Evrim Yazgin
Cosmos science journalist
Crocodiles are often thought of as living fossils – unchanged over millions of years. New research has shown that their evolutionary history is a lot more complicated than that.
Crocodilia is the surviving family of a lineage which emerged about 230 million years ago (mya) called crocodylomorphs. This group split from other reptilian species including those that eventually became dinosaurs. Today, the crocodilia include crocodiles, alligators, caiman and gharials.
Ancestors of modern crocodilians survived through 2 mass extinctions, including the one which spelled the end of the “Age of Dinosaurs” 66 mya.
The new study, published in the journal Palaeontology, shows that the secret to success of crocodylomorphs was their adaptability to new food sources and habitats.
“Lots of groups closely related to crocodilians were more diverse, more abundant, and exhibited different ecologies, yet they all disappeared except these few generalist crocodilians alive today,” says lead author Keegan Melstrom from the University of Central Oklahoma.
Today’s crocodilians are semi-aquatic generalists. The thrive in different habitats and aren’t picky eaters.
It was a different story with ancient crocodylomorphs.
The palaeontologists visited museum collections in 7 countries, across 4 continents to understand the evolution of crocodilian ancestors. They examined the skulls of 99 extinct crocodylomorph species and 20 living crocodilians.
Crocodylomorphs exploded after the end-Triassic mass extinction 201 mya which killed off ancient lineages of hypercarnivores and land-based predators.
“After that, it goes bananas,” says Melstrom. “Aquatic hypercarnivores, terrestrial generalists, terrestrial hypercarnivores, terrestrial herbivores – crocodylomorphs evolved a massive number of ecological roles throughout the time of the dinosaurs.”
Toward the end of the time of the dinosaurs, however, crocodylomorphs started to decline.
Most of the specialised crocodylomorphs had died off by the end of the Cretaceous. Almost all 26 remaining species today are semi-aquatic generalists.
“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,” says co-author Randy Irmis from the Natural History Museum of Utah. “Crocodilians are equipped to survive many future changes – if we’re willing to help preserve their habitats.”
“Extinction and survivorship are 2 sides of the same coin,” Melstrom says. “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?”
The authors say that understanding the persistence of birds, mammals and crocodilians after the last mass extinction 66 mya could aid in efforts to conserve biodiversity as we are going through a 6th mass extinction due to climate change, invasive species and habitat loss.
But they are also cautious about making generalisations based on extinctions millions of years in the past.
“There’s a danger of trying to draw conclusions from millions of years ago and directly apply it to conservation,” says Irmis. “We have to be cautious.
“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.”
