Analysis of paleontological findings unearthed from the Chinle Formation within Arizona’s Petrified Forest National Park indicates that the shuvosaurid species, Sonselasuchus cedrus, which roamed the Earth approximately 215 million years ago during the Triassic period, likely initiated its life cycle employing quadrupedal locomotion before transitioning to a facultative bipedal gait as it reached maturity.
An artistic rendition of Sonselasuchus cedrus inhabiting its ancestral milieu in the region now designated as Petrified Forest National Park, dating back 215 million years. Attribution: Gabriel Ugueto.
Sonselasuchus cedrus belonged to the Shuvosauridae family, a notable lineage of pseudosuchian archosaurs distinguished by a skeletal architecture remarkably convergent with that of ornithomimid theropod dinosaurs.
This ancient creature stood approximately 63.5 cm (25 inches) in height, possessed a keratinous, toothless rostrum, prominent orbits, and a pneumatic skeletal structure.
“Notwithstanding the superficial resemblance to ornithomimid dinosaurs, these specific morphological characteristics would have arisen independently. This convergence is likely attributable to the fact that both crocodylomorph-line and avian-line archosaurs occupied congruent ecological niches within the same ancient ecosystems, leading to parallel evolutionary adaptations,” commented Elliott Armour Smith, a researcher affiliated with the University of Washington.
“Furthermore, despite the presence of traits such as bipedalism, a edentulous beak, pneumatized bones, and enlarged orbits being historically associated with ornithomimid theropod dinosaurs, the existence of shuvosaurids like Sonselasuchus demonstrates that these analogous features also evolved within the crocodylomorph lineage.”
The fossil record for Sonselasuchus cedrus is substantial, comprising over 950 specimens representing a minimum of 36 distinct individuals.
“Through meticulous analysis of the proportional relationships within the appendicular skeletons of various specimens, we have deduced that its propensity for a bipedal posture may have been a consequence of differential ontogenetic growth patterns,” stated Armour Smith.
“Our hypothesis posits that Sonselasuchus cedrus exhibited relatively similar forelimb and hindlimb proportions during its juvenile stages, with subsequent hindlimb elongation and increased robustness occurring throughout its maturation process.”
“In essence, we surmise that these organisms commenced their existence as quadrupedal walkers, subsequently transitioning to facultative bipedalism as they progressed through adulthood. This developmental trajectory is particularly noteworthy.”
The ecological habitat of Sonselasuchus cedrus was forested environments, with its specific epithet, cedrus, being a direct reference to the cedar tree, an evergreen conifer characteristic of Late Triassic woodlands.
“Since the commencement of fieldwork at Petrified Forest National Park in 2014, our excavation efforts have yielded in excess of 3,000 fossils from the Sonselasuchus bonebed, and this rich deposit shows no indications of depletion,” reported Professor Christian Sidor of the University of Washington.
“Beyond the remains of Sonselasuchus, this paleontological site has also provided evidence of piscine, amphibian, and other reptilian taxa, including dinosaurs.”
This significant paleontological discovery is detailed in a scholarly publication in the esteemed 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
