London: Fluctuating sea levels and global cooling led to a significant decline in the number of crocodilian species over millions of years, says a study.
Crocodilians include present-day species of crocodiles, alligators, caimans, gavials and their extinct ancestors.
They first appeared in the Late Cretaceous period, approximately 85 million years ago, and the 250 million years fossil record of their extinct relatives reveals a diverse evolutionary history.
"Crocodilians are known by some as living fossils because they have been around since the time of the dinosaurs," said one of the study lead authors Philip Mannion from Imperial College London.
Many crocodilians survived the mass extinction that wiped out almost all of the dinosaurs 66 million years ago, but only 23 species survive today, six of which are classified by the International Union for Conservation of Nature as critically endangered and a further four classified as either endangered or vulnerable.
"Millions of years ago these creatures and their now extinct relatives thrived in a range of environments that ranged from the tropics, to northern latitudes and even deep in the ocean. However, all this changed because of changes in the climate, and crocodylians retreated to the warmer parts of the world," Mannion said.
Crocodylians are ectotherms, meaning they rely on external heat sources from the environment such as the Sun.
In this study, researchers compiled a dataset of the entire known fossil record of crocodylians and their extinct relatives and analysed data about Earth's ancient climate.
They wanted to explore how the group responded to past shifts in climate, to better understand how the reptiles may cope in the future.
The researchers concluded that at higher latitudes in areas we now know as Europe and America, declining temperatures had a major impact on crocodylians and their relatives.
At lower latitudes the decline of crocodylians was caused by areas on many continents becoming increasingly arid.
For example, in Africa around ten million years ago, the Sahara desert was forming, replacing the vast lush wetlands in which crocodylians thrived.
In the future, the team suggest that a warming world caused by global climate change may favour crocodylian diversification again, but human activity will continue to have a major impact on their habitats.
The study was published in the journal Nature Communications.