Poposaurus

Poposaurus is an extinct genus of pseudosuchian archosaur from the Late Triassic of the southwestern United States. It belongs to the clade Poposauroidea, an unusual group of Triassic pseudosuchians that includes sail-backed, beaked, and aquatic forms. Fossils have been found in Wyoming, Utah, Arizona, and Texas. Except for the skull, most parts of the skeleton are known. The type species, P. gracilis, was described by M. G. Mehl in 1915. A second species, P. langstoni, was originally the type species of the genus Lythrosuchus. Since it was first described, Poposaurus has been variously classified as a dinosaur, a phytosaur, and a "rauisuchian".

Like theropod dinosaurs, Poposaurus was an obligate biped, meaning that it walked on two legs rather than four. However, as a pseudosuchian, it is more closely related to living crocodilians than to dinosaurs.[1] Poposaurus is thought to have evolved this form of locomotion independently, possibly from early archosaurs' ability to high walk.

Description
Poposaurus is an extinct genus of pseudosuchian archosaur from the Late Triassic of the southwestern United States. It belongs to the clade Poposauroidea, an unusual group of Triassic pseudosuchians that includes sail-backed, beaked, and aquatic forms. Fossils have been found in Wyoming, Utah, Arizona, and Texas. Except for the skull, most parts of the skeleton are known. The type species, P. gracilis, was described by M. G. Mehl in 1915. A second species, P. langstoni, was originally the type species of the genus Lythrosuchus. Since it was first described, Poposaurus has been variously classified as a dinosaur, a phytosaur, and a "rauisuchian".

Like theropod dinosaurs, Poposaurus was an obligate biped, meaning that it walked on two legs rather than four. However, as a pseudosuchian, it is more closely related to living crocodilians than to dinosaurs.[1] Poposaurus is thought to have evolved this form of locomotion independently, possibly from early archosaurs' ability to high walk.

History
The first remains of Poposaurus were found in 1904 near Lander, Wyoming. In 1907, paleontologist J. H. Lees described this fossil, an ilium (part of the hip) from the Popo Agie Formation, and identified it as that of the phytosaur Paleorhinus bransoni.[2] In 1915, paleontologist M. G. Mehl named Poposaurus based on more complete material from the Popo Agie Formation, including vertebrae, hips, and limb bones. He cited the holotype as [Walker Museum] 602, but in fact the holotype is UR 357. Mehl concluded that the ilium described by Lees, UR 358, also belonged to Poposaurus. He did not classify Poposaurus as a phytosaur because the shape of its ilium was different and it had more sacral vertebrae fused to the hip. Mehl made comparisons between Poposaurus and the earlier named Dolichobrachium, also from the Triassic of Wyoming. Dolichobrachium was only known from some teeth, a humerus, and part of the pectoral girdle, so Mehl suggested that the Poposaurus and Dolichobrachium material could belong to the same animal. Mehl noted similarities between Poposaurus and theropod dinosaurs, including its hollow leg bones and deep hip socket, but did not consider it a dinosaur because each sacral vertebra supported only one rib (theropods usually have multiple ribs projecting from each sacral vertebra).[3]

In the following years, Poposaurus was assigned to many different groups of reptiles. Hungarian paleontologist Franz Nopcsa classified it as an ornithischian dinosaur in 1921, identifying similarities with iguanodonts and camptosaurs. In 1928, Nopcsa placed it in a new family called Poposauridae and a new suborder called Poposauroidea. To Nopsca, Poposauroidea was one of three suborders that made up the order Ornithopoda. Over the following years, many paleontologists supported this classification. For example, German paleontologist Oskar Kuhn classified Poposaurus in its own suborder of ornithischians, which he called Poposauria. In 1930, American paleontologist Oliver Perry Hay placed Poposaurus in Anchisauridae, a family of sauropodomorph dinosaurs. German paleontologist Friedrich von Huene considered it a very early stegosaur in 1950.[4]

In 1961, American paleontologist Edwin Harris Colbert gave an extensive description of the known material of Poposaurus and classified it as a theropod dinosaur. Colbert thought that Poposaurus could not have been a more primitive archosaur because it had hollow leg bones and complex vertebrae. He placed it in the Carnosauria, but because its ilium was distinct from all other archosaurs, Colbert placed Poposaurus in its own family, Poposauridae. In the same paper, Colbert described an ilium from the Dockum Group of Howard County, Texas, which he assigned to P. gracilis.[4]

In his 1977 study of Late Triassic saurischians, Peter Galton reclassified Poposaurus as a thecodont pseudosuchian. In 1915, Mehl described a "distal femur" in the holotype specimen of Poposaurus, but Galton interpreted this to be the fused end of the hip's pubis bones. Galton noted similarities between the hips of Poposaurus, Arizonasaurus, Bromsgroveia, Postosuchus, and Teratosaurus, and grouped them all in Poposauridae. Like paleontologists before him, Galton distinguished Poposaurus based on the unique shape of its ilium.[5]

In 1995, paleontologists Robert Long and Phillip Murry described new fossils of Poposaurus from the Placerias quarry in the Chinle Formation of Arizona. Among the new material were parts of the lower limb, including the tibia and calcaneum. They removed Postosuchus from Poposauridae, claiming that the material used in this assignment was a chimera, or a collection of bones belonging to different animals. The pubis of Postosuchus was in fact a pubis of Poposaurus, leading to the mistaken classification. Long and Murry separated poposaurids like Poposaurus, Bromsgroveia, and the newly named Lythrosuchus from rauisuchians like Postosuchus, which they held in the family Rauisuchidae

The known material of Poposaurus was again described in 2007, along with two new specimens from the Tecovas Formation of Texas and the Petrified Forest of Arizona. Long and Murry's Lythrosuchus langstoni was reclassified as a new species of Poposaurus, P. langstoni. P. langstoni differs from P. gracilis in that it is larger, it does not have a ridge of bone behind the hip socket, and does not have a pit on the ischium that fits into the ilium.[7] In 2011, a nearly complete specimen of P. gracilis called YPM VP 057100 was found in the Chinle Formation of Grand Staircase-Escalante National Monument, Utah. It includes the forelimbs, hind limbs, hips, ribs, dorsal vertebrae, and much of the tail. Most of the skeleton of Poposaurus is now known, except for the skull.