Modern Crocodiles Are Evolving at a Rapid Rate

Crocodiles look like they belong to another time, an era when reptiles ruled. But appearances can be deceiving. Today’s crocodiles are not holdovers that have gone unchanged since the Jurassic, but are one expression of a great, varied family that’s been around for over 235 million years. More than that, crocodiles are still evolving—and faster than they have at other times in their family’s scaly history.

The seemingly contradictory conclusion about crocodylian evolution comes from a recent study published in Proceedings of the Royal Society B by University College London anatomist Ryan Felice and colleagues. By comparing three-dimensional models to track anatomical landmarks on crocodylian skulls over time, the researchers found that modern crocodile species in Australia, southeast Asia and the Indo-Pacific are evolving quickly despite looking like semi-aquatic antiques. Modern croc species look so similar not because of conserving ancient traits, but because crocodiles are evolving the same skull shapes over and over again through time.

“Crocodylians just seem ancient and primordial and look a bit like Hollywood dinosaurs,” Felice says, not to mention that some species are among the largest reptiles on the planet. American alligators can grow to be 14 feet long, and saltwater crocodiles can get to be over 20 feet in length. On top of that, Felice notes, some fossil crocodiles superficially resemble living species. The spitting reptilian image can create a narrative that crocodiles haven’t changed much at all, content to lurk in the evolutionary backwaters. But that’s not the real story.

Today’s alligators, crocodiles and gharials—grouped together as crocodylians—shared a common ancestor back in the Cretaceous, around 95 million years ago. But crocodile history goes much deeper. Today’s alligators and crocodiles are just the remaining members of a broader, older group of reptiles called pseudosuchians—or crocodiles and their extinct relatives. The first pseudosuchians evolved over 235 million years ago, around the time that the first dinosaurs were making their mark on the Triassic landscape, and have gone through a vast array of changes through time.

Many fossil pseudosuchians were unlike any creatures alive today. Triassic animals called aetosaurs, for example, are sometimes called “armadillodiles” for the way these crocodile relatives resembled large, omnivorous armadillos. Around the same time there lived bipedal crocodile relatives like Poposaurus and Effigia, pseudosuchians that evolved dinosaur-like body plans and habits independently. In fact, during the Triassic there was a greater diversity of pseudosuchian species and body plans than dinosaurs. They, not the “terrible lizards,” were the dominant reptiles on land. That is, until massive volcanic eruptions triggered a mass extinction about 201 million years ago and many forms of pseudosuchians died out, allowing the dinosaurs to step out of the shadows.

But even in Jurassic and Cretaceous heyday of the dinosaurs, surviving lineages of pseudosuchians thrived. Some crocodile relatives spent their whole lives at sea. Sharp-toothed predators like Dakosaurus and Thalattosuchus hunted the oceans for prey alongside ichthyosaurs and plesiosaurs. Others were terrestrial carnivores that were stiff competition for dinosaurs. In prehistoric Brazil around 90 million years ago, land-dwelling crocodiles like Baurusuchus were among the apex predators of their time and bit into prey with blade-like teeth. And semi-aquatic ambush predators existed, too. Some of them were about the size of their modern counterparts, but a few were giants. Deinosuchus, the “terror crocodile,” was an alligator that could grow to be almost 40 feet long and lurked in North America’s swamps between 82 and 73 million years ago.

Even after the asteroid strike that ended the “Age of Dinosaurs,” some species of crocodile survived and continued to evolve in new ways. While mammals were going through a major evolutionary flowering, new terrestrial crocodiles—such as the “hoofed” crocodile Boverisuchus —evolved to chase after them. And even in recent times, until about 4,000 years ago, there lived terrestrial crocodiles in the South Pacific called mekosuchines that had teeth and jaws suited to crunching mollusks and insects. Crocodiles and their relatives haven’t been evolutionary slouches. Instead, these reptiles have rapidly responded to changing times.

To track how crocodile skull shapes changed through the ages, Felice and colleagues used a technique called geometric morphometrics to compare the skulls of 24 living crocodylian species and 19 of their fossil relatives. These three-dimensional models allowed researchers to look at different landmarks on the skulls—such as the placement of the eyes or the length of the snout— and track how anatomy has shifted over time and through evolutionary relationships.

The analysis did more than track shape. Crocodile snout shapes are heavily-influenced by what those species eat. For example, very long, narrow snouts filled with many small teeth are considered clues to a diet heavy in fish. Living gharials have this shape, but so do some extinct marine crocodiles. Felice and colleagues hypothesize that this fish-eating snout shape has evolved at least three times in the history of crocodiles.

Two crocodiles with similar skull shapes, then, might not be close relatives. Instead, distantly-related crocodiles are converging on the same skull shapes because they’re feeding on similar prey and living in similar habitats, with an array of species repeating a small number of skull shapes. The fact that distantly-related branches on the crocodile family tree are converging on similar skull shapes, University of Tennessee paleontologist Stephanie Drumheller-Horton says, suggests that crocodiles are evolving rapidly to repeatedly fill the same set of niches.

And new studies are beginning to track just how quickly crocodiles are changing, especially modern species. Earlier this year Harvard University paleontologist Stephanie Pierce and colleagues reported that some modern crocodylian lineages are evolving rapidly. The study by Felice and coauthors adds to this picture. Crocodile species living through the landmasses of the southern Pacific—like the famous saltwater crocodile—are showing a high rate of evolutionary change over the past two million years. Over and over again, Felice and colleagues found, modern crocodiles are converging on a small set of skull shapes. Africa’s Nile crocodile and Morelet’s crocodile of Central America are not especially close relatives, for example, but they’ve evolved remarkably similar skull shapes. Likewise, today’s broad-snouted caiman of South America has a very similar skull to the extinct crocodile Voay whose remains are found in Madagascar. This is why they seem ancient. It’s not that they’ve gone unchanged, but that crocodiles are evolving into a limited number of forms over vast spans of time. Look at a saltwater crocodile and you’re seeing the revival of one of evolution’s greatest hits.

Recognizing the pattern is just a first step towards answering some deeper evolutionary questions. “With new and more sophisticated analytical techniques,” Pierce says, “we can start to better understand how and why they converged on similar morphologies.” The new study helps lay the groundwork for experts to investigate how particular diets influence the evolution of distinct skull shapes in both modern and fossil crocs. In addition, Pierce notes, a skull can come together during embryonic development in only so many ways. The number of anatomical forms crocodiles can take are influenced by what internal factors, such as growth, allow as well as outside influences like diet.

Digging in deeper to why crocodiles tend to repeat similar forms over time might help explain why today’s alligators, crocodiles and gharials seem so similar to each other. They are all semi-aquatic carnivores that hunt by ambush. There are no more terrestrial crocs, seagoing crocs, crocodiles with a varied toolkit of different-shaped teeth, or species like many of those that used to exist. Felice and colleagues estimate that of all the crocodile skull shapes that have existed, today’s species only represent about a third of that variety. But given the longstanding survival of these reptiles, could crocodiles someday undergo another evolutionary burst and evolve some of these lost forms again?

“It’s possible, but highly unlikely,” Felice says. There might be some characteristics inherent to modern crocodylians that prevent them from reclaiming some of their family’s past diversity. Living crocodylians generally have low, squat bodies with limbs that need to hold them up on land as well as push them through water. The need for an amphibious body makes them less efficient on land than mammalian carnivores and not quite as maneuverable as entirely-aquatic hunters, and so competition from other meat-eaters may keep them restricted. The global climate likely has a factor to play, as well. During warmer global climates, Drumheller-Horton says, crocodylian species spread and diversify. “But the crocodylians living today are the survivors of the last cycles of ice ages, so we’re looking at a fairly restricted sampling of the total diversity they once had,” she says. On top of that, many modern crocodylian species are facing habitat loss and other extinction threats created by humans. If the armored reptiles are to someday thrive again, they’ll need a major ecological shakeup to open a path to a new Age of Reptiles.

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.