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Stupendemys Temporal range: Langhian - Messinian 9.0–7.246 Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N ↓
A young adult Stupendemys specimen
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Testudines
Suborder:	Pleurodira
Family:	Podocnemididae
Genus:	†Stupendemys Wood, 1976
Species
†S. geographica Wood, 1976
Synonyms
†S. souzai Bocquentin & Melo, 2006

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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.

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.^[2] 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.^[3] 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.^[4] 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.^[5] 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.^[5]

The name Stupendemys is a combination of "stupendous", meaning extremely impressive, and the Latin word "emys" for freshwater turtle.^[2] 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).^[2][5]

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.^[5]

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.^[4]

Although initially believed to be a pelomedusid by Wood,^[2] later studies consistently recovered Stupendemys as a podocnemidid turtle instead.^[3][4][5] In 2020 Stupendemys was recovered as a basal member of Erymnochelyinae.^[4] However, this position was influenced by the inclusion of material belonging to Caninemys. In their 2021 publication, Cadena and colleagues again attempted to determine the relationship between Stupendemys and other pan-pleurodiran turtles using the morphological characters established previously by Joyce and colleagues (2021), 268 characters across 104 species of turtles. The analysis was run once with all taxa and once with a focus on Podocnemidoidae, removing all other taxa safe for Proganochelys quenstedti, Notoemys laticentralis, and Platychelys oberndorferi. The single most parsimonious tree resulting from the second analysis recovered Stupendemys as an early branching member of a clade with Peltocephalus dumerilianus at its base. Caninemys, now recognized as a distinct taxon, nested at the base of Erymnochelyinae.^[5] Similar results were later recovered in the 2024 description of Peltocephalus maturin.^[6]

During the Middle Miocene, the area inhabited by Stupendemys was part of an interconnected series of lakes, rivers, swamps and marshes that drained into the Caribbean known as the Pebas Mega-Wetlands, which included the Colombian La Victoria Formation.^[7] The Wetlands provided favorable conditions to the native reptilian fauna, with several lineages of crocodilians reaching enormous sizes during the Mid to Late Miocene and also diversifying in ecology. Some of the enormous crocodilians that coexisted with Stupendemys included the enormous caimain Purussaurus, the bizarre Mourasuchus and large-bodied gharials of the genus Gryposuchus, some species of which reaching lengths of over 10 meters. Some of these crocodilians may have played a role in the evolution of Stupendemys' large body-size, putting pressure on the animal through predation. Bite marks have been found on Colombian and Venezuelan specimens and an isolated tooth was found attached to the ventral surface of CIAAP-2002-01.^[4]

As the Pebas System began to disappear with the onset of the transcontinental Amazon Drainage, Stupendemys persisted in the wetlands of the northern Urumaco Formation and the Solimões Formation in Acre State, Brazil, into the Late Miocene before eventually dying out during the Early Pliocene like much of the large crocodilian fauna of the Miocene wetlands. Besides the aforementioned reptiles the waterways of Late Miocene South America were also inhabited by fish, including catfish such as Phractocephalus and Callichthyidae, characids such as Acregoliath rancii and the tambaqui (Colossoma macropomum), the South American lungfish (Lepidosiren paradoxa), trahiras (e.g. Paleohoplias assisbrasiliensis) and freshwater rays and sharks. Other turtles and tortoises found in the same deposits are Chelus columbiana^[8] (a fossil relative of the mata mata) and Chelonoidis. Further aquatic vertebrates included river dolphins and the large darter "Anhinga" fraileyi.^[3] At least within the Solimões Formation Stupendemys would have inhabited a floodplain or lacustrine environment with savannahs and gallery forests.^[3]

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