A graphic reconstruction of the giant turtle Stupendemys geographicus: male (front) and female individual (left) swimming in freshwater. Credit: Artwork: Jaime Chirinos

Paleobiologists from the University of Zurich have discovered exceptional specimens in Venezuela and Colombia of an extinct giant freshwater turtle called Stupendemys. The carapace of this turtle, which is the largest ever known, measured between 2.4 to almost 3 meters. Moreover, the shell of male Stupendemys had horns—a rare feature in turtles.

The tropical region of South America is one of the world's hot spots when it comes to animal diversity. The region's extinct fauna is unique, as documented by fossils of giant rodents and crocodilians, including crocodiles, alligators, caimans and gavials—that inhabited what is today a desert area in Venezuela. Five to ten million years ago, this was a humid swampy region teeming with life. One of its inhabitants was Stupendemys geographicus, a turtle species first described in the mid-1970s.

Giant turtle 100 times heavier than its closest relative

Researchers of the University of Zurich (UZH) and fellow researchers from Colombia, Venezuela, and Brazil have now reported exceptional specimens of the extinct turtle recently found in new locations across Venezuela and Colombia.

"The carapace of some Stupendemys individuals reached almost three meters, making it one of the largest, if not the largest turtle that ever existed," says Marcelo Sánchez, director of the Paleontological Institute and Museum of UZH and head of the study. The turtle had an estimated body mass of 1,145 kg—almost one hundred times that of its closest living relative, the big-headed Amazon river turtle.

A short video of the largest male specimen of Stupendemys geographicus from Venezuela described in the paper, reaching 286 cm length of the carapace and exhibited at the local museum of Urumaco, Venezuela. Credit: Edwin Cadena

Males carried horns on their carapace

In some individuals, the complete carapace showed a peculiar and unexpected feature: horns. "The two shell types indicate that two sexes of Stupendemys existed—males with horned shells, and females with hornless shells," concludes Sánchez. According to the paleobiologist, this is the first time that sexual dimorphism in the form of horned shells has been reported for any of the side-necked turtles, one of the two major groups of turtles world-wide.

Venezuelan Palaeontologist Rodolfo Sánchez and a male carapace of the giant turtle Stupendemys geographicus, from Urumaco, Venezuela, found in 8 million years old deposits. Credit: Edwin Cadena

Despite its tremendous size, the turtle had natural enemies. In many areas, the occurrence of Stupendemys coincides with Purussaurus, the largest caimans. This was most likely a predator of the giant turtle, given not only its size and dietary preferences, but also as inferred by bite marks and punctured bones in fossil carapaces of Stupendemys.

Turtle phylogeny thoroughly revised

Venezuelan Palaeontologist Rodolfo Sánchez and a male carapace of the giant turtle Stupendemys geographicus, from Urumaco, Venezuela, found in 8 million years old deposits. Credit: Jorge Carrillo

Since the scientists also discovered jaws and other skeleton parts of Stupendemys, they were able to thoroughly revise the evolutionary relationships of this species within the turtle tree of life. "Based on studies of the turtle anatomy, we now know that some living turtles from the Amazon region are the closest living relatives," says Sánchez. Furthermore, the new discoveries and the investigation of existing fossils from Brazil, Colombia and Venezuela indicate a much wider geographic distribution of Stupendemys than previously assumed. The animal lived across the whole of the northern part of South America.

Paleontologists excavating one of the specimens of Stupendemys geographicus at the top of a cliff in northern Venezuela. Credit: Produced by Rio Verde for Edwin Cadena, senior author of the manuscript. Edited by Jonathan Reyes, Universidad del Rosario.

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More information: E.-A. Cadena at Universidad del Rosario in Bogotá, Colombia el al., "The anatomy, paleobiology, and evolutionary relationships of the largest extinct side-necked turtle," Science Advances (2020). advances.sciencemag.org/content/6/7/eaay4593 Journal information: Science Advances E.-A. Cadena at Universidad del Rosario in Bogotá, Colombia el al., "The anatomy, paleobiology, and evolutionary relationships of the largest extinct side-necked turtle,"(2020). DOI: 10.1126/sciadv.aay4593