An ancient hippo-sized reptile may have been surprisingly agile

Some 260 million years ago, before the rise of dinosaurs, bone-crushing anteosaurs reigned as land’s largest predators. A new analysis of an anteosaur skull suggests that these hefty reptiles may have been relatively speedy.

“This contradicts what we knew about anteosaurs before,” says Ashley Kruger, a paleontologist at the Swedish Museum of Natural History in Stockholm. Based on the reptiles’ size, which was around that of today’s hippos or rhinos, researchers had pegged the Permian Period predators as sluggish beasts that waited to ambush prey. The skull of an Anteosaurus magnificus appears to tell a different story.

Relying on CT scans of fossil skull segments excavated in South Africa, Kruger and his team digitally reconstructed the long, bumpy noggin of a juvenile A. magnificus. They found that the animal’s inner ears — bony tubes that help with balance — dwarfed those of its peer predators. The shape of these bones also suggests that anteosaurs may have benefited from a rather large brain region used to coordinate motion while surveilling prey, the researchers report February 18 in Acta Palaeontologica Polonica.

The team compared A. magnificus’ skull with that of its head-butting, herbivorous relation Moschognathus whaitsi. While M. whaitsi’s skull slopes downward, A. magnificus appears to have held its head more level, allowing it to more easily scan the environment. All of these findings suggest that Anteosaurus was an agile hunter, Kruger says, with the ability to move quickly and track its prey.

These are reasonable conclusions, but “it’s not the smoking gun” that anteosaurs were fleet-footed, says Z. Jack Tseng, a paleontologist at the University of California, Berkeley who was not involved with the work. The study draws on analyses of the inner ears of modern mammals, distant relatives of the group of reptiles that includes anteosaurs. But even in today’s beasts, scientists don’t know exactly how inner ears influence different types of motion. Additional information from the rest of the skeleton would help us better understand how anteosaurs may have moved, he says.

Much of what’s known about Anteosaurus beyond the skull comes from its close relatives, says Christian Kammerer, a paleontologist at the North Carolina Museum of Natural Sciences in Raleigh who was not part of the study. Anteosaurus probably had leaner limbs than related herbivores so it seems that this animal could have been capable of running bursts, he says.

“Whether it was an ambush or pursuit predator is a very difficult thing to address, and perhaps unknowable,” given that animals at that time were quite different from modern ones. The swift herbivores of the Serengeti today would outpace Anteosaurus, Kammerer says, but perhaps the chase was on in a world where big plant-munchers moved like tortoises.