Croc ancestor started life on 4 legs before it began walking on 2

By Stephen Beech

A "peculiar” ancient relative of the crocodile started life on four legs before it began walking on two, according to new research.

The poodle-sized reptile that lived over 200 million years ago was among creatures that had physical features similar to dinosaurs it lived beside, say scientists.

The unusual proportions of some of the fossils led he American research team to believe that the creature had to learn how to walk on two feet.

Named Sonselasuchus cedrus, the team explained that the ancestor of today's crocodiles was part of the shuvosaurid group.

Most members of the group an appearance mimicking that of the ornithomimid dinosaurs that it shared the landscape with during the Late Triassic period around 201 to 225 million years ago.

The new study, published in the Journal of Vertebrate Paleontology, was led by experts from University of Washington Department of Biology and Burke Museum in Washington, DC.

Lead author Elliott Armour Smith said: “By analysing the proportions of the limb skeletons of different animals, they determined its bipedal stance - standing on two feet - may have been the result of a differential growth pattern.

“We think that Sonselasuchus had more proportional forelimbs and hindlimbs as young, and their hindlimb grew longer and more robust through adulthood.

“Essentially, we think these creatures started out their lives on four legs… they then started walking on two legs as they grew up.

“This is particularly peculiar.”

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Graduate student Armour Smith conducted the study alongside Burke Museum colleague Professor Christian Sidor who was among the dig team that unearthed the 950 Sonselasuchus fossils, in 2014, from Arizona’s Petrified Forest National Park.

The researchers said Sonselasuchus’ fossils revealed multiple clues about its appearance and 25-inch tall size.

They believe it had a toothless beak, a large eye socket and hollow bones.

Armour Smith said: “Although similar to the ornithomimid dinosaurs these features would have evolved separately.

"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.

“Also, despite the fact that features like bipedalism, a toothless beak, hollow bones and a large orbit are characteristic of ornithomimid theropod dinosaurs, shuvosaurids like Sonselasuchus show that these features evolved on the croc-line as well.”

He says Sonselasuchus would have lived in the forest, and its name cedrus represents the cedar tree, an evergreen conifer similar to those of Late Triassic forests.

The Sonselasuchus part of the name is in recognition of the geologic unit from which the species originates.

Sidor said: “Since starting fieldwork at Petrified Forest in 2014, we have collected over 3,000 fossils from the Sonselasuchus bonebed, and it doesn’t seem to show any signs of petering out."

He added: “In addition to Sonselasuchus, the bonebed has yielded fossils of fish, amphibians, as well as dinosaurs and other reptiles.

"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.”