Ancient croc relative shifted from four legs to two as it matured

by Clarence Oxford
Los Angeles CA (SPX) Mar 09, 2026
A peculiar ancient relative of modern crocodiles that appears to have started life on four legs before adopting a two legged stance in adulthood has been described in a new study. The reptile, named Sonselasuchus cedrus, is a member of the shuvosaurid group and lived alongside early dinosaurs in what is now Petrified Forest National Park in Arizona during the Late Triassic, around 225 to 201 million years ago.

Researchers from the University of Washington Department of Biology and the Burke Museum analyzed limb proportions from hundreds of fossils to reconstruct how the animal moved as it grew. They conclude that this poodle sized reptile likely began life walking on all fours, then shifted to bipedal locomotion as its hind limbs became longer and more robust through growth.

"By analyzing the proportions of the limb skeletons of different animals, they determined its bipedal stance (standing on two feet) may have been the result of a differential growth pattern," explains lead author Elliott Armour Smith. "We think that Sonselasuchus had more proportional forelimbs and hindlimbs as young, and their hindlimb grew longer and more robust through adulthood. Essentially, we think these creatures started out their lives on four legs... they then started walking on two legs as they grew up. This is particularly peculiar."

The fossils of Sonselasuchus were excavated from a rich bonebed in the Sonsela Member of the Upper Triassic Chinle Formation at Petrified Forest National Park. The genus name honors this geologic unit, while the species name cedrus refers to a cedar tree, reflecting the evergreen conifers that characterized Late Triassic forests in the region.

Professor Christian Sidor of the Burke Museum, who coauthored the study, was part of the team that began excavating the site in 2014. Over a decade of fieldwork has yielded about 950 bones attributable to Sonselasuchus and more than 3,000 fossil bones in total from the bonebed. The site has also produced fossils of fish, amphibians, dinosaurs and other reptiles, and more than 30 University of Washington students and volunteers have taken part in the excavations.

The anatomy of Sonselasuchus provides important clues to its appearance and lifestyle. The animal is estimated to have stood about 25 inches tall and possessed a toothless beak, a large eye socket and hollow bones. These features recall those of ornithomimid theropod dinosaurs, which evolved much later, but the authors emphasize that the similarities evolved independently in croc line and bird line archosaurs.

"Although similar to the ornithomimid dinosaurs these features would have evolved separately," Armour Smith notes, "and this similarity was probably due to the fact that croc-line and bird-line archosaurs evolved in the same ecosystems and converged upon similar ecological roles. Also, despite the fact that features like bipedalism, a toothless beak, hollow bones and a large orbit are characteristic of ornithomimid theropod dinosaurs, shuvosaurids like Sonselasuchus show that these features evolved on the croc-line as well."

The discovery of Sonselasuchus adds to evidence that croc line archosaurs experimented with diverse body plans and locomotor strategies during the Triassic. The differential growth pattern inferred from the limb proportions suggests that developmental changes played a key role in the transition from quadrupedal to bipedal stance in this lineage. The fossil material also highlights how extensive bonebeds can illuminate individual species and broader Late Triassic ecosystems.

Research Report:Osteology and relationships of a new shuvosaurid (Pseudosuchia, Poposauroidea) from the Upper Triassic Chinle Formation of Petrified Forest National Park, Arizona, U.S.A.