Climate cooling caused decline in crocodiles

Fluctuating sea levels and global cooling caused a significant decline in the number of crocodylian species over millions of years, a new study has found.
Crocodylians include present-day species of crocodiles, alligators, caimans and gavials and their extinct ancestors.
They first appeared in the Late Cretaceous period, approximately 85 million years ago, and the 250 million year fossil record of their extinct relatives shows a diverse evolutionary history.
Extinct crocodylians and their relatives came in all shapes and sizes, including giant land-based creatures such as Sarcosuchus, which reached around 12 metres in length and weighed up to eight metric tonnes.

Crocodylians also roamed the ocean - for example, thalattosuchians were equipped with flippers and shark-like tails to make them more agile in the sea.
Researchers from Imperial College London, the University of Oxford, the Smithsonian Institution and the University of Birmingham compiled a dataset of the entire known fossil record of crocodylians and their extinct relatives and analysed data about Earth's ancient climate.

Crocodylians are ectotherms, meaning they rely on external heat sources from the environment such as the Sun.
The researchers conclude 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.
In Africa around ten million years ago, the Sahara desert was forming, replacing the vast lush wetlands in which crocodylians thrived.
In South America, the rise of the Andes Mountains led to the loss of a proto-Amazonian mega wetland habitat that crocodylians lived in around five million years ago.
The team found that fluctuations in sea levels exerted the main control over the diversity of marine species of crocodylians.
At times when the sea level was higher it created greater diversity because it increased the size of the continental shelf, providing the right conditions near the coast for them and their prey to thrive.

The Cretaceous-Paleogene mass extinction event, which wiped out many other creatures on Earth nearly 66 million years ago including nearly all of the dinosaurs, had positive outcomes for the crocodylians and their extinct relatives.
The team found that while several groups did go extinct, the surviving groups rapidly radiated out of their usual habitats to take advantage of territories that were now uninhabited.
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.