Late Triassic crocodile relative transitioned from four legs to bipedal movement during maturation

Fossils from Arizona reveal Sonselasuchus cedrus, an ancient reptile that began life on four legs before transitioning to a two-legged stance as an adult.

By: AXL Media

Published: Apr 23, 2026, 8:27 AM EDT

Source: Information for this report was sourced from Taylor & Francis Group

A Rare Developmental Shift in Locomotion

The discovery of a peculiar prehistoric reptile is challenging long-held assumptions about how ancient vertebrates navigated their environments. Researchers have identified a specific crocodile relative that appears to have undergone a radical physical transformation as it matured, shifting from a quadrupedal gait to an upright, bipedal stance. According to Elliott Armour Smith, a graduate student at the University of Washington, this transition was driven by a differential growth pattern where the hindlimbs became disproportionately longer and more robust during adulthood. This developmental flexibility is exceptionally rare in the fossil record and suggests that certain Triassic species occupied highly specialized ecological niches as they aged.

Massive Fossil Assembly at Petrified Forest

The species, named Sonselasuchus cedrus, was identified following the excavation of an extraordinarily rich bonebed in Arizona’s Petrified Forest National Park. Since 2014, a team led by Professor Christian Sidor has recovered more than 3,000 fossils from the site, including 950 bones belonging specifically to the new shuvosaurid. This extensive collection has allowed paleontologists to compare individuals at various stages of growth, providing a comprehensive view of the animal's life cycle. The site remains a fertile ground for discovery, yielding not only the remains of these crocodile relatives but also those of ancient fish, amphibians, and early dinosaurs.

Convergent Evolution and Dinosaur Mimicry

Despite its striking physical resemblance to ornithomimid dinosaurs, Sonselasuchus is technically a member of the "croc-line" of archosaurs. The reptile possessed several features typically associated with bird-line dinosaurs, such as hollow bones, large eye sockets, and a toothless beak. However, scientists emphasize that these traits evolved independently through a process known as convergent evolution. Because shuvosaurids and early dinosaurs shared the same ecosystems, they likely converged upon similar survival strategies and ecological roles, leading to the development of comparable anatomical structures in entirely different lineages.