Ancient Crocodile Ancestor Walked on Two Legs as Adult

Newly discovered fossil reveals fascinating details about a 200-million-year-old reptile's changing gait.

Published on Mar. 9, 2026

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A newly discovered fossil of an ancient relative of crocodiles, named Sonselasuchus cedrus, reveals that this reptile from the Late Triassic period changed its walking style as it matured - starting on four legs and then transitioning to a two-legged, bipedal stance as an adult. Researchers found this species bore a striking resemblance to ornithomimid dinosaurs, despite not being closely related, due to convergent evolution.

Why it matters

This rare finding in the fossil record provides insights into how ancient reptiles evolved and adapted their locomotion over their lifetimes. The discovery also highlights the rich paleontological resources found in Petrified Forest National Park, Arizona, where hundreds of Sonselasuchus fossils have been uncovered.

The details

Analysis of the proportions of Sonselasuchus' leg bones suggests its change in walking style was due to differing growth patterns between the forelimbs and hindlimbs. When young, the front and back legs were relatively balanced, but as the animal matured, its hindlimbs grew longer and stronger, leading it to transition to a two-legged, bipedal stance.

  • The Sonselasuchus fossils were discovered in 2014 as part of a decade-long excavation project in Petrified Forest National Park, Arizona.
  • Sonselasuchus cedrus lived during the Late Triassic period, approximately 225–201 million years ago.

The players

Sonselasuchus cedrus

An ancient relative of crocodiles that belonged to the shuvosaurid group and lived during the Late Triassic period.

Elliott Armour Smith

The lead researcher from the University of Washington Department of Biology and Burke Museum who studied the Sonselasuchus fossils.

Petrified Forest National Park

The national park in Arizona where the Sonselasuchus fossils were discovered, known for its abundance of petrified wood and other paleontological resources.

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What they’re saying

“By analyzing the proportions of skeletons from different animals, we found that the two-legged posture was likely a result of different growth patterns.”

— Elliott Armour Smith, Lead researcher (Journal of Vertebrate Paleontology)

“Essentially, we think this creature started life walking on four legs... then switched to walking on two legs as they grew up.”

— Elliott Armour Smith, Lead researcher (Journal of Vertebrate Paleontology)

What’s next

Researchers plan to continue studying the Sonselasuchus fossils to gain further insights into the evolution and adaptations of this ancient reptile.

The takeaway

The discovery of Sonselasuchus cedrus and its unique transition from four-legged to two-legged locomotion highlights the remarkable diversity and adaptability of ancient reptiles, and underscores the importance of ongoing paleontological research in places like Petrified Forest National Park.