''Stupendemys'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of freshwater side-necked

turtle Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...

, belonging to the family

Podocnemididae Podocnemididae is a family of pleurodire (side-necked) turtles, once widely distributed. Most of its 41 genera and 57 species are now extinct. Seven of its eight surviving species are native to South America: the genus '' Peltocephalus'', with tw ...

. It is the largest freshwater turtle known to have existed, with a

carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...

over 2 meters long. Its

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

s have been found in northern

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

, in rocks dating from the Middle

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

to the very start of the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58MCNC-244, the

medial Medial may refer to: Mathematics * Medial magma, a mathematical identity in algebra Geometry * Medial axis, in geometry the set of all points having more than one closest point on an object's boundary * Medial graph, another graph that repr ...

portion of a large sized

with associated left

femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...

scapula The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...

coracoid and a

cervical vertebra In tetrapods, cervical vertebrae (: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In sauropsid s ...

. 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 The turtle shell is a shield for the ventral and dorsal parts of turtles (the Order (biology), order Testudines), completely enclosing all the turtle's vital organs and in some cases even the head. It is constructed of modified bony elements such ...

and various other bones. The fossils were unearthed by a paleontological excavation of the

Harvard University Harvard University is a Private university, private Ivy League research university in Cambridge, Massachusetts, United States. Founded in 1636 and named for its first benefactor, the History of the Puritans in North America, Puritan clergyma ...

Venezuela Venezuela, officially the Bolivarian Republic of Venezuela, is a country on the northern coast of South America, consisting of a continental landmass and many Federal Dependencies of Venezuela, islands and islets in the Caribbean Sea. It com ...

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 Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

, 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 A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

. 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

Colombia Colombia, officially the Republic of Colombia, is a country primarily located in South America with Insular region of Colombia, insular regions in North America. The Colombian mainland is bordered by the Caribbean Sea to the north, Venezuel ...

n 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 The National Geographic Society, headquartered in Washington, D.C., United States, is one of the largest nonprofit scientific and educational organizations in the world. Founded in 1888, its interests include geography, archaeology, natural sc ...

. 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

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...

e 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. Stupendemys geographicus replica Osaka

The

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

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. It is the key structural material making up Scale (anatomy), scales, hair, Nail (anatomy), nails, feathers, horn (anatomy), horns, claws, Hoof, hoove ...

ous 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 Vienna ( ; ; ) is the capital city, capital, List of largest cities in Austria, most populous city, and one of Federal states of Austria, nine federal states of Austria. It is Austria's primate city, with just over two million inhabitants. ...

-specimen of the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

sea turtle ''

Archelon ''Archelon'' is an extinct marine turtle from the Late Cretaceous, and is the largest turtle ever to have been documented, with the biggest specimen measuring from head to tail and in body mass. It is known only from the Pierre Shale and ...

,'' 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.

Phylogeny

Although initially believed to be a pelomedusid by Wood, later studies consistently recovered ''Stupendemys'' as a podocnemidid turtle instead. In 2020 ''Stupendemys'' was recovered as a basal member of Erymnochelyinae. 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. Similar results were later recovered in the 2024 description of '' Peltocephalus maturin''.

Paleobiology

Paleoecology

Following the 2021 research of Cadena and colleagues, the Pebas Mega-Wetlands housed at least two species of giant side-necked turtles: ''Stupendemys'' and ''Caninemys''. Despite their similar size (both sporting a carapace length greater than 2 meters), they vary greatly in skull morphology, with ''Caninemys'' proposed to have been deploying a vacuum feeding strategy combined with a strong bite supported by tooth-like structures of the maxilla, while a more durophagous-omnivorous diet has been suggested for ''Stupendemys''. This difference in diet and feeding strategy would be in accordance with Gause's Law, by which two species competing for the same ecological niche cannot coexist with one another for a long period of time without either differentiating or one dominating over the other in the long run. In addition to the different skull morphology, the two taxa may have also been able to coexist due to the sheer size of the Pebas Mega-Wetlands they inhabited, as this ecosystem stretched over most of northern South America during the Middle Miocene. This reason may also prevent the two taxa from being in direct competition over nesting grounds and basking spaces. The diet of ''Stupendemys'' may have been very diverse and broad, possibly including molluscs and other hard shelled prey as well as vertebrate prey as suggested by Meylan and colleagues for ''Caninemys''. At its size it would have been easily capable of consuming various fish, snakes and small crocodilians. A broad dietary width would have helped ''Stupendemys'' in maintaining its large body size. Furthermore, Cadena and colleagues also highlight the role of turtles as seed-dispersers in modern-day Amazonia, consuming fruit of palms for example (

Arecaceae The Arecaceae () is a family (biology), family of perennial plant, perennial, flowering plants in the Monocotyledon, monocot order Arecales. Their growth form can be climbing palm, climbers, shrubs, tree-like and stemless plants, all commonly k ...

), seasonally sometimes in great quantities, even if they are not typically part of their standard diet. With its wide gape, ''Stupendemys'' would qualify as a megafaunal frugivore and seed disperser.

Sexual dimorphism

The absence of horns on most ''Stupendemys'' specimens indicates that they were not used as a defense mechanism. However, their forward-facing position on the carapace may indicate that they were used in intraspecific combat. Cadena and colleagues hypothesize that the horns may have been a

sexually dimorphic Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

trait exclusively found in males, suggesting them to have been used similar to the horns and antlers found in artiodactyls. Among extant turtles similar behavior can be found in

snapping turtles The Chelydridae is a Family (biology), family of turtles that has seven extinct and two extant genera. The extant genera are the snapping turtles, ''Chelydra'' and ''Macrochelys''. Both are Endemic (ecology), endemic to the Western Hemisphere. Th ...

, some of the largest freshwater turtles alive, which are known to fight for dominance in overlapping territories. This hypothesis is supported by the presence of a deep, elongated scar along the left horn of CIAAP-2002-01, which could have been left by the horn of a rival male that engaged it in combat. The authors further suggest that in ''Stupendemys'' the males may have been the larger sex, similar to the condition seen in the modern podocnemids. However other traditionally sexually dimorphic traits of the turtle shell, such as a deeper anal notch or a xiphiplastral concavity, have not yet been observed in ''Stupendemys'' fossils. Stupendemys ontogeny

Ontogeny

Prior to the 2021 study of Cadena and colleagues, only adult specimens of ''Stupendemys'' had been described. The discovery of a specimen with a carapace length smaller than 1 meter gives an insight into the changes the animal undergoes while reaching maturity. In addition to its small size, the animal is identified as a juvenile to young adult based on the absence of large horns and shallow anal notch. The inner nuchal notch, anterior expansion of the peripherals 1 and 2, irregular nodular contours, inner contact between the 7th and 8th costals and the relative size of the plastral lobes and their arrangement (except for the pectorals) remains relatively consistent with size. One of the most significant changes of the carapace of ''Stupendemys'' is its height. With age the shell of the turtle grows significantly flatter, while the nuchal region develops a pronounced upturn of its anterior margin and peripheral 1, creating a wide and deep anteromedial embayment of the carapace. The 2nd and 3rd vertebral scutes grow narrower as the animal matures from juvenile to adult, similar to the extant ''

Podocnemis ''Podocnemis'' is a genus of aquatic turtles, commonly known as South American river turtles, in the Family (biology), family Podocnemididae. The genus consists of six Extant taxon, extant species occurring in tropical South America.Erymnochelys'' and '' Peltocephalus''. The 5th vertebral scute meanwhile belongs the longest and widest of the series in adults while keeping its trapezoid shape. This

ontogenetic Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the stu ...

change of the vertebral scutes means that phylogenetic coding using the width of the vertebral scutes in relation to the pleural scutes should be treated with care due to the variable nature of these features as shown by ''Stupendemys''.

Paleoenvironment

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. 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. 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,

, 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 Catfish (or catfishes; order (biology), order Siluriformes or Nematognathi) are a diverse group of ray-finned fish. Catfish are common name, named for their prominent barbel (anatomy), barbels, which resemble a cat's whiskers, though not ...

such as '' Phractocephalus'' and

Callichthyidae Callichthyidae is a family of catfishes (order Siluriformes), called armored catfishes due to the two rows of bony plates (or scutes) along the lengths of their bodies. It contains some of the most popular freshwater aquarium fish, such as many ...

, characids such as '' Acregoliath rancii'' and the

tambaqui The tambaqui (''Colossoma macropomum'') is a large species of freshwater fish in the family Serrasalmidae. It is native to tropical South America, but kept in aquaculture and Introduced species, introduced elsewhere. It is also known by the names ...

(''Colossoma macropomum''), the

South American lungfish The South American lungfish (''Lepidosiren paradoxa''), also known as the American mud-fish and scaly salamanderfish, is the single species of lungfish found in swamps and slow-moving waters of the Amazon Basin, Amazon, Paraguay River, Paraguay, ...

(''Lepidosiren paradoxa''), trahiras (e.g. '' Paleohoplias assisbrasiliensis'') and freshwater rays and

shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

s. Other turtles and

tortoise Tortoises ( ) are reptiles of the family Testudinidae of the order Testudines (Latin for "tortoise"). Like other turtles, tortoises have a shell to protect from predation and other threats. The shell in tortoises is generally hard, and like o ...

s found in the same deposits are '' Chelus columbiana'' (a fossil relative of the

mata mata Matamata () is a town in Waikato, New Zealand. It is located near the base of the Kaimai Ranges, and is a thriving farming area known for Thoroughbred horse breeding and training pursuits. It is part of the Matamata-Piako District, which tak ...

) and '' Chelonoidis''. Further aquatic vertebrates included

river dolphin River dolphins are a polyphyletic group of fully aquatic mammals that reside exclusively in freshwater or brackish water. They are an informal grouping of dolphins, which itself is a paraphyletic group within the infraorder Cetacea. Extant rive ...

s and the large

darter The darters, anhingas, or snakebirds are mainly tropical waterbirds in the family Anhingidae, which contains a single genus, ''Anhinga''. There are four living species, three of which are very common and widespread while the fourth is rarer and c ...

'' "Anhinga" fraileyi''. At least within the Solimões Formation ''Stupendemys'' would have inhabited a floodplain or lacustrine environment with savannahs and gallery forests.

References

{{Testudines Prehistoric turtles of South America Podocnemididae Prehistoric turtle genera Extinct turtles Miocene turtles Miocene genus first appearances Pliocene extinctions Miocene reptiles of South America Pliocene reptiles of South America Huayquerian Montehermosan Miocene Brazil Pliocene Brazil Fossils of Brazil Miocene Colombia Pliocene Colombia Fossils of Colombia Honda Group, Colombia Miocene Venezuela Pliocene Venezuela Fossils of Venezuela Fossil taxa described in 1976