and evolved steadily for the last 80 million years.
However, there is no evidence for a slow-down in their evolution and they are not “living fossils”, as once thought.
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.”
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.
Researchers say 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 added: “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.”