Postosuchus, Ancient Greek, 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 all archosaurs more closely related to birds than to crocodilians). 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 that probably preyed on large, bulky herbivores such as dicynodonts and many other creatures smaller than itself (such as early dinosaurs). The skeleton of Postosuchus is large and robust, with a deep skull and a long tail. It was a large animal, up to 5–6 m (16–20 ft) long or even more. The extreme shortness of the fore limbs relative to the hind limbs, the very small fore paws, and measurements of the vertebrae suggest that Postosuchus may have been committed to bipedal locomotion.
Description
Postosuchus was one of the largest carnivorous reptiles during the late Triassic. The neck of Postosuchus consists of at least eight cervical vertebrae followed by 16 dorsals, while two co-ossified sacral vertebrae supported the hips (it was previously described as four, but these were actually from Shuvosaurus). Along with remains of the skeleton, paleontologists also identified osteoderms, which were thick plates forming scales on its back, neck, and possibly above or under the tail. The pelvis with the hooked pubis and the rod-like ischium looked like those of carnosaurs. 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.
Skull Postosuchus had a massively built skull, bearing dagger-like teeth, which was constructed narrow in front, and extended wide and deep behind. The holotype skull was 55 cm in length and 21 cm broad and deep. Like more derived archosaurs, the lower jaw had mandibular fenestrae (at the lower jaw), formed by the junction of the dentary with other jaw bones (surangular and angular). Inside the skull, under the nostrils, a hollow is seen that may have contained a 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 17 teeth on each side, with each premaxilla bearing only four teeth and each maxilla 13 teeth. The lower jaw had over 30 teeth. Replacement activity in Postosuchus was different from that of crocodiles, since the replacement tooth did not fit directly in the pulp cavity of the old tooth, but grew until resorption of the old tooth was complete.
Limbs and posture With the forelimbs being about 64% the size of the hind limbs, Postosuchus had small hands bearing five toes, of which only the first digit bore a claw. Due to the diminutive size of the hands, whether this claw was especially predominant in predation is uncertain, but it may have helped in grappling prey. The feet were much larger than the hands, with the fifth metatarsal forming a hook shape. As it was 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' two forelimbs was slightly over half the size of the hind limbs. This characteristic of short fore limbs can usually be seen in bipedal reptiles. Chatterjee suggested that Postosuchus could walk in an erect stance, since the short fore limbs 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 fore limbs. 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 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. However, a 2015 study noted several load-bearing adaptations present in the manus of Postosuchus, substantiating the view that its manus was used for support. A 2021 study looked at the influence of femur shape on archosaur posture and found conflicting results for Postosuchus: a larger femur specimen overlapped with fully bipedal archosaurs, while a smaller pair of femora overlapped with quadrupeds. In a 2022 article, Postosuchus was considered predominantly bipedal, but probably still capable of supporting its weight on the fore limbs at low speeds, and an ontogenetic shift was noted, with the shortening of the arms as individuals aged, suggesting that at least juveniles were facultatively quadrupedal, as the smaller paratype specimen (TTUP 9002, which is likely a juvenile) possesses proportionally larger arms then the presumably adult holotype (TTUP 9000).
History
Postosuchus kirkpatricki
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 the Cooper Canyon Formation (Dockum Group), unearthed many remains of late Triassic terrestrial fauna.
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