Ancient Crocodile Species Reveals Bizarre Walking Pattern Shift
Paleontologists have uncovered a remarkably unusual ancient crocodile species that walked on four legs during its infancy before switching exclusively to two legs as it matured into adulthood. The discovery, based on fossils more than 200 million years old excavated in Arizona, challenges long-held assumptions about the evolutionary pathways of crocodilian ancestors.
A Peculiar Transition in Locomotion
Named Sonselasuchus cedrus, this prehistoric reptile inhabited Earth during the Late Triassic period, approximately 225 to 201 million years ago. Researchers from the University of Washington Department of Biology and the Burke Museum of Natural History and Culture have published their findings in the Journal of Vertebrate Paleontology, detailing how the creature's limb bone proportions indicate a dramatic change in walking behavior over its lifespan.
Lead author Elliott Armour Smith explained, "We believe that Sonselasuchus possessed more proportional forelimbs and hindlimbs as juveniles, with the hindlimbs growing significantly longer and more robust through adulthood. Essentially, these creatures started their lives moving on four legs and then began walking exclusively on two legs as they grew older. This is particularly peculiar and unprecedented in the fossil record."
Striking Resemblance to Dinosaurs
The reptile, estimated to have stood around 64 centimeters (25 inches) tall, exhibited several distinctive features including a toothless beak, large eye sockets, and hollow bones. These characteristics bear an uncanny resemblance to ornithomimid dinosaurs, often referred to as "ostrich dinosaurs." However, Armour Smith emphasized that these traits evolved independently within the crocodile lineage.
"This similarity likely resulted from croc-line and bird-line archosaurs evolving within the same ecosystems and converging upon similar ecological roles," he noted. Shuvosaurids like Sonselasuchus demonstrate that attributes such as bipedalism, a toothless beak, hollow bones, and large orbits also emerged independently within the crocodile evolutionary line.
Rich Fossil Discovery in Arizona
The fossils of Sonselasuchus cedrus were first discovered in 2014 at Petrified Forest National Park in Arizona by a team including Professor Christian Sidor, a colleague of Armour Smith. The excavation site has proven exceptionally fruitful, yielding over 3,000 bones from a decade of meticulous digging and preparation.
Professor Sidor reflected on the ongoing discoveries, stating, "Since commencing fieldwork at Petrified Forest in 2014, we have collected more than 3,000 fossils from the Sonselasuchus bonebed, and it shows no signs of diminishing." The bonebed has also provided fossils of fish, amphibians, dinosaurs, and various other reptiles, with contributions from over 30 University of Washington students and volunteers continuously unearthing new and intriguing specimens.
Naming and Environmental Context
The species name cedrus references the cedar trees that characterized the forested environments of the Late Triassic, where the reptile is believed to have thrived. The genus name Sonselasuchus acknowledges the Sonsela Member of the Upper Triassic Chinle Formation, the specific geological layer where the fossils were found.
This groundbreaking research not only sheds light on the diverse adaptations of ancient crocodilians but also underscores the complexity of evolutionary convergence between different archosaur lineages. The study continues to reveal how prehistoric creatures adapted their locomotion in response to environmental pressures and ecological niches.
