Approximately 215 million years ago, a small reptile may have undergone a remarkable transformation during its lifetime: beginning its existence on four legs, it would have gradually adopted walking on its two hind legs upon reaching adulthood.
Belonging to the family Shuvosauridae, a particular branch of archosaurs related to crocodilians,
Sonselasuchus cedrus shared its environment with fast dinosaurs reminiscent of ostriches. Although a cousin of crocodiles, its appearance strongly recalled that of certain dinosaurs, a resemblance due to convergent evolution. These two groups coexisted during the Late Triassic, long before the appearance of modern forms of crocodiles.
Artistic reconstruction of Sonselasuchus cedrus in its environment, in what is now Petrified Forest National Park, 215 million years ago.
Credit: Artwork by Gabriel Ugueto Research published in the
Journal of Vertebrate Paleontology indicates that this ability to change its posture resulted from a differential growth model. Examination of the proportions of the fossil limbs shows that the hind legs became longer and more robust with age. Elliott Armour Smith, lead author of the study, explains that young animals had limbs with more balanced proportions, facilitating quadrupedal locomotion, while adults instead opted for bipedalism.
From a morphological standpoint,
Sonselasuchus cedrus measured about 2 feet 1 inch (approximately 64 cm) tall and sported a toothless beak, large eye sockets, and hollow bones. These traits, comparable to those of ornithomimid dinosaurs, developed independently in the crocodilian lineage, illustrating how distinct animals can acquire analogous forms to occupy similar ecological functions.
The species name,
cedrus, refers to cedars, evergreen conifers that populated the forests of the Late Triassic. The genus name
Sonselasuchus derives from the Sonsela geological formation, where the fossils were identified. This wooded environment offered a favorable habitat for this reptile and other creatures of the time.
The discovery of these fossils took place in Petrified Forest National Park in Arizona, where over 3,000 bones have been extracted since 2014. Christian Sidor, an associate professor involved in the excavations, highlights the richness of this site, which has yielded remains of fish, amphibians, and other reptiles.
Research conducted with the National Park Service is ongoing, involving numerous students and volunteers. The site continues to provide new fossils.