Fossils from the Chinle Formation of Petrified Forest National Park in Arizona, the United States, reveal that Sonselasuchus cedrus, a species of shuvosaurid that lived about 215 million years ago (Triassic period), likely began life walking on four legs before shifting to a bipedal stance as it matured.
An artist’s reconstruction of Sonselasuchus cedrus in its environment in what is now 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 strikingly convergent with that of ornithomimid theropod dinosaurs.
The animal was about 63.5 cm (25 inches) tall, had a toothless beak, a large eye socket and hollow bones.
“Although similar to the ornithomimid dinosaurs these features would have evolved separately, 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,” said Elliott Armour Smith, a researcher at the University of Washington.
“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.”
Sonselasuchus cedrus is represented by over 950 fossil specimens from a minimum of 36 individuals.
“By analyzing the proportions of the limb skeletons of different animals, we determined its bipedal stance may have been the result of a differential growth pattern,” Armour Smith said.
“We think that Sonselasuchus cedrus 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.”
Sonselasuchus cedrus would have lived in the forest, and its specific name cedrus represents the cedar tree, an evergreen conifer similar to those of Late Triassic forests.
“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,” said University of Washington’s Professor Christian Sidor.
“In addition to Sonselasuchus, the bonebed has yielded fossils of fish, amphibians, as well as dinosaurs and other reptiles.”
The discovery is described in a paper in the Journal of Vertebrate Paleontology.
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Elliott Armour Smith & 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, U.S.A. Journal of Vertebrate Paleontology, published online march 8, 2026; doi: 10.1080/02724634.2025.2604859
