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 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 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 length of the paratype is estimated up to long, and an individual of such length would have measured tall at the head when stood upright, and weighed around . The holotype is estimated up to long, and the largest known individual may measure up to long or more based on a complete cervical series specimen (TTU-P 9235).

The neck of ''Postosuchus'' consists of at least eight cervical vertebrae followed by 16 dorsals, while four co-ossified sacral vertebrae supported the hips. The neck was elongated, expanding to a short torso and long tail. 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. It is thought to have had over 30 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 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.Peyer ''et al.'' (2008), p. 370.

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. Many fenestrae (openings) are present in the bones that lighten the skull, providing space for the muscles. 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).

''Postosuchus'' likely had very good long-distance sight, due to large orbits, supporting large and sharp eyes, and strong olfaction provided by elongated nostrils. 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.Peyer ''et al.'' (2008), p. 368. ''Postosuchus'' possessed heterodonty dentition, which means each tooth was different in size and shape from the others. The upper jaw contained 17 teeth, 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 ia uncertain, but it may have helped in grappling prey. Peyer ''et al.'' (2008), p. 380. The feet were much larger than the hands, with the fifth metatarsal forming a hook shape. The innermost two digits were less robust than the other toes, and likely could not touch the ground. 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 ''Postosuchuss 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.Long and Murry (1995), p. 139. 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 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. 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. 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 hatchlings and juveniles were facultatively quadrupedal.

History

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 (1995), pp. 148–162. 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''.Long and Murry (1995), pp. 154–162. Furthermore, in 2006 Nesbitt and Norell argued that ''Chatterjeea'' is a junior synonym of '' Shuvosaurus''.Nesbitt and Norrell (2006), pp. 1045–1048.

In 2008, Peyer ''et al.'', described a new species of ''Postosuchus'', ''P. alisonae'' that was discovered by two UNC undergrad students, Brian Coffey and Marco Brewer in 1992 in Triangle Brick Co. Quarry, Durham County, North Carolina.Peyer ''et al.'' (2008), p. 365. The remains were prepared and reconstructed between 1994 and 1998 by the Department of Geological Sciences at the University of North Carolina.Peyer ''et al.'' (2008), p. 363. 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. Moreover, the well-preserved remains of ''P. alisonae'' shed new light on parts of ''Postosuchus'' anatomy, which were previously not well known. Specifically, the differences between the manus bones of ''P. kirkpatricki'' and ''P. alisonae'' confirm the chimera theory (associated fossils belonging to different animals) suggested by Long and Murry. The holotype specimen of ''P. alisonae'' ( UNC 15575) is also unusual in its preservation of gut contents: bones from at least four other animals, including a partial skeleton of an aetosaur, 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. ''P. alisonae'' represents the largest suchian reptile recovered from the quarry and the first articulated specimen of 'rauisuchian' archosaur found in eastern North America.

Putative occurrences

Specimens similar to ''Postosuchus'' were discovered in Crosby County, Texas, in 1920, and described by paleontologist Ermine Cowles Case

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