Stupendemys geographicus: Paleontologists Find New Fossils of Gigantic Freshwater Turtle
An international team of paleontologists has unearthed several well-preserved shells and the first known jaw specimen of Stupendemys geographicus, a species of freshwater side-necked turtle that lived 5-10 million years ago (Miocene Epoch) in South America. Together, the fossils shed new light on the biology, past distribution, and phylogenetic position of the gigantic turtle.
“Since the extinction of dinosaurs, the northern Neotropics have harbored now-extinct vertebrates that have been at the extreme of large size within their respective clades,” said team leader Dr. Marcelo Sánchez, director of the Paleontological Institute and Museum at the University of Zurich, and colleagues.
“Among them are the largest snake, caimanine crocodile, gharial, and some of the largest rodents.”
“One of the most iconic of these species is the gigantic turtle Stupendemys geographicus, as it is the largest non-marine turtle ever known from a complete shell.”
“Stupendemys geographicus was first described in 1976 from the Urumaco Formation in northwestern Venezuela, but our knowledge of this animal has been based on partial specimens that have resulted in a problematic taxonomy, especially due to a lack of specimens with associated skull and shell elements.”
Dr. Sánchez and co-authors unearthed and examined new specimens of Stupendemys geographicus in the Urumaco region in Venezuela and La Tatacoa Desert in Colombia.
The finds included the largest shell reported for any extant or extinct turtle, with a carapace length of 2.4 m (8 feet) and estimated mass of 1.145 kg, almost 100 times the size of its closest living relative.
“The carapace of some Stupendemys geographicus individuals reached almost 3 m (10 feet), making it one of the largest, if not the largest turtle that ever existed,” Dr. Sánchez said.
In some specimens, the researchers observed a peculiar and unexpected feature: horns.
“The two shell types indicate that two sexes of Stupendemys geographicus existed: males with horned shells and females with hornless shells,” Dr. Sánchez said.
“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.”
The scientists were also able to 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,” Dr. Sánchez said.
“Furthermore, the new discoveries and the investigation of existing fossils from Brazil, Colombia and Venezuela indicate a much wider geographic distribution of Stupendemys geographicus than previously assumed. The animal lived across the whole of the northern part of South America.”
“Despite its tremendous size, the turtle had natural enemies,” the authors added.
“In many areas, the occurrence of Stupendemys geographicus 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 geographicus.”
The research is described in a paper in the journal Science Advances.
E.-A. Cadena et al. 2020. The anatomy, paleobiology, and evolutionary relationships of the largest extinct side-necked turtle. Science Advances 6 (7): eaay4593; doi: 10.1126/sciadv.aay4593