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 long or even more.
Description
thumb|left|P. kirkpatricki compared to a human
Postosuchus was one of the largest carnivorous reptiles during the late Triassic. The length of the paratype is estimated up to long, 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 two co-ossified sacral vertebrae supported the hips (it was previously described as four, but these were actually from Shuvosaurus). 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.
Skull
thumb|Skull cast
thumb|A tooth belonging to Postosuchus from the Upper Triassic [[Chinle Formation]]
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 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 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. Peyer et al., 2008, argued that the thick pectoral girdle served for locomotion of the fore limbs. 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 hatchlings and juveniles were facultatively quadrupedal.
History
Postosuchus kirkpatricki
thumb|340px|Postosuchus have been discovered only in the United States within the states of Arizona, New Mexico, North Carolina, and Texas.
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. 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, which they named Chatterjeea elegans. Furthermore, in 2006 Nesbitt and Norell argued that Chatterjeea is a junior synonym of Shuvosaurus.
Postosuchus alisonae
thumb|Reconstructed skeleton of Postosuchus alisonae
In 2008, Peyer and colleagues described a new species of Postosuchus, P. alisonae. This new species was discovered in 1992 by two UNC undergrad students, Brian Coffey and Marco Brewer, at Triangle Brick Co. Quarry, in Durham County, North Carolina. The remains were prepared and reconstructed between 1994 and 1998 by the Department of Geological Sciences at the University of North Carolina. The specific name is in reference to Alison L. Chambers, who worked to popularize paleontology in North Carolina. 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. 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. 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. 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). These early findings, from 1932 to 1943, were initially referred to as a new phytosaur reptile, but assigned 40 years later to Postosuchus. 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.
In their description of Vivaron, Lessner et al. (2016) questioned the historical standard of referring all rauisuchid material from the southwestern US to Postosuchus. They argue that the discovery of a second valid rauisuchid from the area (Vivaron) stresses the need for a reappraisal of all material from localities younger or older than unequivocal remains of Postosuchus and Vivaron.
At least one valid Postosuchus specimen is known from the Chinle Formation at Petrified Forest National Park. This fossil, PEFO 55550, is a collection of fragmentary skull material from the Lot's Wide Bed in the Sonsela Member. Despite its fragmentary nature, it shows diagnostic Postosuchus traits such as a prominent ridge on the maxilla. The 2026 study which described this specimen mentioned other recently-collected rauisuchid fossils from Petrified Forest National Park, including a nearly-complete skull, though further study is required to evaluate whether they belong to Postosuchus or another genus. Plants of the Dockum group are not well known since the oxidizing of the environment has destroyed most of the plant fossils.
Postosuchus was one of the largest animals in that ecosystem and preyed on herbivores in the uplands, such as the dicynodont Placerias. The fauna found in Dockum group confirm that lakes and/or rivers existed containing fish such as the cartilaginous Xenacanthus, the lobe-finned Chinlea, and the dipnoan Ceratodus. On the margins of these rivers and in the uplands lived labyrinthodonts (Latiscopus) and reptiles such as Malerisaurus and Trilophosaurus, and even the archosaurs Coelophysis, Desmatosuchus, Typothorax, Leptosuchus, Nicrosaurus and Rutiodon.