Postosuchus, terror of the late Triassic
Long before Tyrannosaurus rex, before Giganotosaurus, before the Allosaurs and the rest of the familiar theropod carnivores dominated their respective patches of Jurassic and Cretaceous time and turf, a killer stalked the plains of what is now Arizona. The American southwest 220 million years ago was a section of Pangaea, the western portion of which had fairly well come together by that time. The climate was warm with little seasonal fluctuation and low lying forests and marshes dominated the landscape.
Postosuchus - the name means “crocodile from Post”, a reference to the Post Quarry where a number of fossilized specimens have been found – was not a dinosaur nor was it a crocodile although the two are related. Rather, Postosuchus was a member of the Rauisuchian order of the group Archosauria, a large reptilian group that includes the dinosaurs, crocodiles both living and extinct, the pterosaurs and extinct as well as present day birds.
Absent competition from the dinosaurs, which were represented only by small and relatively feeble emergent specimens at this point in time, carnivorous reptiles like the Rauisuchians were dominant. Postosuchus was the ruler of the mid to late Triassic roost.
The Rauisuchian branch of the Archosauria were a fleet footed, generally quadruped group who unlike lizards and crocodiles walked upright on limbs that descended straight from their bodies, without the characteristic lizard sprawl seen in modern crocodiles. The forward looking eyes in their large orbits, shielded by bony growths indicate very keen, predator style eyesight. Evidence drawn from the skull structure reveals a well developed sense of smell as well.
Weighing as much as a ton, nearly 18 feet nose to tail with a mouthful of sharp and wickedly curved teeth, Postosuchus would have made easy prey of most of his contemporaries. This prey would have consisted of fledgling dinosaurs like Coelophysis, large to mid sized amphibians like the Capitosaurs, as well as other Archosaurs with the bad luck to cross its path. One specimen discovered in Durham County South Carolina had portions of four different contemporary creatures digesting in its gut at the time at the time of its death; these were fossilized along with their voracious killer.
Postosuchus carried a large, sharp hooked claw on each hand, these “hands” and forelegs spur a lively debate among paleontologists which has yet to be definitively settled.
Quadruped or Biped:
Many Rauisuchians were quadrupeds, although there are ample examples of bipedalism as well, some dating back to the earliest members of the order. The forelegs of Postosuchus were only 60 percent the length of the hind limbs, and armed as previously noted with claws designed to clutch and hold prey. Depicted in an upright stance Postosuchus mimics a smallish Tyrannosaur in appearance and indeed was thought to be an ancestral theropod dinosaur when first discovered. There are some who state with some confidence that Postosuchus was in fact a biped.
Others assert with equal confidence that the difference in limb size is not large enough to be conclusive evidence and that given the bulk of the fore body; a quadruped gait is more likely.
It seems certain that whatever the primary form of locomotion might have been, Postosuchus was at least capable of bipedalism for attack, defense, to peer around and perhaps to walk at least part of the time. It also seems likely that whatever the primary gait, creatures coexisting with Postosuchus would have been delighted to see one use it to leave the neighborhood, without stopping for lunch.