A newly discovered fossil is shedding light on the evolutionary history of crocodiles, revealing a surprising adaptation in one ancient relative. Sonselasuchus cedrus, which lived over 200 million years ago, appears to have transitioned from a four-legged gait in its youth to walking on two legs as an adult.
The unusual reptile, belonging to the shuvosaurid group, roamed the Earth during the Late Triassic period, roughly between 225 and 201 million years ago. This discovery offers a rare glimpse into how ancient reptiles adapted to their environments, and highlights the complex pathways of evolution.
Researchers from the University of Washington Department of Biology and Burke Museum published their findings in the Journal of Vertebrate Paleontology, detailing how changes in limb proportions suggest this shift in locomotion. The team analyzed skeletal features to identify Sonselasuchus cedrus as a shuvosaurid using a specific identification approach.
A Peculiar Change in Locomotion
The analysis of the reptile’s leg bone proportions indicates a gradual change in body shape as it matured. Lead author Elliott Armour Smith explained that the transformation was likely driven by differing growth patterns between the forelimbs and hindlimbs.
“By analyzing the proportions of the skeletal feet of various animals, we found that a two-legged stance was likely the result of different growth patterns,” Smith said. “We suspect Sonselasuchus had a more balanced set of front and back legs when young, but its hind legs grew longer and stronger as it aged.”
This shift in anatomy directly impacted how the animal moved. “Essentially, we think these creatures started out their lives on four legs… they then started walking on two legs as they grew up,” Smith stated. “This represents particularly peculiar.”
Such a dramatic change in locomotion is rarely observed in the fossil record of reptiles. The finding underscores the adaptability of early archosaurs – the group that includes crocodiles, dinosaurs, and birds – and their ability to exploit diverse ecological niches.
Resemblance to Dinosaurs and a Rich Fossil Site
Despite being a crocodile relative, Sonselasuchus cedrus shared striking similarities with ornithomimid dinosaurs, often nicknamed “ostrich dinosaurs” due to their long legs and bird-like appearance. Still, researchers emphasize that this resemblance is a case of convergent evolution, where unrelated animals independently develop similar traits in response to similar environmental pressures.
“Even though similar to the ornithomimid dinosaurs, these features would have evolved separately,” explained 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.”
The fossils of Sonselasuchus cedrus were unearthed at Petrified Forest National Park in Arizona. A team discovered approximately 950 fossils from the species at the site in 2014, as part of a long-term excavation project spanning over a decade. The species name, cedrus, references the cedar-like trees that grew in the Late Triassic forests where the reptile lived.
The discovery provides valuable insights into the ecosystems of the Late Triassic, a period marked by significant environmental changes and the rise of early dinosaurs. The ongoing research at Petrified Forest National Park continues to reveal new details about the ancient world and the creatures that inhabited it.
