Stupendemys is an extinct genus of freshwater side-necked turtle, belonging to the family Podocnemididae. It is the largest freshwater turtle known to have existed, with a carapace over 2 meters long. Its fossils have been found in northern South America, in rocks dating from the Middle Miocene to the very start of the Pliocene, about 13 to 5 million years ago. Male specimens are known to have possessed bony horns growing from the front edges of the shell and the discovery of the fossil of a young adult shows that the carapace of these turtles flattens with age. A fossil skull described in 2021 indicates that Stupendemys was a generalist feeder.
History and naming Stupendemys was first named in 1976 by Roger C. Wood based on specimen MCNC-244, the medial portion of a large sized carapace with associated left femur, scapulacoracoid and a cervical vertebra. Wood also described several other specimens he referred to Stupendemys, which includes MCZ(P)-4376. This specimen preserves much of the carapace alongside a fragmented plastron and various other bones. The fossils were unearthed by a paleontological excavation of the Harvard University in Venezuela in 1972. In 2006 a second species, Stupendemys souzai was described by Bocquentin and Melo based on material from the Solimões Formation in Acre State in Brazil, also home to the giant Caninemys. In February 2020, Cadena and colleagues published a paper describing material discovered during the routine excavations in the Urumaco Formation, which have been ongoing since 1994. The material includes a relatively complete carapace that set a new maximum size for the genus and was designated as the allotype, meaning the specimen is of the opposite sex of the holotype. Venezuela also yielded fossils of a lower jaw, which has been used to lump Caninemys into Stupendemys in the 2020 study. The authors likewise consider S. souzai to be synonymous with S. geographica. However, more fossils were discovered in the Colombian Tatacoa Desert and formally described by Cadena and colleagues in 2021, including the first definitive skull remains as well as the first remains of a juvenile or early adult specimen (carapace length under 1 meter). The La Victoria Formation also yielded the remains of an adult female as well as more fossils of Caninemys. With definitive skull remains of Stupendemys known in association with a carapace and new fossils of Caninemys, the referral of Caninemys' skull to Stupendemys was contested and the former was re-established as a valid genus. The name Stupendemys is a combination of "stupendous", meaning extremely impressive, and the Latin word "emys" for freshwater turtle. The species name meanwhile honors the National Geographic Society. However, the name Stupendemys geographicus, as coined by Wood, is grammatically incorrect, as Stupendemys constitutes a feminine generic name. The name was eventually corrected to Stupendemys geographica in 2021 in accordance with the International Code of Zoological Nomenclature (ICZN).
Description
The skull of Stupendemys is roughly triangular in top view and the edges of the jaws converge at the front of the snout in a straight edge. The skull is dorsally extremely inflated by the prefrontals that make up a large area of the front region of the skull, forming a vertical wall above the bony nostril. Following the prefrontals and orbits the skull slopes down drastically before ascending again through the parietals. The orbits are relatively small and oriented to the sides. When viewed from below the premaxillae bear a deep concavity at their center. In this view the premaxillae form most of the anteromedial edge of the skull, meeting each other towards the middle of the skull and narrowing just before the deep concavity. In front view, the premaxillae form the bottom margin of the bony nostrils, tapering as they move down.
The carapace of adult Stupendemys can reach a straight midline length of greater than 2 meters with a low-arched profile. The nodular contours on the surface are irregular and the frontal margin of the shell is characterized by a deep notch flanked by large horns in male specimens. These horns are deeply grooved, suggesting that they were covered by a keratinous sheath. In addition to these horns, the front margin of the nuchal-peripheral bones is notably thickened and upturned. The surface is smooth to striated or lightly pitted. The margins of the posterior peripheral bones are moderately scalloped. The costal scutes of the carapace are relatively thin. In overall shape, the carapace of Stupendemys is longer than it is wide.
Size
Stupendemys is the largest known species of freshwater turtle currently known to science, with several specimens reaching a carapace length exceeding 2 meters. The largest specimen of Stupendemys is CIAAP-2002-01, an almost complete carapace with a parasagittal length of 2.86 meters. This exceeds the size of the Vienna-specimen of the Cretaceous sea turtle Archelon, the largest known turtle, (carapace length 2.20 meters). The weight of Stupendemys was estimated based on the straight carapace length, with calculations indicating a weight of 871 kg for CIAAP-2002-01 and 744 kg for MCZ(P)-4376, the former largest known specimen of Stupendemys. However, these estimates do not compensate for the large embayment present at the front of the shell. A more precise body mass estimate might be achieved by calculating the average between the results of weight estimates based on midline length and parasagittal length. Applying this method yields a weight of 1,145 kg for the largest Stupendemys specimen. The evolution of such an enormous size may have been multi-facetted and caused by a combination of factors including pressure from predators, habitat size and favorable climatic conditions, although Stupendemys temporal range indicates that it managed to survive through times of global cooling following the middle Miocene climatic transition (MMCT). Lastly, the giant size could have a phylogenetic link and be ancestral to Stupendemys, with several other related forms being known to have possessed gigantic proportions.
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