This ancient crocodile relative walked on four legs as a juvenile, then switched to two

Roughly 220 million years ago, a poodle-sized reptile with a toothless beak, hollow bones, and oversized eye sockets moved through forests of ancient conifers in what is now Arizona. It looked remarkably like the ornithomimid dinosaurs it shared the landscape with. But it was not a dinosaur at all. It was a relative of crocodiles.

Researchers from the University of Washington and the Burke Museum have named this creature Sonselasuchus cedrus (pronounced "sawn-SAY-la-SOOK-us") and published their analysis in the Journal of Vertebrate Paleontology. The name references the Sonsela Member of the Chinle Formation, the geologic unit in Arizona's Petrified Forest National Park where 950 fossils of the animal were excavated beginning in 2014.

But the most unusual finding was not the animal's appearance. It was how it walked -- or rather, how it changed the way it walked as it grew up.

A shift from four legs to two

Lead author Elliott Armour Smith, a graduate student at the University of Washington, analyzed limb proportions across fossils representing different growth stages. The data pointed to a pattern: younger individuals had forelimbs and hindlimbs of roughly proportional size, while adults had disproportionately longer and more robust hindlimbs.

The interpretation is that Sonselasuchus started life walking on four legs and transitioned to bipedal locomotion as it matured. This kind of developmental locomotor shift is, as Smith put it, "particularly peculiar." It has not been previously documented in shuvosaurids -- the broader group of crocodile-line archosaurs to which Sonselasuchus belongs.

"We think that Sonselasuchus had more proportional forelimbs and hindlimbs as young, and their hindlimb grew longer and more robust through adulthood," Smith said. "Essentially, we think these creatures started out their lives on four legs -- they then started walking on two legs as they grew up."

Crocodile relatives that looked like dinosaurs

Sonselasuchus belongs to the shuvosaurid group, members of the crocodile evolutionary lineage (croc-line archosaurs) rather than the bird-dinosaur lineage. Yet many shuvosaurids evolved physical features that closely mirror those of ornithomimid dinosaurs: bipedal stance, toothless beaks, hollow bones, and large eye sockets.

This is a textbook case of convergent evolution -- unrelated lineages independently developing similar features, presumably because they occupied similar ecological roles in the same environments. During the Late Triassic (approximately 225 to 201 million years ago), both croc-line and bird-line archosaurs were diversifying, and some converged on the same body plans despite evolving along completely separate branches of the reptile family tree.

"Although similar to the ornithomimid dinosaurs these features would have evolved separately," Smith explained, "and this similarity was probably due to the fact that croc-line and bird-line archosaurs evolved in the same ecosystems and converged upon similar ecological roles."

A fossil site that keeps producing

The Sonselasuchus bonebed at Petrified Forest National Park has yielded more than 3,000 fossil bones since excavation began in 2014. The site contains not just shuvosaurids but fish, amphibians, dinosaurs, and other reptiles -- a cross-section of Late Triassic terrestrial life.

Professor Christian Sidor, Smith's co-author and a member of the original dig team, noted that the site shows no signs of being exhausted. "Over 30 University of Washington students and volunteers have been involved over the years. It's exciting to see that the site continues to produce new and interesting fossils."

Limits of the fossil evidence

Inferring locomotor behavior from fossil limb proportions involves assumptions. The researchers did not observe Sonselasuchus walking -- they reconstructed its gait from bone dimensions across growth stages. Alternative explanations for the limb proportion changes, such as sexual dimorphism or variation between populations, would need to be ruled out with additional specimens and analysis.

The 950 fossils provide a large dataset by paleontological standards, but assigning individual bones to specific growth stages introduces uncertainty, especially when dealing with disarticulated material from a bonebed where bones from multiple individuals are mixed together.

The "cedrus" species name references cedar trees, chosen because ancient conifers similar to cedars dominated the Late Triassic forests where Sonselasuchus lived -- a detail that connects the animal to its environment and reminds us that these creatures inhabited a world very different from the arid Arizona landscape the fossils were pulled from.