Postosuchus

Postosuchus was a massive, carnivorous reptile that ruled as an apex predator during the Late Triassic period.

As a member of the clade Pseudosuchia, it belonged to the lineage of archosaurs that includes modern crocodiles. This distinguishes it from the Avemetatarsalia, the archosaur lineage more closely related to birds. Postosuchus lived in what is now North America and was much larger than the small dinosaur predators of its era, such as Coelophysis. It likely hunted large, bulky herbivores like dicynodonts, as well as smaller creatures like early dinosaurs.

The anatomy of Postosuchus was built for power and sensory precision. Its skull was massively constructed, appearing narrow at the front but extending wide and deep toward the back. The holotype skull measured 55 cm in length and 21 cm in both breadth and depth. To keep the skull light, it featured several fenestrae, which are openings in the bone that provide space for muscles.

The animal likely possessed excellent long-distance sight due to its large orbits, which housed large, sharp eyes. It also had elongated nostrils that provided strong olfaction, or the sense of smell. Inside the skull, a hollow area under the nostrils may have contained a Jacobson's organ, a sensory organ used to detect chemical signals.

One of the most striking features of Postosuchus was its heterodonty dentition. Heterodonty means that the teeth varied in size and shape throughout the mouth.

The upper jaw held 17 teeth, with 4 in the premaxilla and 13 in the maxilla. The lower jaw contained over 30 teeth. These teeth were large, sharp, and serrated, with some developing into hooked sabers. A complete tooth found in North Carolina reached a height of 7.2 cm. Unlike modern crocodiles, the tooth replacement process in Postosuchus involved a new tooth growing until the resorption of the old tooth was complete.

The body structure of Postosuchus included an elongated neck and a long tail. The neck was composed of at least eight cervical vertebrae, followed by 16 dorsal vertebrae. The torso was relatively short, and the tail was quite long, containing over 30 vertebrae that decreased in size toward the tip.

To protect itself, the animal had osteoderms, which are thick bony plates that formed scales on its neck, back, and possibly its tail. The ribcage followed a typical archosaur structure with large, slender, curved ribs. In some fossils, these ribs were found alongside gastralia, which are dermal bones located in the ventral or belly region of the body.

There has been significant scientific debate regarding the locomotion and posture of Postosuchus. Because the forelimbs were only about 64% the size of the hind limbs, many researchers have studied whether it was bipedal or quadrupedal.

Some scientists, like Chatterjee, suggested an erect, bipedal stance where the short forelimbs were used only for slow movement. Conversely, Long and Murry argued in 1995 that the animal was heavily built and quadrupedal. A 2013 study suggested it may have been an obligate biped, meaning it walked only on two legs, due to its pelvis and spine. However, a 2022 study proposed it was predominantly bipedal but could support its weight on all four limbs at low speeds. This may have changed as the animal grew, an ontogenetic shift where juveniles were likely facultatively quadrupedal.

The history of Postosuchus discovery is tied to several important sites in the United States. Much of what we know comes from the Post Quarry in Texas, where a 1980 expedition found many well-preserved specimens. Sankar Chatterjee described the species P. kirkpatricki in 1985, naming it after the Kirkpatrick family who assisted his fieldwork. Earlier, in 1922, Ermine Cowles Case found fossils in Texas but mistakenly identified them as the dinosaur Coelophysis. Other significant remains were collected by Charles Lewis Camp in Arizona's Petrified Forest National Park. These discoveries helped paleontologists eventually move the specimens from being classified as phytosaurs to being recognized as Postosuchus.

A second species, Postosuchus alisonae, was described following a 1992 discovery in North Carolina by undergraduate students Brian Coffey and Marco Brewer.

This species provided vital clues about the animal's life and diet. The holotype specimen of P. alisonae was unusual because it preserved gut contents, including bones from an aetosaur and a traversodontid cynodont. It was also found positioned on top of a skeleton of the sphenosuchian Dromicosuchus, which showed tooth marks on the skull and neck. This find confirmed the "chimera theory," suggesting that some previously grouped fossils actually belonged to different animals. The discovery of P. alisonae remains the first articulated specimen of a rauisuchian archosaur found in eastern North America.