Newly Discovered Crocodile Ancestor Sonselasuchus Cedrus Transitioned From Four Legs to Two During Adulthood

New fossil study of Sonselasuchus cedrus shows this ancient crocodile relative started life on four legs and became bipedal as an adult.

By: AXL Media

Published: Mar 9, 2026, 6:09 AM EDT

Source: The information in this article was sourced from Taylor & Francis Group

The Evolutionary Shift From Quadrupedalism to Bipedal Stance

A peculiar relative of the modern crocodile has challenged established views on reptilian development by demonstrating a rare postural shift during its lifespan. According to a study published in the Journal of Vertebrate Paleontology, the newly named Sonselasuchus cedrus began its life walking on four legs before transitioning to a two-legged gait as it reached maturity. Lead author Elliott Armour Smith, a graduate student at the University of Washington, explains that this transition was likely driven by a differential growth pattern where the hindlimbs became disproportionately longer and more robust over time. This unique biological trajectory suggests that the animal's physical requirements changed significantly as it navigated the Late Triassic landscape.

Convergent Evolution and the Mimicry of Early Dinosaurs

Sonselasuchus cedrus belonged to the shuvosaurid group, a lineage of reptiles that shared an uncanny resemblance to the ornithomimid dinosaurs of the same era. Despite being on the "croc-line" of evolution rather than the "bird-line" associated with dinosaurs, this species developed hollow bones, large eye sockets, and a toothless beak. Experts from the Burke Museum note that these features evolved independently, a classic example of convergent evolution where different species adapt similar traits to fill comparable ecological roles. The existence of Sonselasuchus proves that many characteristics traditionally associated with theropod dinosaurs actually appeared much earlier in the crocodile family tree.

A Decadelong Excavation in the Petrified Forest National Park

The discovery is the result of over ten years of meticulous fieldwork led by Professor Christian Sidor at Arizona’s Petrified Forest National Park. Since the initial unearthing of the Sonselasuchus bonebed in 2014, the site has yielded more than 3,000 fossil bones, including 950 specifically belonging to this poodle-sized reptile. The geologic unit from which the fossils were extracted, the Sonsela Member of the Chinle Formation, provided the inspiration for the creature's name. According to Sidor, the site remains incredibly productive, offering a diverse record of fish, amphibians, and early dinosaurs that lived alongside this ancient crocodile relative 215 million years ago.