Uncovering the Surprisingly Complex History of Crocodiles

Crocodiles have a deep and varied evolutionary past. Now researchers are peeling back the layers to discover how the surviving species came to be.

There are about 28 living species of crocodilians found in tropical and subtropical regions of the world.

But this is only a small fraction of the number of crocodile species that existed.

Two new papers have investigated this rich evolutionary history, tracing the origin of this group and how it spread around the world, as well as exploring the origin of its typically slow growth. Researchers found that the larger, modern group of crocodilians likely first appeared in Europe 145 million years ago.

Following this, the ancestors of crocodiles and crocodiles separated in North America, with the ability of crocodiles to tolerate salt water meaning they were able to spread much further around the world. This discovery was published in Royal Society Open Science.

“It seems more likely that the ancestors of today’s alligators and crocodiles evolved in North America and later into alligators. [which includes alligators and caimans] they stay more or less in the Americas, while crocodiles go everywhere else”, explains Professor Paul Barrett, a paleontologist at the Natural History Museum who worked on both articles.

“It appears that the ability to traverse bodies of salt water has allowed crocodiles to become much more dispersed than crocodiles: crocodiles are found throughout the world, including in tropical oceans, while crocodiles are confined to freshwater and unable to reach some areas. Subgroups appear to have thrived and originated in different regions.”

Furthermore, Paul and his colleagues discovered that the slow growth rate of crocodiles was a secondary adaptation that was not found in their distant relatives. This research, published in Current Biologyit also shows that crocodiles and birds, which are each other’s closest living relatives, have had completely opposite physiological strategies for more than 220 million years.

Various crocodilians

With their thick, armored skin, large teeth, slow-moving lifestyles and predatory habits, crocodiles are often considered unchanged from when dinosaurs roamed the earth hundreds of millions of years ago. But this ignores its rich and varied evolutionary history.

The largest group that contains living crocodiles, alligators and gharials, called Crocodylomorpha, also includes hundreds of species that are no longer with us. Although the only surviving animals are carnivorous species, largely adapted to freshwater, this has been far from the case for the last 200 million years.

“Some of these animals were large predators that preyed on dinosaurs,” explains Paul. “But others were very small, fleet-footed animals that probably preyed on things like insects. There were even herbivorous crocodilians of various types, with very complicated teeth – almost mammal-like – that could have chewed plants before swallowing.”

This included animals like Simosuchus, which looked a bit like a modern short-snouted armadillo but probably fed on fruits, tubers and ferns. There were also purely marine predatory crocodilians, such as thalattosuchians, which had flippers instead of feet, and some small, lightly built runners, such as Terrestrisuchus, which looked a bit like reptilian whips.

“Crocodiles and their relatives were really experimenting with many different ways of life,” says Paul. “They are doing a surprising number of things. This contrasts greatly with what we know about living crocodiles, which are all predators limited to living in the tropics with semiaquatic or amphibious lifestyles.”

“Living crocodiles are really a pale shadow of the diversity they and their relatives once had.”

This enormous diversity allowed researchers to explore the life histories of different animals. This refers to how an animal lives, grows and behaves.

Although we think of large reptiles as slow-moving, slow-growing animals, this was not always the case. For example, some dinosaurs were huge animals that we might expect to grow slowly throughout their lives. But instead, it is now thought that many of these gigantic animals were growing much faster than might be assumed from their size.

And it turns out that the life history of crocodiles is just as complex.

Secondarily slow

Let’s go back in time to the Triassic Period and the ancient crocodilians alive at that time were far from being heavy, lethargic animals. Fossils of these creatures reveal that they were, in fact, active and fast-growing animals.

So when did crocodiles start taking things slower? It has been suggested that this occurred with movement into the water, that their growth slowed with the adoption of semi-aquatic lurking habits.

“The first question was: is the growth slowdown because of the crocodiles’ aquatic habits or does it predate that?” asks Paul. “And second, at what point in crocodile evolution do they turn off their ancestrally high metabolism and reevolve in what appears to be a reversion to a more primitive condition of slow growth?”

Fossils of an ancient crocodile-like relative, dating back to 220 million years ago, appear to rule out the aquatic lifestyle theory. Despite being the first known crocodilian to have a slow growth rate, this new animal lived long before crocodiles began exploring the water. But the reason why this land animal developed a slower metabolism is still up for debate.

“Maybe it had to do with the resources available at the time – they lived in fairly resource-poor environments,” explains Paul. “But interestingly, they also live directly alongside another major branch of archosaur evolution: dinosaurs.”

“Dinosaurs were doing something completely different, which was growing fast. So although today we have crocodiles and slow-growing birds, this difference has existed between these two large groups since at least the Late Triassic. But the reasons why they adopted these strategies different is still guesswork.”

Paul and his colleagues were also able to trace the origin of Crocodylomorpha. They showed that this group first appeared in what would become modern Europe, before the ancestors of crocodiles and alligators split somewhere in what is now North America. From here, salt-adapted crocodiles were better able to spread around the world and colonize Africa, Asia, and Oceania, while crocodiles and their relatives were typically limited to the Americas.