Exaeretodon

Cynodonts were one of the few lineages of synapsids (“protomammals”) to survive through the Great Dying mass extinction into the Triassic. And while a major branch of cynodonts known as probainognathians would eventually go on to produce the ancestors of modern mammals, for much of the Triassic a separate branch called cynognathians were initially much more diverse and numerous.

Exaeretodon argentinus was a large traversodontid cynognathian, growing up to about 1.8m long (~6′), known from the Late Triassic (~234-227 million years ago) of what is now northwestern Argentina. It was a low-slung animal with short stocky limbs, sprawling at the front and semi-upright at the back, and had a large head with a fairly short narrow snout and wide flaring cheekbones accommodating massive jaw muscles.

Although it it had large fang-like canine teeth, further back in its jaws wide molar-like grinding teeth show it was a specialized herbivore – at least as an adult. Different skull proportions in juveniles suggest that young Exaeretodon may have actually started out life as omnivorous or carnivorous, with jaws better suited for crushing hard-shelled invertebrate prey.

One Exaeretodon specimen shows evidence of severe rib injuries that would have hindered its mobility and made it very difficult to forage for food or avoid predators. But in this case those injuries were healed, suggesting this species may have lived in social groups that helped to protect each other.

References:

• Chinsamy, Anusuya, and Fernando Abdala. “Palaeobiological implications of the bone microstructure of South American traversodontids (Therapsida: Cynodontia): Research Letters.” South African Journal of Science 104.5 (2008): 225-230. https://hdl.handle.net/10520/EJC96794
• Doneda, Ana Laura, Lívia Roese–Miron, and Leonardo Kerber. “Bony injuries in a Late Triassic forerunner of mammals from Brazil.” The Science of Nature 112.3 (2025): 1-7. https://doi.org/10.1007/s00114-025-01984-2
• Kerber, Leonardo, et al. “New insights into the postcranial anatomy of Exaeretodon riograndensis (Eucynodontia: Traversodontidae): phylogenetic implications, body mass, and lifestyle.” Journal of Mammalian Evolution 32.1 (2025): 2. https://doi.org/10.1007/s10914-024-09741-4
• Ruta, Marcello, et al. “The radiation of cynodonts and the ground plan of mammalian morphological diversity.” Proceedings of the Royal Society B: Biological Sciences 280.1769 (2013): 20131865. https://doi.org/10.1098/rspb.2013.1865
• Wynd, Brenen, Fernando Abdala, and Sterling J. Nesbitt. “Ontogenetic growth in the crania of Exaeretodon argentinus (Synapsida: Cynodontia) captures a dietary shift.” PeerJ 10 (2022): e14196. https://doi.org/10.7717/peerj.14196