Crocodiles might look tough, but new research suggests they're more sensitive to climate change than we thought.

By studying thousands of fossils, a team of palaeontologists has been tracing the impacts of fluctuating sea levels, global temperature changes and evolving landscapes on the ancient crocodile lineage – and their findings offer clues about how modern-day crocs might be affected as the planet continues to warm.

The fossil record is our gateway to understanding the evolution of crocodiles, whose long and complex history stretches back nearly 250 million years. During this time, crocodiles and their ancestors (collectively known as crocodyliforms) diversified into all sorts of shapes and sizes, from 12-metre-long giants to dwarf species no bigger than a cat. On land, members of the Notosuchia group looked like a bizarre cross between a crocodile and an armadillo, while others took to the seas, evolving sleek, dolphin-like forms, complete with flippers!

Sarcosuchus, an extinct crocodyliform from North Africa and South America, reached 12 metres in length. Image: Robert Nicholls/Imperial College London

But fast forward to the present, and only 23 crocodile species still exist today (almost half of them are at high risk of extinction). “While [crocodylians] 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,” says Imperial College London’s Dr Phillip Mannion, who co-led the new study. “This may help us to determine how they will cope with future changes.”

To better understand this connection, the team analysed thousands of fossils from the Paleobiology Database, and compared them with data about the Earth's past climate. 

When modern crocodiles first emerged about 80 million years ago, they marched into a hot "greenhouse world" – and they thrived. But things began to change from about 50 million years ago: the earth cooled at high latitudes and its lower regions became drier. In Africa, for example, the Sahara began to form around 10 million years ago, replacing the lush wetlands in which crocodiles once thrived.

For these ectotherms – animals that require external sources of heat, like the sun’s warmth, to survive – life was becoming increasingly difficult.

Ancient crocodiles once shared the oceans with a range of bizarre marine reptiles. Image: Craig Dyke/Flickr

And the changes didn't just affect crocodiles on land. Just like it does today, climate change millions of years ago had an impact on the oceans, too.

A changing climate alters the sea levels (something we see happening right now), and those ups and downs can affect habitats close to shore. As the sea level rises, it can actually allow these habitats to become more diverse – but when it falls, these same habitats can be destroyed completely. This is what likely sealed the fate of the freely swimming crocodiles that once inhabited the oceans alongside a range of bizarre marine reptiles.

Today, scientists are predicting a global temperature rise of up to four degrees over the next century, and we know that ocean levels are already rising. So what could this mean for crocodiles in the future? Initially, it could mean a boost for their diversity, and it's possible these reptiles could actually eke out new territories outside of the tropics.

"We already have one living crocodile that often goes out to sea – the saltwater crocodile, or ‘salty’," explains Mannion. “It may well be possible that this species ... could one day give rise to a group of marine species.” 

This might sound like great news, but with human activities already destroying crocodile habitats, any future expansion carries a risk, putting more crocs in our paths – and human-wildlife conflict is something that's already taking a toll. These ruling reptiles have never been more threatened.

The next step for the researchers will be to look at similar patterns in other animal groups with long evolutionary histories, including birds and mammals, to examine how climate change helped shape modern ecosystems.

“Detailed study of biodiversity during [past] intervals ... [of] very rapid ... global warming, potentially [like] what we're experiencing today, might provide clues as to how organisms respond on a more similar timescale," says Mannion. 

Note: The author of this article is a co-author of the study.

Top header image: Paul Williams/Flickr