Ancient Crocodile Relative Walked on Two Legs as Adult | 200-Million-Year-Old Fossil Discovery

A newly discovered fossil is shedding light on the evolutionary history of ancient crocodile relatives, revealing a surprising shift in locomotion. Researchers have found that this prehistoric reptile, Sonselasuchus cedrus, transitioned from walking on four legs as a juvenile to walking on two legs as an adult.

The species belongs to a group called shuvosaurids, ancient reptiles that lived during the Late Triassic period, approximately 225–201 million years ago. This discovery offers valuable insights into the adaptability and evolutionary pathways of early archosaurs, the group that includes crocodiles, dinosaurs, and birds.

Scientists noted that Sonselasuchus cedrus shared a striking resemblance in body plan to ornithomimid dinosaurs – often nicknamed “ostrich dinosaurs” – known for their swift running abilities. Though, this similarity isn’t due to a close evolutionary relationship, but rather a case of convergent evolution, where different species independently develop similar traits in response to similar environmental pressures.

The findings were published on Monday, March 9, 2026, in the journal Journal of Vertebrate Paleontology by a team of researchers. The research team included members from the University of Washington Department of Biology and Burke Museum.

An Unusual Change in Gait

One of the most compelling aspects of the research is the documented change in Sonselasuchus’s mode of locomotion throughout its life. Analysis of the proportions of its leg bones suggests a significant shift in body structure as the animal grew.

Lead researcher Elliott Armour Smith explained that this change was driven by differing growth patterns between the forelimbs and hindlimbs. “By analyzing the proportions of the skeletons of different animals, we found that bipedal posture was likely the result of different growth patterns,” Smith said.

The researchers believe that young Sonselasuchus had relatively balanced forelimbs and hindlimbs. “We hypothesize that Sonselasuchus had more balanced forelimbs and hindlimbs when young, but its hindlimbs grew longer and stronger as it matured,” Smith added.

the animal’s gait also evolved. “Essentially, we think this creature started life walking on four legs… then switched to walking on two legs as it grew,” Smith stated. This type of shift is rare in the fossil record of reptiles.

A Remarkable Fossil Site

The Sonselasuchus fossils were unearthed at Petrified Forest National Park in Arizona, United States.

In 2014, researchers discovered approximately 950 fossils belonging to this species at the site, representing a significant locate within a long-term excavation project spanning more than a decade. The fossils represent a minimum of 36 individuals, most of which were skeletally immature, and were found within a bonebed containing mostly disarticulated elements.