A 25-Inch Crocodile Relative Walked on Two Legs in Late Triassic Forests 225 Million Years Ago
Some animals learn new skills as they get older. This ancient relative of the crocodile may have taken this idea to the extreme: starting life on four legs before eventually learning to walk on two.
The unusual reptile, named Sonselasuchus cedruslived at the end of the Triassic, approximately 225 to 201 million years ago. The reptile likely lived in forest environments that covered much of the region at this time. The name of the species Cedar refers to the cedar-like trees that grew in the forests of the Late Triassic. In a study published in the Journal of Vertebrate PaleontologyThe researchers report that changes in the animal’s limb proportions suggest that it gradually transitioned from walking on four legs as a juvenile to walking on two legs as an adult.
“Essentially, we think these creatures started life on four legs…they then began walking on two legs as they grew,” lead author Elliott Armor Smith said in a press release. “It’s particularly special.”
Learn more: Jurassic predators feasted on long-necked baby dinosaurs 150 million years ago
A Triassic crocodile relative that evolved a dinosaur-like body
Although it belongs to the crocodile branch of the reptile family tree, Sonselasuchus would have looked a lot like a dinosaur.
The animal was about 25 inches tall, about the size of a small dog, and probably had a toothless beak, large eye sockets and hollow bones. At first glance, these features resemble those of ornithomimid dinosaurs, long-legged animals sometimes compared to ostriches.
But the resemblance appears to be a case of convergent evolution, when unrelated animals independently develop similar traits.
“Although similar to ornithomimid dinosaurs, these features would have evolved separately,” Armor Smith explained, “and this similarity was likely because archosaurs of the crocodile lineage and the bird lineage evolved in the same ecosystems and converged on similar ecological roles.”
In other words, animals from different evolutionary branches may have evolved similar bodies because they fulfilled similar roles in the same ancient environments.
How this reptile went from four legs to two
Evidence of the reptile’s unusual movement comes from the proportions of its limb bones.
By comparing the skeleton of Sonselasuchus Along with those of related reptiles, researchers found that the animal’s hind legs appear to have become longer and stronger compared to its forelimbs as it grew.
This change may have shifted the animal’s balance toward its hind legs, allowing adults to walk upright while younger individuals likely moved on all fours.
The researchers describe this as a differential growth pattern, meaning that different parts of the body grew at different rates as the animal developed.
Arizona Petrified Forest Fossil Site Produces Thousands of Triassic Bones
The fossils of Sonselasuchus were discovered in Arizona’s Petrified Forest National Park, one of the most important Late Triassic fossil sites in North America.
Excavations at the site began in 2014 and over the past decade, researchers have discovered more than 3,000 fossil bones, including around 950 specimens belonging to the newly identified reptile.
“In addition to Sonselasuchusthe bone bed yielded fossils of fish, amphibians, as well as dinosaurs and other reptiles. More than 30 University of Washington students and volunteers have been involved over the years. It’s exciting to see that the site continues to produce interesting new fossils,” said Christian Sidor, co-author of the study.
For researchers, discoveries like Sonselasuchus highlight how diverse ancient reptiles were and how much we are still learning about the animals that lived alongside the first dinosaurs.
Learn more: 90-million-year-old fossil reveals adult dinosaur that weighed less than 2 pounds
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