Poposaurus

Poposaurus Fossil range: Late Triassic
Poposaurus gracilis
Scientific classification
Class:	Sauropsida
Order:	Rauisuchia
Superfamily:	Poposauroidea
Family:	Poposauridae
Genus:	Poposaurus Mehl, 1915
Species:	P. gracilis Mehl, 1915 type species P. langstoni Long & Murry, 1995
Synonyms:	Lythrosuchus

Poposaurus is an extinct genus of reptile from the Late Triassic. It was a crurotarsan archosaur which lived in what is now Wyoming, Utah, Arizona, New Mexico and Texas. The type species, Poposaurus gracilis, was described by M. G. Mehl in 1915. The second species, P. langstoni was originally the type species of the genus Lythrosuchus, however discovery of new specimens has led to it being reclassified within Poposaurus.

Poposaurus gives its name to Poposauridae, a family of Late Triassic crurotarsans. Other members of the clade include Shuvosaurus, Sillosuchus and Effigia.

Description

Poposaurus was about 4 metres (13 ft) long, with the tail comprising about half the body length. It is estimated to have weighed around 60 to 75 kilograms (132 to 165 lb), with the largest individuals reaching 90 to 100 kilograms (200 to 220 lb). The body of Poposaurus is laterally compressed, with a long and narrow hip structure. The pubis and ischium are elongated. The end of the pubis forms a distinct hook that is unique to Poposaurus and a few other early pseudosuchians. Poposaurus has five sacral vertebrae connecting the spine to the hip, three more than most early archosaurs. The hind legs are about twice as long as the arms and placed close together. Five digits are present on the foot, but the fifth is reduced to a small splint of bone next to the metatarsals. The middle three toes are well-developed, giving the foot a tridactyl appearance. The calcaneum bone extends far from the ankle to form a distinct heel.

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. 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).

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.

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.

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.

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.

Gallery

External links

• Poposaurus at the Paleobiology Database
• Poposauridae at Palaeos.com
• Dinosaur Mailing List entry which discusses the Poposauridae