Discovery of Sonselasuchus cedrus: A Bipedal Ancestor of Modern Crocodiles

Paleontologists from the University of Washington and Seattle’s Burke Museum of Natural History and Culture have identified a new species of ancient reptile, named *Sonselasuchus cedrus*, which lived approximately 225 to 201 million years ago during the Late Triassic period. This discovery was made in the Petrified Forest National Park, a site renowned for its extensive fossil wood deposits. The *Sonselasuchus cedrus*, a poodle-sized creature, exhibits a unique evolutionary trait: it transitioned from a quadrupedal to a bipedal stance as it matured.

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The research team, led by Elliott Armour Smith, noted that the bipedalism of *Sonselasuchus cedrus* may have resulted from a differential growth pattern. Initially, these reptiles had more proportional forelimbs and hindlimbs, but as they aged, their hindlimbs became longer and more robust. This evolutionary adaptation is significant as it parallels similar features found in ornithomimid dinosaurs, suggesting a convergent evolution where different species develop similar traits due to comparable ecological roles.

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Since 2014, over 3,000 fossils from the *Sonselasuchus* bonebed have been collected, indicating a rich site for ongoing paleontological research. Christian Sidor, a vertebrate paleontologist at the Burke Museum, highlighted the involvement of more than 30 University of Washington students and volunteers in the excavation efforts, emphasizing the site's potential for yielding further discoveries.

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The species name, *cedrus*, pays homage to the cedar trees that would have been prevalent in the Late Triassic forests, while the genus name, pronounced “sawn-SAY-la-SOOK-us,” references the Sonsela Member of the Upper Triassic Chinle Formation, the geological unit from which these fossils were extracted. This nomenclature reflects both the ecological context of the species and its geological origins.

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Elliott Armour Smith remarked on the peculiar nature of *Sonselasuchus cedrus*, stating, “Essentially, we think these creatures started out their lives on four legs… they then started walking on two legs as they grew up.” Christian Sidor added, “It’s exciting to see that the site continues to produce new and interesting fossils,” indicating the ongoing importance of the Petrified Forest for paleontological studies.

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While the discovery has been met with enthusiasm within the scientific community, some critics argue that the implications of bipedalism in ancient reptiles require further investigation to fully understand the evolutionary pathways that led to such adaptations.

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• “Essentially, we think these creatures started out their lives on four legs… they then started walking on two legs as they grew up,” — Elliott Armour Smith, University of Washington

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• “Over 30 University of Washington students and volunteers have been involved over the years. It’s exciting to see that the site continues to produce new and interesting fossils.” — Christian Sidor, Burke Museum

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The discovery of *Sonselasuchus cedrus* not only enriches our understanding of reptilian evolution but also underscores the significance of the Petrified Forest National Park as a vital site for paleontological research.

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