Triassic Crocodile Relative May Have Learned to Walk on Two Legs

Fossils from the Chinle Formation of Petrified Forest National Park in Arizona, United States, reveal that Sonselasuchus cedrusa species of shuvosaurid that lived around 215 million years ago (Triassic period), probably began life walking on four legs before transitioning to a bipedal stance as it matured.

An artistic reconstruction of Sonselasuchus cedrus in its environment in what is now the Petrified Forest National Park, 215 million years ago. Image credit: Gabriel Ugueto.

Sonselasuchus cedrus was a type of Shuvosauridae, a small group of pseudosuchian archosaurs characterized by a body plan surprisingly convergent with that of ornithomimid theropod dinosaurs.

The animal was approximately 63.5 cm (25 inches) tall, had a toothless beak, a large eye socket, and hollow bones.

“Although similar to those of ornithomimid dinosaurs, these features would have evolved separately, and this similarity was likely because archosaurs of the crocodile lineage and the bird lineage evolved in the same ecosystems and converged on similar ecological roles,” said Elliott Armor Smith, a researcher at the University of Washington.

“Furthermore, despite the fact that features such as bipedalism, a toothless beak, hollow bones, and a large eye socket are characteristic of ornithomimid theropod dinosaurs, shuvosaurids like Sonselasuchus show that these characteristics also evolved in the crocodile lineage.

Sonselasuchus cedrus is represented by more than 950 fossil specimens from a minimum of 36 individuals.

“By analyzing the proportions of the limb skeletons of different animals, we determined that its bipedal stance could be the result of a differential growth pattern,” Armor Smith said.

“We think that Sonselasuchus cedrus had more proportionate forelimbs and hindlimbs when young, and their hindlimbs became longer and sturdier into adulthood.

“Essentially, we think these creatures started life on four legs…they then started walking on two legs as they grew up. It’s particularly strange.”

Sonselasuchus cedrus would have lived in the forest, and its specific name Cedar represents cedar, an evergreen conifer similar to those of Upper Triassic forests.

“Since fieldwork at Petrified Forest began in 2014, we have collected more than 3,000 fossils from the Petrified Forest. Sonselasuchus the bed of bones, and he doesn’t seem to show any signs of shortness of breath,” said Professor Christian Sidor of the University of Washington.

“In addition to Sonselasuchusthe bone bed yielded fossils of fish, amphibians, as well as dinosaurs and other reptiles.

The discovery is described in an article in Journal of Vertebrate Paleontology.

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Elliott Armor Smith and Christian A. Sidor. 2026. Osteology and relationships of a new shuvosaurid (Pseudosuchia, Poposauroidea) from the Upper Triassic Chinle Formation of Petrified Forest National Park, Arizona, USA Journal of Vertebrate Paleontologypublished online March 8, 2026; doi: 10.1080/02724634.2025.2604859