Recently excavated fossils from the Shinle Formation in Petrified Forest National Park, Arizona, USA, have unveiled a fascinating creature: Sonserascus cedrus, a member of the Shubosaurus lineage that roamed the Earth approximately 215 million years ago during the Triassic period. It is believed that this dinosaur began its life walking on four legs before transitioning to a bipedal posture as it matured.
Reconstruction of Sonserascus cedrus 215 million years ago, showcasing the environment of what is now Petrified Forest National Park. Image credit: Gabriel Uguet.
Sonserascus cedrus is part of the Shubosauridae family, a unique group of pseudoarchosaurs known for their body shape resembling that of ornithomimid theropod dinosaurs.
This dinosaur stood approximately 63.5 centimeters (25 inches) tall, featuring a toothless beak, large eye sockets, and hollow bones.
University of Washington researcher Elliott Armor-Smith stated, “Although Sonserascus cedrus shares characteristics with ornithomimid dinosaurs, these features seem to have evolved independently due to similar ecological roles in a shared environment of crocodilian and avian archosaurs.”
Armor-Smith added, “Bipedalism, toothless beaks, hollow bones, and large orbits are typically associated with ornithomimid theropod dinosaurs, but Sonserascus cedrus demonstrates that these traits can also evolve within crocodiloforms.”
More than 950 fossil specimens representing at least 36 individuals of Sonserascus cedrus have been discovered.
“By examining the limb proportions of various specimens, we propose that their bipedal posture may arise from distinct growth patterns,” Armor-Smith noted.
“We believe as Sonserascus cedrus aged, its forelimbs and hindlimbs maintained proportionality, with hindlimbs ultimately becoming longer and more robust,” he continued.
“Essentially, these creatures likely started life as quadrupeds, transitioning to bipedalism as they matured, which is quite intriguing.”
The name Sonserascus cedrus reflects its connection to cedar trees, evergreen conifers that thrived in Late Triassic forests.
“Since initiating fieldwork at Petrified Forest in 2014, we have collected over 3,000 fossils from the area, including those of Sonserascus cedrus,” stated Professor Christian Sydow from the University of Washington. “These bones are remarkably well-preserved, showing no signs of decay.”
“In addition to Sonserascus cedrus, we have also uncovered fossils of fish, amphibians, dinosaurs, and other reptiles within the bone beds.”
For further details on this remarkable discovery, refer to the research paper published in the Journal of Vertebrate Paleontology.
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Elliott Armor Smith and Christian A. Sydow. 2026. Osteology and relationships of a new Shubosaurid (Pseudoschia, Poposaurinae) excavated from the upper Triassic Chinle Formation of Petrified Forest National Park, Arizona, USA Journal of Vertebrate Paleontology published online March 8, 2026. doi: 10.1080/02724634.2025.2604859
Source: www.sci.news
