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), 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 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, 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.
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,
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 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==