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Stupendemys geographicus lived in South American wetlands between 5 million and 14 million years ago.

Jaime Chirinos

Battle-scarred fossils suggest giant turtles fought each other—and crocodiles three times their size

In the swamps and rivers of northern South America 12 million years ago, some of the largest turtles that ever lived fought epic battles for mates and territory. New fossils unearthed in Colombia and Venezuela reveal that the 1100-kilogram males of the species, Stupendemys geographicus, bore unusual horns on the front of their 2.4-meter-long shells, which they likely used to fight each other and fend off crocodiles more than three times their size. At the same time, a detailed look at the turtles’ gigantic jaws suggest they might not have been the ferocious predators some scientists presumed, but instead ate hard-shelled mollusks and large fruits.

This is the first time such horned remains have been found, even though the species was first described in 1976. At the time, scientists had uncovered only fragments of S. geographicus shells.

But over the past 6 years, paleontologist Edwin Cadena of Del Rosario University and his colleagues uncovered several complete S. geographicus fossils in northern Venezuela and in Colombia’s Tatacoa desert, which for the first time included fragments of a jaw. Those fragments closely matched jaw pieces from fossils in Brazil and Peru that had been assigned to other species.

The jaws are a close enough match that all the remains should be classified as S. geographicus, Cadena and his colleagues propose in a paper published today in Science Advances. That means this species, which weighed nearly as much as a hippopotomus, once ranged across a large swath of territory, from northwest Brazil, through Peru, Colombia, and all the way to the coast of Venezuela. At the time, the region was a vast series of rivers and wetlands called the Pebas system.

Despite their similarities to the older fossils, several of the largest new shells have one unique feature: They carry prominent horns on their fronts. The horns, one of which has a long scar on its side “are really striking and bizarre,” says Walter Joyce, a paleontologist at the University of Fribourg, who was not involved in the research.

The researchers conclude that the horns belonged to S. geographicus males, which may have used them to fight each other. Joyce agrees they were likely a male trait. That idea is backed by evidence from modern tortoises, the authors note. Some males fight over mates, aiming to turn the opponent onto its back. Because S. geographicus was largely aquatic, it’s likely they fought differently, Joyce says. But little is known about the behavior of S. geographicus’s closest living relative, a “bizarre” turtle living in the middle of the Amazonian jungle. “There’s no Jane Goodall of turtles,” Joyce laments.

The newly unearthed jaw pieces also hint that S. geographicus may not have been the snapping turtle–like predator that earlier studies suggested. The new fossils have a wide surface on the roof of the mouth for grinding, suggesting the animal might have used its powerful jaws to crunch mollusk shells. The turtle’s wide gape—nearly 30 centimeters across—may have also let it swallow large palm fruits and spread their seeds, as some turtles do today.

Such a varied diet might have helped the animal grow as large as it did, Cadena says. That size was probably a useful defense against the 10-meter crocodilelike creatures also living in the Pebas system, he adds. Several of the fossil turtle shells have bite marks, and one has direct evidence of a near miss: a crocodilelike tooth still embedded in the shell.