Could global warming lead to bigger, badder crocodiles?

Since they arrived on the scene nearly 85 million years ago, crocodiles and their relatives have often suffered when things got cool or sea levels fluctuated. The changing climatic conditions cause scores of species to go extinct.

So with the world warming up, could this be a renaissance for crocodiles, reptiles that date back to the time of the dinosaurs?

That is one of the questions Philip Mannion, from the Imperial College London, and his colleagues seek to answer in a new Nature Communications paper. The study looks at the rise and fall of these living fossils that begins with the emergence of their now extinct relatives pseudosuchians around 250 million years ago, during the Triassic. That gave way to crocodylians around 85 million years ago, during the Late Cretaceous period.

Both groups were incredibly diverse - there were as many as 500 pseudosuchian species alone - and they came in all shapes and sizes.

Among the crocodylians, there were giant land-based creatures such as Sarcosuchus, which reached around 12 meters (39 feet) in length and weighed up to eight metric tons. Crocodylians also roamed the ocean, including the thalattosuchians, which were equipped with flippers and shark-like tails to make them more agile in the sea.

Most crocodylians managed to survive the extinction that killed off dinosaurs 66 million years ago and some species even thrived soon after, expanding their ranges amid a dearth of competitors.

But over time, the numbers of crocodylian species has dropped to 23, including 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," Mannion said. "However, all this changed because of changes in the climate, and crocodylians retreated to the warmer parts of the world. While they have a fearsome reputation, these creatures are vulnerable and looking back in time we've been able to determine what environmental factors had the greatest impact on them."

A big contributor to the decline of this sun-loving family was cooler conditions, especially at higher latitudes in areas of modern day Europe and America, where declining temperatures had a major impact on crocodylians and their relatives.

They also suffered at lower latitudes due to increasingly arid conditions in places like Africa around 10 million years ago, where 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.

Fluctuation in sea levels also hit marine species of crocodylians, which were once widespread across the oceans. They appeared to do best when sea levels were high because it increased the size of the continental shelf, providing the right conditions near the coast for them and their prey to thrive.

The researchers suggested a warming world could lead to crocodiles, alligators and their relatives expanding their ranges - much as they did in the past during the Paleogene that lasted from 66 million years ago to about 23 million ago - and even leading to increased species diversification.

But the researchers cautioned that much of that would depend on what impact humans have on their habitats, by way of coastal development, pollution and hunting.

"As the Earth continues to warm, perhaps heading towards a greenhouse world comparable to that of the early Paleogene, we might therefore expect that higher temperatures should promote long-term increases in crocodylian biodiversity and the expansion of the group's latitudinal range outside of the tropics, as was the case for much of their Mesozoic and early Cenozoic history," the researchers wrote.

"However, in contrast to these earlier times, predictions of the distribution of their future biodiversity are complicated by the impact of human activity on habitat loss and fragmentation, which are likely to reduce the rate and magnitude of crocodylian range expansion, especially into populated regions," they wrote.