At a glance, it would be tempting to call Poposaurus gracilis a dinosaur. This 225-million-year-old reptile stood on two legs, had small forelimbs, and sported a long, tapering tail that allowed it to balance while walking and running about the Late Triassic landscape. But Poposaurus wasn’t a dinosaur at all. It was much more closely related to the forerunners of crocodiles, and, according to a new study, its curious mode of walking  challenges a leading hypothesis about why dinosaurs were so successful.
First described over a century ago, Poposaurus is a “rauisuchian,” part of an extinct lineage of reptiles whose diverse array of members included the precursors of crocodiles and their closest relatives. Rauisuchians differed from crocodiles as we know them today in holding their limbs upright beneath their bodies rather than out to the side. This arrangement made them more efficient at walking and running on land, and, until recently, all rauisuchians were thought to have walked on all fours.
Then in 2006, paleontologists Sterling Nesbitt and Mark Norell of the American Museum of Natural History in New York City described a bipedal rauisuchian they called Effigia okeeffeae. The discovery showed that at least some of the rauisuchians adapted a very dinosaurlike posture. Now, a new skeleton of Poposaurus described by Nesbitt, Yale University paleontologist Jacques Gauthier, and co-authors in the current edition of the Bulletin of the Peabody Museum of Natural History confirms that it, too, walked like a dinosaur. Instead of being an evolutionary fluke, the new find suggests that Effigia was part of a specialized subgroup of bipedal crocodile cousins that diversified at the same time as the early dinosaurs.
The newly described Poposaurus was found in the Triassic Chinle Formation of southern Utah’s Grand Staircase-Escalante National Monument. The fossil included most of the postcranial skeleton from midway down the neck to the tip of the tail, stretching about 4.5 meters in all. According to the new research, the anatomy of the creature’s pelvis and hind limbs shows that Poposaurus walked upright, planting its feet close to the midline of its body. In fact, the hip anatomy would have made it impossible for Poposaurus to sprawl its limbs out to the side, like living crocodiles, although the exact way the rauisuchian’s feet touched the ground is unclear. Poposaurus may have stood on tiptoe, walked with the whole foot touching the ground, or it might have alternated between both foot postures depending on how fast it was moving.
Discoveries of bipedal crocodile cousins such as Effigia and Poposaurus have also raised questions about how dinosaurs rose to dominance near the end of the Triassic. Paleontologists such as Robert Bakker, curator of the Houston Museum of Natural Science in Texas, had previously credited the upright, bipedal posture of the dinosaurs for their success over the variety of crocodile relatives that flourished during the Triassic. By holding their limbs directly beneath their bodies, the argument went, dinosaurs would have moved faster and more efficiently than the cousins of crocodiles and relatives of early mammals that also lived at the time. The subgroup of rauisuchians that includes Poposaurus independently evolved the same posture—and competed with early dinosaurs such as Coelophysis in the same environments—but why they perished while dinosaurs thrived is a mystery.
“This new specimen is important because it is so complete,” says Randall Irmis, curator of paleontology at the Utah Museum of Natural History at the University of Utah in Salt Lake City, who found the skeleton in 2003 but was not involved in the new study. In addition to finally illustrating what the body of Poposaurus looked like, he says, the newly described specimen “helps us understand that transition to bipedality” among the rauisuchians. Whereas Effigia had a toothless beak, Irmis points out, Poposaurus had sharp, recurved teeth and was clearly a carnivore like other rauisuchians, “so it shows poposauroids became bipedal before they evolved an herbivorous habit.”