Tens of millions of years ago, an apex predator resembling a giant crocodile stalked the humid freshwater floodplains of southern Patagonia. Measuring up to 11.5 feet (3.5 meters) long and weighing about 550 pounds (250 kilograms), it ate whatever it could catch. Including some dinosaurs.
Near the southern tip of South America, in Argentina, scientists recently discovered a skeleton — including the skull and jaws — of this hypercarnivore, an animal whose diet is at least 70% meat. They named it Kostensuchus atrox and described the reptile as a new species of peirosaurid crocodyliform, an ancient relative of modern alligators and crocodiles. This is the first crocodyliform found in Argentina’s Chorrillo Formation, which dates to about 70 million years ago, toward the end of the Cretaceous period (145 million to 66 million years ago); it is the most complete fossil ever found for this group.
The second-biggest extinct predator found in the region to date, K. atrox is also one of the largest of this type of crocodyliform, researchers reported on August 27 in the journal PLOS One.
“It is equivalent to a lion among felids,” the study’s lead author, Dr. Fernando Novas, a paleontologist with the Félix de Azara Natural History Foundation at Maimónides University in Buenos Aires, said in an email.
Other fossils previously found in the Chorrillo Formation indicate that K. atrox lived in a diverse habitat, home to dinosaurs, other reptiles, amphibians and even a Cretaceous ancestor of the modern platypus. With its giant head, powerful jaws and large teeth, K. atrox could have easily hunted plant-eating dinosaurs and defended its kills against carnivorous theropod dinosaurs, making crocodyliforms an important part of terrestrial ecosystems, according to Novas.
Discovering K. atrox so far to the south greatly expands the known range of this ancient and fearsome reptile group, suggesting that peirosaurid crocodyliforms were “not only restricted to warm and dry regions in Brazil and northern Patagonia, but also to temperate environments,” Novas said.
‘Puncturing and slicing’
Novas was among the team of paleontologists who found the K. atrox skeleton in March 2020 while excavating the fossil of a large plant-eating dinosaur, and they were immediately struck by its superb preservation. The skeleton was nearly complete, missing only the tail and some limb bones, and the skull was in such good condition that its teeth still bore a coating of enamel, Novas said.
The teeth were pointed with serrated edges that would have been used for “puncturing and slicing through the flesh of sizable prey,” the study authors reported. Though the animal’s posture was more upright than that of modern crocs, the position of the forelimbs suggest it may have been semiaquatic.
“It is a really interesting animal, partially in that it looks so similar to many other croc relatives,” said paleontologist Dr. Keegan Melstrom, an assistant professor at the University of Central Oklahoma who was not involved in the new research. Such similarities are examples of convergent evolution, when species that are not closely related evolve to have similar traits.
“This may seem counterintuitive, but crocs from this time often look different and have different ecologies from modern crocs,” Melstrom told CNN in an email.
“What this fossil shows is different groups of crocodile relatives repeatedly and independently evolving a modern croc-like look and lifestyle,” Melstrom said. “They (crocodyliforms) keep going from terrestrial carnivores to semiaquatic predators and evolving similar shapes while doing it. The authors do a lovely job of telling this story.”
Still, K. atrox differs from modern crocodilians in some important ways. Its nostrils were forward-facing, and its eyes were positioned on the sides of the skull. By comparison, the eyes and nostrils in modern crocodiles sit higher on the skull, enabling them to lie in wait for prey while keeping most of the body underwater.
“The anatomical evidence indicates that Kostensuchus searched for prey on land, where medium-sized herbivore dinosaurs were present,” Novas said.
Being a massive meat eater helped K. atrox dominate its floodplain habitat, but it also may have been a factor in what doomed these ancient crocs. While K. atrox and other large crocodyliforms did not survive the mass extinction at the end of the Cretaceous, other croc groups did, perhaps in part due to their smaller size and more varied diets, said Melstrom, who studies how crocodilians’ eating habits shaped their evolution.
“I hypothesized earlier this year that being a hypercarnivore made a group more likely to die off in a mass extinction than a smaller species,” he said, “and that appears to happen here.”
Next, to learn more about the animal’s diet, and where it lived and hunted, scientists want to recover isotope information from the fossil’s teeth. Imaging the internal structure of its bone tissue could reveal more about its growth rate and age. And abnormalities in the vertebrae also “need to be investigated with participation of veterinary pathologists,” Novas said, adding that the “skeleton of Kostensuchus may reveal more secrets in the years to come!”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine. She is the author of “Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind-Control” (Hopkins Press).
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