Newly Discovered Sonselasuchus Cedrus Fossils in Arizona Reveal Body Transformation During Growth

The fossil remains of Sonselasuchus cedrus, unearthed in Arizona, reveal a rare developmental shift in locomotion, where the animal transitioned from walking on four legs as a juvenile to walking on two legs as an adult.

Discovered in the Chinle Formation at Kaye Quarry within Petrified Forest National Park, the fossils date back to the Late Triassic period, approximately 215 million years ago. Over 3,000 fossils have been collected from the site since 2014, including more than 950 disarticulated bones representing at least 36 individuals, most of them juvenile.

Sonselasuchus cedrus belongs to the shuvosaurid group, a lineage of pseudosuchian archosaurs that are distant relatives of modern crocodilians. Despite this lineage, the animal bore little resemblance to today’s crocodiles in appearance or posture.

Researchers determined the shift in locomotion by analyzing limb proportions across numerous specimens. Juveniles showed limb proportions consistent with quadrupedal movement, while adults exhibited elongated hind limbs and reduced forelimb development, indicating a shift to bipedalism during growth.

The fossil bed also contains remains of fish, amphibians, dinosaurs, and other reptiles, indicating a rich and diverse ecosystem. There are no signs of depletion in the fossil yield after over a decade of excavation, suggesting continued productivity of the site.

Tom Kaye first discovered the material in 2014. The fossils were later formally described by paleontologists Elliott Armour Smith and Christian A. Sidor in 2026. The holotype specimen, a left maxilla cataloged as PEFO 47305/UWBM 119436, was key to identifying the new genus and species.

Sidor noted the site’s ongoing significance: “Since starting fieldwork at Petrified Forest in 2014, we have collected over 3,000 fossils from the Sonselasuchus bonebed, and it doesn’t seem to show any signs of petering out.”

The discovery highlights the ecological experimentation among Triassic archosaurs, where lineages like shuvosaurids evolved traits convergent with later dinosaur groups, such as the ostrich-like ornithomimids, despite not being closely related.