Postosuchus

Postosuchus was a basal archosaur which lived in what is now North America during the middle through to the late Triassic period (228-202 million years ago). It was a rauisuchian, a cousin of crocodiles and came from the same ancestry as dinosaurs. Its name means "crocodile from Post", named after the Post Quarry in Texas, where many fossils of the species were found. It was one of the top 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.

Postosuchus was a quadrupedal reptile with a wide skull and a long tail. This carnivore attacked with its large curved claws.[1] It was about 4-5 meters long (12-15 feet),[1] and was held up by columnar legs (a quite uncommon feature in reptiles). A crocodile-like snout, filled with many large-sized dagger-like teeth, was used to kill its prey. Rows of protective plates covering its back formed a defensive shield.

General description
Postosuchus was one of the largest carnivorous reptiles during the late Triassic. Adults reached around 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).[3] It had a massively built skull bearing dagger-like teeth.[4] The neck was elongated, expanding to a short torso and long tail.

Posture
Each one of Postosuchus's two forelimbs was slightly over half the size of the hindlimbs.[5] 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.[5] However, in 1995 Robert Long and Phillip A Murry stated that Postosuchus was heavily built and quadrupedal.[6] There is debate over whether Postosuchus was bipedal or quadrupedal and scientists aren't certain, yet, about its gait. However, it has also been suggested by some palaeontologists that Postosuchus might possibly have been a facultative biped, meaning that it alternated between walking bipedally, occasionally, and quadrupedally.

Skull
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.[4] 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).[7] 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 hollowing that probably was for the Jacobson's organ, an olfactory sensory organ sometimes referred as the "sixth sense".[8] The jaws held large and sharp serrated teeth and in some points were developed even larger to operate as hooked sabers.[9] A complete tooth found among Postosuchus remains in North Carolina measured about 7.2 cm in height.[10] Postosuchus possessed heterodonty dentition, which means each tooth was different in size and shape from the others. The upper jaw composed of seventeen teeth, with each premaxilla bore only four teeth and each maxilla thirteen teeth.[9] In the lower jaw were counted over thirty teeth.[9] 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.

Postcranial anatomy
The neck of Postosuchus consists of at least eight cervical vertebrae followed by sixteen dorsals, while four co-ossified sacral vertebrae supported the hips.[12] It is thought to be over thirty vertebrae in the tail decreasing in size to the end.[12] The pelvis with the hooked pubis and the rod-like ischium looked like those of carnosaur dinosaurs.[13] 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 ribcage of Postosuchus had typical archosaur structure, composed of large and slender, curved ribs.[14] In some discoveries ribs were found associated with gastralia, dermal bones which located in the ventral region of the body.[15] The limbs were located underneath the body giving Postosuchus an upright stance.[5] With the forelimbs being approximately 64% of the hindlimbs, Postosuchus had small hands bearing five toes.[5] 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.[16] Peyer et al. 2008, argued that the thick pectoral girdle served for locomotion of the forelimbs.[16] However, this doesn't detract from the theory that Postosuchus could walk bipedally.[16] The feet were much larger than the hands, with the fifth metatarsal forming a hook shape.[5] The halluxes were more slender than the other toes and the marginal ones couldn't touch the ground.[5] As crurotarsan, the heel and ankle of Postosuchus resemble those of modern crocodiles.

P. alisonae
A partial skeleton of Postosuchus from Durham County, North Carolina was discovered in 1994 and named in 2008 as new species P. alisonae. The specific name is in reference to Alison L. Chambers, who worked to popularize paleontology in North Carolina. The skeleton of P. alisonae consists of a few cranial bones, seven neck, one back, and four tail vertebrae, ribs, gastralia ("belly ribs"), chevrons, bony scutes, much of the shoulder girdles, most of the forelimbs except the left wrist and hand, most of the hindlimbs except for the thigh bones, and pieces from the hip. This specimen is unusual in preserving gut contents: bones from at least four other animals, including a partial skeleton of an aetosaurid, a snout, coracoid, and humerus of the traversodontid cynodont Plinthogomphodon, two phalanges from a dicynodont, and a possible temnospondyl bone. Furthermore, the Postosuchus was positioned on top of a skeleton of the sphenosuchian Dromicosuchus, which included tooth marks on the skull and neck.