Turtles are a successful model of evolution – they have existed for 220 million years and have inhabited land, sea and inland water. But paleontologists have now discovered turtle fossils in Venezuela that are exceptional in several respects: the shell of the extinct freshwater turtle is almost three meters long and the entire animal weighed more than a ton. Stupendemys geographicus is the largest known turtle species. Also unusual: the back armor of the males carried two large horns at the front end, which they probably used to fight rivals or predators.
The tropical north of South America is one of the world’s biodiversity hotspots – and it was millions of years ago. After the end of the Cretaceous period around 66 million years ago, some unusually large animals developed in the humid, lush habitats of this region, as fossil finds show. These included the largest known representatives of snakes, caimans, gavials and rodents, as Edwin Alberto Cadena from the University of Rosario in Bogota and his colleagues explain. The extinct freshwater turtle Stupendemys geographicus was among the giant animals of this region around five to ten million years ago. The first fossils of this kind were found in the northwest of Venezuela in 1976. But because only a few fragments and no complete shells and skulls were preserved from these specimens, their characteristics and the phylogenetic assignment have so far largely been unknown.
Largest known turtle
This has changed now. Because Cadena and his colleagues have found further remains of this prehistoric turtle in the Urumaco region in northern Venezuela, including for the first time complete shells and lower jaws of this type. These fossils now reveal for the first time the true size of this extinct reptile: “The shell of the Stupendemys reached a length of almost three meters for some individuals, ”reports senior author Marcelo Sanchez from the University of Zurich. “It is one of the largest, if not the largest, turtle ever. The turtle had an estimated body weight of more than a ton – that’s almost a hundred times the mass of its closest living relative, the large-headed rail turtle living in the Amazon.
It is not clear why Stupendemys geographicus grew so big. However, Cadena and his colleague suspect that several factors contributed to this. For one, the giant tortoise lived in an area in which large wetlands and lakes stretched over large areas. “It appears that the size of these wetland habitats during the Miocene in northern South America favored the occurrence of gigantism – and not just in turtles,” the researchers explain. Another factor could have been the warm climate of that time, but also the presence of similarly large predators. Because the giant caiman Purussaurus also lived in the same area as the giant tortoise. Most of the locations of Stupendemys match those of the largest South American alligator relative. Its dietary preferences as well as bite marks and pierced bones in Stupendemys fossils indicate that these giant caimans hunted the giant tortoises.
Big horns on the tank
In addition to its enormous body size, Stupendemys geographicus surprises with another feature: Some of the fossil tanks have two large horn-like projections at their front ends. Closer analyzes showed that these are real horns, the structure of which consists of a bony core with a keratin coating similar to that of cow horns. Up until now, such horns on the shell were only known from a turtle living during the Cretaceous period. But what were these protrusions used for? The orientation of the horns suggests that they served not only for protection, but also for the fight, ”explain Cadena and his colleagues. In some of the turtles living today, for example, the males use similar protrusions to lever out rivals in battle and turn them over on their backs. The researchers therefore suspect that Stupendemys could have used his horns in such rivalries.
This is also supported by the fact that not all fossils of this turtle species had the striking curb horns. The paleontologists therefore assume that only the males of Stupendemys geographicus wore these protrusions on their armor. “This hypothesis fits the observation that horn-like structures also occur in other vertebrate groups in the males,” the researchers explain. If this is confirmed, this would be the first example of curb horns as a sex characteristic in the so-called necked turtles, to which Stupendemys belongs. These do not pull their head back into the tank, but put it under the side.
Source: Edwin Alberto Cadena (Universidad del Rosario, Bogota) et al., Science Advances, doi: 10.1126 / sciadv.aay4593