Ancient Crocodile Relative Sonselasuchus cedrus Transitioned from Four Legs to Two

A new study has revealed an ancient crocodile relative, Sonselasuchus cedrus, which is believed to have evolved from a quadrupedal (four-legged) stance in its youth to a bipedal (two-legged) stance in adulthood.

This reptile belongs to the shuvosaurid group and lived during the Late Triassic period, approximately 225-201 million years ago.

Research Findings

Findings published in the Journal of Vertebrate Paleontology by experts from the University of Washington Department of Biology and Burke Museum indicate that the unusual proportions of some Sonselasuchus fossils led to the hypothesis of its bipedal development. Lead author Elliott Armour Smith explained that the bipedal stance may have resulted from a differential growth pattern.

Researchers propose that Sonselasuchus had more proportional forelimbs and hindlimbs when young, with its hindlimbs growing longer and more robust into adulthood. This suggests the creatures began life on four legs and transitioned to walking on two legs as they matured.

Discovery and Characteristics

Armour Smith conducted the study with Professor Christian Sidor, who was part of the dig team that unearthed 950 Sonselasuchus fossils in 2014 from Arizona's Petrified Forest National Park. The species name "cedrus" represents the cedar tree, similar to those in Late Triassic forests where the creature lived. The genus name "Sonselasuchus" refers to the Sonsela Member of the Upper Triassic Chinle Formation, the geologic unit where it was found.

The fossils suggest Sonselasuchus was about 25 inches tall, with a toothless beak, large eye sockets, and hollow bones. While these features are characteristic of ornithomimid theropod dinosaurs, experts believe they evolved separately in Sonselasuchus due to convergence in similar ecological roles within shared ecosystems.

Fossil Site Significance

Professor Sidor noted that the Sonselasuchus bonebed, a result of over a decade of fieldwork, has yielded over 3,000 fossils since 2014, including those of fish, amphibians, dinosaurs, and other reptiles. Over 30 University of Washington students and volunteers have contributed to the excavations.