“Peculiar” ancient ancestor of the crocodile started life on four legs in adolescence before it began walking on two
Newly discovered Late Triassic reptile was among creatures that had physical features mimicking the late-evolving dinosaurs it lived beside
Artist's reconstruction of Sonselasuchus cedrus in its environment in what is now Petrified Forest National Park, 215 million years ago (Artwork by Gabriel Ugueto)
A “peculiar” ancient relative of the crocodile which experts believe began life on four legs before, in adulthood, it learnt how to walk on just two has been revealed in a new study.
Named Sonselasuchus cedrus, this archaic reptile was part of the shuvosaurid group, most of which had an appearance mimicking that of the ornithomimid dinosaurs that it shared the landscape with during Late Triassic time (approximately 225-201 million years ago).
In peer-reviewed findings, published today in the Journal of Vertebrate Paleontology, experts from University of Washington Department of Biology and Burke Museum reveal that unusual proportions of some of the fossils led them to believe that this poodle-sized creature had to learn how to walk on two feet.
“By analyzing 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,” explains lead author Elliott Armour Smith.
“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.”
Armour Smith, a graduate student, carried out the study alongside Burke Museum colleague Professor Christian Sidor.
Professor Sidor was among the dig team that unearthed the 950 Sonselasuchus fossils, in 2014, from Arizona’s Petrified Forest National Park – an extraordinary fossil site which in 10 years of excavation and preparation has revealed more than 3,000 fossil bones.
Sonselasuchus’ fossils also reveal many clues about its appearance and 25-inch tall size. It had a toothless beak, a large eye socket, hollow bones, the experts believe.
“Although similar to the ornithomimid dinosaurs these features would have evolved separately,” explains Armour Smith, “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.”
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 (pronounced “sawn-SAY-la-SOOK-us”) is in recognition of the geologic unit (the Sonsela Member of the Upper Triassic Chinle Formation) from which the animal originates.
This bedrock has presented many finds to-date.
For Professor Sidor, this project is a culmination of over a decade of fieldwork in collaboration with the National Park Service.
“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,” Professor Sidor states.
“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.”