Poposaurus

***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.

The known Poposaurus material was described once again in 2007, along with two new specimens from the Tecovas Formation of Texas and the Petrified Forest National Park of Arizona by Weinbaum and Hungerbühler. 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, does not have a rim behind the acetabulum, and does not have a pit in the ischium that fits into the iliacus. In 2011, a nearly complete specimen of P. gracilis , identified as YPM VP 057100, was found in the Chinle Formation in Grand Staircase-Escalante National Monument, Utah. This includes the front limbs, the hind limbs, the hips, ribs, dorsal vertebrae and almost the entire tail. Except for the skull, most of the skeleton of Poposaurus is now known.

## Classification

Poposaurus is a member of the family Poposauridae, part of the larger pseudosuchian group Poposauroidea. It is a close relative of other Triassic pseudosuchians such as ctenosauriscids and shuvosaurids. Like Poposaurus , shuvosaurids were bipedal. When specimen YPM VP 057100 was described by Gauthier et al. In 2011, Poposaurus was included in a phylogenetic analysis. Poposaurus was placed within Poposauroidea as the sister taxon of the enormous herbivore Lotosaurus and the shuvosaurids. This means that Poposaurus is more closely related to Lotosaurus and the shuvosaurids than to any other pseudosuchian. The analysis found that ctenosauriscids and the aquatic Qianosuchus were successively more basal poposauroids. Below is the cladogram from Gauthier et al. (2011):

## Paleobiology

### Locomotion

When MG Mehl originally named Poposaurus in 1915, he described it as "a muscular creature of light weight, possibly of occasional bipedal posture, and most certainly of rapid movement." Mehl based his description on the long limb bones and the deep acetabulum, two characteristics that related it to bipedal dinosaurs. However, since the 1970s Poposaurus has been considered a pseudosuchian archosaur, phylogenetically closer to crocodiles than to dinosaurs. Many of their close relatives (such as the large rauisuchids and ctenosauriscids) were exclusively quadrupedal, they could not stand on two legs. Although its complete skeleton was unknown, Poposaurus was expected to be similar in appearance to its relatives. In 2011, the nearly complete specimen YPM VP 057100 was described. This specimen confirmed Mehl's description, revealing that Poposaurus was indeed bipedal. The skeleton preserves both the front and hind limbs, showing that Poposaurus had arms shorter than its legs.

Although Poposaurus and the first dinosaurs were bipedal, this method of locomotion developed independently in each group. Their independent origin can be seen through several differences in the skeletons of Poposaurus and dinosaurs. Unlike these, Poposaurus had the distinctive crurotarsal ankle of pseudosuchians, usually associated with quadrupedal locomotion. Poposaurus also had an "upright pillar" posture in which the acetabulum or hip fossa is oriented downward and placed directly over the head of the femur. In contrast, dinosaurs have an "upright buttress" hip structure in which the acetabulum is oriented laterally and the head of the femur is angled to fit into the hip.

Despite the independent origin of bipedalism in Poposaurus and dinosaurs, they inherited the tendency to move upright propelled by their hind legs from a common ancestor. The posture of this ancestral archosaur can be inferred from a method called phylogenetic support. Archosauria is a crown group represented today by birds and crocodilians, meaning that the first archosaur was the last common ancestor of birds. and the crocodiles. All birds have a fixed upright posture, and crocodilians have the ability to walk upright with their limbs raised. If upright posture is a homologous trait in birds and crocodiles, phyogenetic support implies that they inherited that characteristic from their common ancestor and that that ancestor also walked upright. With this reasoning, it is assumed that the first archosaurs could walk without dragging their bodies. Poposaurus and the dinosaurs achieved a bipedal posture by increasing the size of their hind legs, strengthening their hips and adapting their columns to dorsoventral flexion. Other adaptations that may have facilitated bipedal locomotion include the development of a heart divided into chambers and lungs with unidirectional airflow (both of which are assumed to be present in Poposaurus through phylogenetic support).

A 2011 study presented the reconstruction of the limb musculature of Poposaurus , based on the marks of the muscle insertions on the bones and on phylogenetic inferences. 26 muscles, three ligaments and two connective tissue structures were described. Although the reconstructed muscles of Poposaurus share several aspects with those of birds, they are most similar to those of crocodilians. Poposaurus is assumed to have had a pubioischiotibial muscle, absent in birds and probably also in non-avian dinosaurs. Short extensor digitorum muscles were probably present in the foot of Poposaurus , but have disappeared in birds. The external pubioischiofemoral muscle of Poposaurus is also similar to that of modern crocodilians. Other aspects of Poposaurus ' muscles differ from those of crocodilians. For example, the internal pubioischiofemoral muscle originates from the spine in crocodilians and from the hip in Poposaurus . That it originates in the hip is a trait considered the ancestral condition in archosaurs, since it has also been seen this way in birds and non-avian dinosaurs. It is believed that Poposaurus had adductor muscles that were even larger than those of dinosaurs, and their insertion zone ran the entire length of the femur.

## Breathing

The 2011 study of Poposaurus leg muscles also suggested a distinctive form of breathing involving abdominal muscles. The hypothesis presented is that Poposaurus had an ischiotroncal muscle, which went from the ischium, at the back of the hip, through the pubis and reached the gastralia of the abdomen. In a form of breathing called cuirasal breathing, the hamstring might contract and compress the trunk, pumping the lungs.

## Appearance in other media

### *Jurassic Park*

* *Poposaurus* has been appear in *Jurassic World: Epic Evolution* toyline.

## References

1. ↑ Mehl MG. 1915. *Poposaurus gracilis*, a new reptile from the Triassic of Wyoming. *Journal of Geology* **23**: 516-522.
2. ↑ Long RA, Murry PA. 1995. Late Triassic (Carnian and Norian) tetrapods from the Southwestern United States. *New Mexico Museum of Natural History & Science Bulletin* **4**: 1-254.
3. ↑ Weinbaum JC, Hungerbühler A. 2007. A revision of *Poposaurus gracilis* (Archosauria: Suchia) based on two new specimens from the Late Triassic of the southwestern U.S.A. *Palaeontologische Zeitschrift* **81**(2):131-145.
4. ↑ Nesbitt SJ, Norell MA. 2006. Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). *Proceedings of the Royal Society of London B* **273**: 1045–1048.
5. ↑ Nesbitt S. 2007. The anatomy of *Effigia okeeffeae* (Archosauria, Suchia), theropod-like convergance, and the distribution of related taxa. *Bulletin of the American Museum of Natural History* **302**: 84 pp.