Postosuchus

Skull cast

A tooth belonging to Postosuchus from the Upper Triassic Chinle Formation.

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.[6] 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).[11] 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".[12] The jaws held large and sharp serrated teeth, of which some were developed even larger to operate as hooked sabers.[13] A complete tooth found among Postosuchus remains in North Carolina measured about 7.2 cm in height.[14] 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.[13] In the lower jaw were over thirty teeth.[13] 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.[15]

With the forelimbs being approximately 64% of the hindlimbs, Postosuchus had small hands bearing five toes.[16] Only the first toe bore a large claw, which was used as an offensive weapon, and the forelimbs were robust, probably to hold prey.[17] The feet were much larger than the hands, with the fifth metatarsal forming a hook shape.[16] The halluxes were slenderer than the other toes and the marginal ones could not touch the ground.[16] As it was a crurotarsan, the heel and ankle of Postosuchus resemble those of modern crocodiles.

Restoration showing quadrupedal pose

The limbs were located underneath the body giving Postosuchus an upright stance.[16] 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.[16] 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.[16] In 1995 Robert Long and Phillip A. Murry argued that Postosuchus was heavily built and quadrupedal.[18] Peyer et al. 2008, argued that the thick pectoral girdle served for locomotion of the forelimbs.[17] They noted that this does not, however, detract from the theory that Postosuchus could also walk bipedally.[17] 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.[3] 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.[19] In a 2022 article Postosuchus was considered predominantly bipedal, but probably still capable of supporting its weight on the forelimbs 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.[20]

Specimens similar to Postosuchus were discovered in Crosby County, Texas in 1920, and described by paleontologist Ermine Cowles Case in 1922.[29][30] The fossils were composed only of an isolated braincase (UM 7473) and fragments of pelvic bones (UM 7244). Case then mistakenly assigned these specimens to the dinosaur genus Coelophysis.[31] In the case of the braincase later assigned to Postosuchus, in 2002 paleontologist David J. Gower argued that the specimen is not complete and may belong to an ornithodire.[32] Between 1932 and 1934, Case discovered other fossils of caudal vertebrae (UMMP 13670) in Rotten Hill, Texas, and a complete pelvis (UCMP V72183/113314) near Kalgary, Texas.[28] Within the same period, paleontologist Charles Lewis Camp collected over a hundred "rauisuchian" bones, from what is now the Petrified Forest National Park of Arizona, which belong to at least seven individuals (UCMP A296, MNA 207C).[28] Later, more remains came to light. In 1943, Case again described a pelvis along with a pubis (UM 23127) from the Dockum Group of Texas, which dates from the Carnian through the early Norian stages of Late Triassic period.[33] These early findings, from 1932 to 1943, were initially referred to as a new phytosaur reptile, but assigned forty years later to Postosuchus.[34]

The first articulated skeleton referred to P. kirkpatricki (CM 73372) was recovered by David S. Berman of the Carnegie Museum of Natural History, in Coelophysis Quarry at Ghost Ranch, New Mexico, between 1988 and 1989.[28] This specimen was composed of a well-preserved skeleton without skull and was described by Long and Murry in 1995, Weinbaum in 2002 and Novak in 2004.[35][36][37] The specimen represents a skeletally immature individual because none of the neural sutures are closed. It was referred to P. kirkpatricki by Long and Murry (1995) without specific justification, and more recent studies accepted this referral.[36][37][26] Nevertheless, Nesbitt (2011) noted that these studies failed to note any synapomorphies unique to P. kirkpatricki and CM 73372. Weinbaum (2002) and Novak (2004) even noted that the preacetabular process of the ilium in CM 73372 was much longer than that of P. kirkpatricki. Nesbitt (2011) also noted that CM 73372 differs from P. kirkpatricki and Rauisuchus in possessing a concave ventral margin of the ilium, and from P. alisonae in processing an asymmetrical distal end of the fourth metatarsal. Nesbitt (2011) couldn't differentiate CM 73372 and Polonosuchus as they overlap only in the caudal vertebrae. A phylogenetic analysis conducted by Nesbitt (2011), one of the most extensive on archosaurs, found CM 73372 to be the most basal crocodylomorph, thus referable neither to P. kirkpatricki nor to Rauisuchidae.[38]

In their description of Vivaron, Lessner et al. (2016) questioned the random referral of all rauisuchid material from the southwestern US to Postosuchus, saying that the discovery of Vivaron stresses the need for a re-appraisal of all material from localities younger or older than unequivocal remains of Postosuchus and Vivaron.[39]