Giant Extinct Turtle With Nearly 3-Meter Horned Shell Roamed South America 8 Million Years Ago

The male on the right is seen with the horn-like shells on the carapace, possibly to protect its head when fighting other males, while the female on the left has none. (c) Jaime Chirinos

New fossil specimens of the largest land turtle to have ever lived are giving up more of its secrets, revealing brand new insights into these giants that roamed Earth 8 million years ago.

The new specimens found in northern Venezuela and, for the first time, Colombia expands its range across South America, suggests a form of sexual dimorphism not seen before in any side-necked turtle, and include the largest carapace (upper shell) discovered yet.

First described in the 1970s, Stupendemys geographicus was one of the many behemoths found in tropical South America 5-10 million years ago. The region would have been warm, humid, and swampy; perfect for a range of megafauna that included giant crocodiles, alligators, caimans, and gavialis. Though it was likely preyed on by these, its own size is not to be sniffed at.

Reporting in Science Advances, researchers have analyzed several new shells – as well as the first-ever lower jaw – including the largest S. geographicus carapace ever found at 2.4 meters long (7.8 feet). Based on this, the researchers think the hefty turtle weighed around 1,145 kilograms (2,520 pounds), 100 times that of its closest living relative, the big-headed Amazon river turtle. This makes it the largest land turtle we know of, and rivals the famous Late Cretacious marine turtle Archelon in size.

Stupendemys is for sure the largest recorded side-necked turtle, one of the two kinds of turtles there are,” Professor Marcelo Sánchez, director of the Paleontological Institute and Museum of UZH and head of the study, told IFLScience. “In the other group, the Cryptodira, Archelon is the biggest, a marine turtle. Archelon is the largest turtle there has been based on available specimens, but Stupendemys was very close.”

Venezuelan Palaeontologist Rodolfo Sánchez and a male carapace of the giant turtle Stupendemys geographicus from Urumaco, Venezuela, found in 8 million-year-old deposits. (C) Edwin Cadena

The first discovery of Stupendemys at the La Venta archaeological site in the Tatacoa Desert of Colombia, and confirmation that specimens previously thought to be another species found in Brazil are in fact Stupendemys, hugely expands the turtle's range to include the whole northern part of South America. The researchers also think this shows that S. geographicus was probably the only giant turtle roaming the northern Neotropics during the Late Miocene.

Perhaps, most excitingly of all, the researchers discovered curious differences in some of the shells. Some of them had horn-like shells at the front of the carapace. The team thinks this indicates sexual dimorphism in the species, rather than more than one species; the males had horns, possibly to protect their massive heads when engaged in combat with other males, while the females had none. It’s the first time horns have been found as a form of sexual dimorphism in any side-necked turtles.

“This kind of dimorphism is indeed peculiar in that horns in a carapace are very rare,” Sánchez told IFLScience. “One could then question if indeed we have two species instead of two sexes, but everything speaks for two sexes.“

This tropical region of South America is famous for its extinct megafauna. What was once warm, extensive wetlands and freshwater lakes provided a habitat that supported predators and prey. The researchers think this ecosystem would have allowed the turtle to get so big, but it may have also been influenced by the bigger, toothier creatures that also inhabited the area. They found bite marks on some of the carapaces, and in one case, even a tooth, that they think came from one of the giant crocodilians of the time – potentially a Purussaurus, the largest caiman at 13 meters (43 feet).

We wouldn't want to meet anything that could take on a 3-meter (9.8-foot) armored turtle.

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