Postosuchus

Postosuchus, meaning 'Crocodile from Post', is an extinct genus of rauisuchid reptiles comprising two species, P. kirkpatricki and P. alisonae, that lived in what is now North America during the Late Triassic. Postosuchus is a member of the clade Pseudosuchia, the lineage of archosaurs that includes modern crocodilians (the other main group of archosaurs is Avemetatarsalia, the lineage that includes non-avian dinosaurs and their descendants, birds). Its name refers to Post Quarry, a place in Texas where many fossils of the type species, P. kirkpatricki, were found. It was one of the apex predators of its area during the Triassic, larger than the small dinosaur predators of its time (such as Coelophysis). It was a hunter which probably preyed on dicynodonts and many other creatures smaller than itself. The skeleton of Postosuchus is large and robust with a deep skull and a long tail. The total body length is about 4 to 5 metres (13 to 16 ft). The extreme shortness of the forelimbs relative to the hind limbs, the very small hands, and measurements of the vertebrae suggest that Postosuchus may have been committed to bipedal locomotion. Postosuchus was one of the largest carnivorous reptiles during the late Triassic. Adults reached around 1.2 m (3.9 ft) in height, 4 m (13 ft) in length from snout to tail tip and their mass might have ranged from 250 to 300 kg (550 to 660 lb). It had a massively built skull bearing dagger-like teeth. The neck was elongated, expanding to a short torso and long tail. Along with remains of the skeleton, paleontologists also identify osteoderms, which were thick plates forming scales. These were on its back, neck, and possibly above or under the tail. The neck of Postosuchus consists of at least eight cervical vertebrae followed by sixteen dorsals, while four co-ossified sacral vertebrae supported the hips. It is thought to be over thirty vertebrae in the tail decreasing in size to the end. The pelvis with the hooked pubis and the rod-like ischium looked like those of carnosaur dinosaurs. The ribcage of Postosuchus had typical archosaur structure, composed of large and slender, curved ribs. In some discoveries ribs were found associated with gastralia, dermal bones located in the ventral region of the body. The skull of Postosuchus was constructed narrow in front and extended wide and deep behind. It was 55 cm in length and 21 cm broad and deep. There are many fenestrae (openings) present in the bones that lighten the skull, providing space for the muscles. Like more derived archosaurs, the lower jaw had mandibular fenestrae (openings at the lower jaw), formed by the junction of the dentary with other jaw bones (surangular and angular). Postosuchus had very good long distant sight, due to large orbits, supporting large and sharp eyes, and strong olfaction provided by elongated nostrils. Inside the skull, under the nostrils, there was a hollow that may have contained the Jacobson's organ, an olfactory sensory organ sometimes referred as the 'sixth sense'. The jaws held large and sharp serrated teeth, of which some were developed even larger to operate as hooked sabers. A complete tooth found among Postosuchus remains in North Carolina measured about 7.2 cm in height. Postosuchus possessed heterodonty dentition, which means each tooth was different in size and shape from the others. The upper jaw contained seventeen teeth, with each premaxilla bearing only four teeth and each maxilla thirteen teeth. In the lower jaw were over thirty teeth. Replacement activity in Postosuchus was different from that of crocodiles, since the replacement tooth didn't fit directly in the pulp cavity of the old tooth, but grew until resorption of the old tooth was complete. With the forelimbs being approximately 64% of the hindlimbs, Postosuchus had small hands bearing five toes. Only the first toe bore a large claw, which was used as an offensive weapon, and the forelimbs were robust, probably to hold the prey. The feet were much larger than the hands, with the fifth metatarsal forming a hook shape. The halluxes were more slender than the other toes and the marginal ones could not touch the ground. Being a crurotarsan, the heel and ankle of Postosuchus resemble those of modern crocodiles. The limbs were located underneath the body giving Postosuchus an upright stance. Historically, there has been debate over whether or not rauisuchids like Postosuchus were mainly bipedal or quadrupedal. Each one of Postosuchus's two forelimbs was slightly over half the size of the hindlimbs. This characteristic of short forelimbs can usually be seen in bipedal reptiles. Chatterjee suggested that Postosuchus could walk in an erect stance, since the short forelimbs were probably used only during slow locomotion. In 1995 Robert Long and Phillip A. Murry argued that Postosuchus was heavily built and quadrupedal. Peyer et al. 2008, argued that the thick pectoral girdle served for locomotion of the forelimbs. They noted that this does not, however, detract from the theory that Postosuchus could also walk bipedally. In 2013, a major study of the skeletal structure concluded that Postosuchus may have been an obligate biped based on evidence from the anatomy of the digits, vertebrae, and pelvis. The proportions of the limbs and weight-bearing sections of the spine were very similar to many theropod dinosaurs, nearly all of which are thought to have been strictly bipedal. During an expedition in 1980, paleontologists of the Texas Tech University discovered a new geological site rich in fossils near Post, Garza County, Texas, US, where a dozen well-preserved specimens belonging to a new rauisuchid were found. In the following years further excavation in the Post Quarry, in Cooper Canyon Formation (Dockum Group), unearthed many remains of late Triassic terrestrial fauna. The holotype of P. kirkpatricki (TTUP 9000), representing a well-preserved skull and a partial postcranial skeleton, was described along with other findings of this new genus by paleontologist Sankar Chatterjee in 1985. A paratype, TTU-P 9002, representing a well-preserved skull and a complete skeleton was also assigned to this species. Chatterjee named the species after Mr. and Mrs. Jack Kirkpatrick who helped during his fieldwork. Subsequently, some specimens (such manus and toe bones) were re-assigned to Chatterjeea and Lythrosuchus; Long and Murry pointed out that many of the juvenile skeletons (TTUP 9003-9011), which Chatterjee assigned to P. kirkpatricki, belong to a distinct genus, named Chatterjeea elegans. Furthermore, in 2006 Nesbitt and Norell argued that Chatterjeea is a junior synonym of Shuvosaurus.