Stupendemys geographicus: Turtle Knights of the Miocene

Article by: Adam Manning
Edited by: J. D. Dixon
and Harry T. Jones

Name: Stupendemys geographicus
Name Meaning: Stupendous Turtle
Age: Middle Miocene – Early Pliocene
Diet: Omnivore
Size: Carapace (shell) length of over 3 m
Weight: 1145 kg approx.
Location: Venezuela and Colombia

Stupendemys geographicus male (front) and female (middle-left), with the large catfish Phractocephalus nassi alongside, and giant Purussaurus mirandai overhead. Artwork by Jamie Chirinos. Taken from Cadena, et al. (2020).

Turtles are some of the most iconic and beloved marine creatures alive today, and can reach humongous sizes. The Leatherback sea turtle (Dermochelys coriacea) is the largest turtle alive today, at two metres long and 700 kg, but even this was dwarfed by some of its ancient ancestors. One of the largest of these prehistoric turtles was Stupendemys geographicus.

Stupendemys was one of the biggest turtles ever to exist, weighing in at over a ton. It lived in the freshwater rivers and wetland that dominated northern South America during the Miocene. Stupendemys was originally thought to have been similar to a snapping turtle due its large skull, with a carnivorous diet of aquatic creatures. However, more recent analyses suggest that it had a much more varied diet, capable of crushing hard-shelled prey like mollusks as well as eating plants. In fact, it has been hypothesised that Stupendemys was a seed disperser for plants, especially fruit bearing ones. Its closest modern relative, the big-headed Amazon river turtle Peltocephalus dumerilianus, does this today, especially with high energy fruit. Eating a wide variety of foods would have helped allow Stupendemys to grow to such massive sizes.

The fossil carapace (B, C, F and G) and the plastron (D, E, H, I) of a male (B, C, D and E) and female (F, G, H and I) Stupendemys geographicus. Figure taken from Cadena, et al. (2020).

Perhaps the most curious aspects of Stupendemys are the large horns that stick out from its ‘shoulders’. These horns have large grooves on them, suggesting that in life the horn was covered in a keratinous sheath, giving it further strength and protection. One hypothesis was that these horns were to protect the neck and head of the turtles from potential predators, until researchers noticed that some species of Stupendemys lacked them. This is indicative of sexual dimorphism, when the males and females have a different physical morphology, often to help them in mating. Researchers argue that the horned shells of Stupendemys are from males of the species, who used their horns to joust each other over the right to mate with females.

One question still remains though, why and how did these turtles get so big? Several factors contribute to gigantism, firstly habitat. As mentioned above, northern South America was dominated by wetland and rivers during this time, which, coupled with a warm climate, gave more space and resources for the cold-blooded Stupendemys to become huge. Stupendemys likely evolved to be big because its large size would have helped protect it from predators, like giant crocodilians such as Purussaurus mirandai, whose tooth marks can be found on the carapaces of Stupendemys specimens.

Stupendemys geographicus remains one of the largest turtles ever discovered so far. This important fossil shows that palaeontologists need to keep an open mind when examining ancient organisms and to review old specimens. If we don’t, we may never realise the amazing traits, like sexual dimorphism, in some of the most charismatic prehistoric animals we have found.

Image References
[1] Stupendemys geographicus male (front) and female (middle-left), with the giant Purussaurus mirandai overhead and the large catfish Phractocephalus nassi alongside. Artwork by Jamie Chirinos. Taken from Cadena, et al. (2020).
[2] The fossil carapace (B, C, F and G) and the plastron (D, E, H, I) of a male (B, C, D and E) and female (F, G, H and I) Stupendemys geographicus. Figure taken from Cadena, et al. (2020).

Information References and Further Sources
[1] Cadena, E. -A., Scheyer, T. M., Carrillo-Briceño, J. D., Sánchez, R., Aguilera-Socorro, O. A., Vanegas, A., Pardo, M., Hansen, D. M., and Sánchez-Villagra, M. R. (2020). ‘The anatomy, paleobiology, and evolutionary relationships of the largest extinct side-necked turtle’, Science Advances, 6 (7). Accessed 20th January 2021. Click Here.
[2] Scheyer, T. M., and Sánchez-Villagra, M. R. (2007). ‘Carapace bone histology in the giant pleurodiran turtleStupendemys geographicus: Phylogeny and function’, Acta Palaeontologica Polonica, 52 (1), pp. 137-154. Accessed 20th January 2021.