An ancient creature that learned to walk on two legs

(CN) – An ancient crocodilian relative may have started life walking on four legs before switching to two as it matured, a strange developmental twist revealed by fossils found in Arizona.

The animal, a newly described crocodile relative called Sonselasuchus cedrus, lived during the Late Triassic period about 225 million to 201 million years ago.

The researchers say the tiny reptile appears to have changed the way it moved as it grew, a rare developmental change that has not been clearly documented in similar ancient species.

findings, published Monday in the Journal of Vertebrate Paleontology, suggests that the animal’s hind legs became longer and stronger as it matured, eventually allowing it to walk upright.

“By analyzing the proportions of the limb skeletons of different animals, they determined that bipedalism (standing on two legs) may have been the result of a differential growth pattern,” lead author Elliott Armor Smith, a graduate student at the University of Washington, said in a press release. “We think that Sunselasuchus had more proportional forelimbs and hindlimbs when young, and their hindlimbs grew longer and stronger during adulthood.

Researchers believe that these creatures started out on four legs and later began to walk on two legs as they grew.

“This is particularly special,” he said.

Sonselasuchus belonged to a group of reptiles known as shuvosaurids, distant relatives on the crocodile branch of the reptile family tree.

Despite this ancestry, many shuvosaurids closely resembled the fast-running, bird-like dinosaurs called ornithomimids that lived in the same environments.

The researchers say the similarity likely reflects convergent evolution, a process in which unrelated species develop similar traits because they occupy similar ecological roles.

“Although similar to ornithomimid dinosaurs, these characteristics would have evolved separately,” said Armor Smith. “And this similarity was probably due to the fact that croc-line and bird-line archosaurs evolved in the same ecosystems and converged on similar ecological roles.

Traits such as bipedalism, toothless beaks, hollow bones, and large eye sockets are commonly associated with ornithomimid theropod dinosaurs, but fossils show that these traits also evolved independently among crocodilian relatives.

Based on the fossils, the animal was about 25 inches long and likely resembled a small, lightly built reptile with a toothless beak, large eye sockets, and hollow bones.

Researchers say these traits likely helped it move quickly through forested environments during the Late Triassic.

The fossils were discovered in an extremely rich site in Arizona’s Petrified Forest National Park. University of Washington researchers began excavating the site in 2014 and have since found thousands of bones.

Among them were 950 fossils Sunselasuchus making it one of the most abundant animals in the country.

“Since fieldwork began in the Petrified Forest in 2014, we have collected over 3,000 fossils from the Sonselasuchus bone bed, and it does not appear to show any signs of extinction,” Christian Sidor, professor at the University of Washington’s Burke Museum, said in the press release.

The site has yielded a wide range of fossils, including fish, amphibians, dinosaurs and other reptiles, giving scientists a detailed look at ecosystems that existed more than 200 million years ago.

“In addition to Sonselasuchus, the bone bed has yielded fossils of fish, amphibians, as well as dinosaurs and other reptiles,” he said. “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.”