How crocodiles survived Earth's biggest mass extinctions

Most people imagine crocodiles as relics from the dinosaur era – unchanged, prehistoric predators lurking in murky waters. But their survival story is much more complex.

New research from the University of Central Oklahoma and the University of Utah shows that these animals aren’t just fossils with teeth. They’ve made it through two mass extinctions, thanks in part to one big advantage: adaptability.

Why crocodiles survived

Crocodylians – today’s crocodiles, alligators, and gharials – are the last surviving members of a larger group called crocodylomorphs, which goes back 230 million years.

These ancient creatures included many now-extinct species that once lived in a range of environments.

The research is the first to closely examine the diets of these extinct animals to understand how some survived while others did not.

“Lots of groups closely related to crocodylians were more diverse, more abundant, and exhibited different ecologies, yet they all disappeared except these few generalist crocodylians alive today,” said Keegan Melstrom, lead author and assistant professor at UCO.

“Extinction and survivorship are two sides of the same coin. Through all mass extinctions, some groups manage to persist and diversify. What can we learn by studying the deeper evolutionary patterns imparted by these events?”

Today, many scientists believe Earth is entering its sixth mass extinction, fueled by habitat loss, invasive species, and climate change. Studying past survivors, like crocodylians, may offer clues on how to protect today’s wildlife.

Crocodylians vs. mammals

For years, mammals have been the go-to group for studying extinction and survival. Their flexible diets and ability to live in many habitats seemed to explain their resilience. But crocodylomorphs have long been overlooked in this conversation. This crocodylian study changes that.

“There’s a danger of trying to draw conclusions from millions of years ago and directly apply it to conservation. We have to be cautious,” said co-author Randy Irmis.

“If people study mammals and reptiles and find the same patterns with respect to extinction survival, then we might predict that species with a generalist diet may do better. That information helps us make predictions, but it’s unlikely we’ll ever be able to pick out which individual species will survive.”

Shrinking diversity of crocodile ancestors

Modern crocodylians are known for living in water and eating just about anything that crosses their path – fish, small mammals, even fellow crocodiles. But this lifestyle is only a narrow snapshot of their past.

In the Late Triassic Period, crocodylomorph ancestors lived mostly on land. They were small-to-medium-sized predators feeding on small animals. Other reptile groups were more dominant at the time, showing off a wide range of sizes and shapes.

But when the end-Triassic extinction struck, these dominant groups vanished. Only crocodylomorphs, especially those with flexible diets, pulled through.

“After that, it goes bananas,” said Melstrom. “Aquatic hypercarnivores, terrestrial generalists, terrestrial hypercarnivores, terrestrial herbivores – crocodylomorphs evolved a massive number of ecological roles throughout the time of the dinosaurs.”

For a while, crocodylomorphs thrived in every corner of their ecosystems. But during the Late Cretaceous Period, their diversity began to shrink. By the time a meteor ended the age of dinosaurs 66 million years ago, only a few crocodylian lineages survived – most of them semi-aquatic generalists.

The evolution of crocodylians

To understand what these animals ate, the researchers studied the shape of fossilized teeth and skulls.

Sharp, pointed teeth suggested a meat-heavy diet, while flatter, grinding teeth pointed to a plant-based menu. Skull structure also offered clues about how the animals hunted and fed.

The crocodylian study was massive in scope. The researchers traveled to museum collections in seven countries across four continents.

The experts examined the skulls of 99 extinct crocodylomorph species and 20 living crocodylian species. To compare, they also analyzed dietary traits in 89 mammals and 47 lizards. This fossil record, stretching over 230 million years, revealed how crocodylians evolved into a variety of forms.

Some were fast-moving land predators. Others grazed like herbivores. Many returned to the water. Today, that wide-ranging history is mostly hidden behind the toothy grin of a croc.

Will crocodiles survive future changes?

While modern crocodylians continue to thrive in rivers and swamps, many face serious threats. The gharial of the Himalayan foothills and the Cuban crocodile in the Zapata Swamp are critically endangered. Their survival may still hinge on one of their oldest traits – a flexible diet.

“When we see living crocodiles and alligators, rather than thinking of ferocious beasts or expensive handbags, I hope people appreciate their amazing 200+ million years of evolution, and how they’ve survived so many tumultuous events in Earth history,” said Irmis.

“Crocodylians are equipped to survive many future changes – if we’re willing to help preserve their habitats.”

The full crocodylian study was published in the journal Palaeontology.

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