Not exactly what the little guy anticipated:
A warm misty morning in what will many millions of years in the future become the State of Arizona. A small – less than 6 feet total length, much of that tail – early dinosaur known to science as Coelophysis scampers down a narrow trail between the cycads, ginkgos, ferns and conifers that make up the vast low lying forest that is its home. The little dinosaur is concerned with its breakfast, and is not paying enough attention to the fact that it could become breakfast for something else.
A blood chilling sound, half cough, half roar freezes the little creature for just a second. The ferns are thrust aside and a nightmare shape lurches into view, balanced on two sturdy hind legs, a muscular foreleg smashes down, one terrible hooked claw pins the struggling Coelophysis to the ground. The massive, heavily muscled head darts forward, the dagger sharp teeth do their work.
In little more than a minute, and with barely a squeak, the Coelophysis has vanished.
All but forgotten apex predator:
The fortunate hunter is not a Tyrannosaur or an Allosaur or even a Velociraptor for this is the late Triassic and dinosaurs are in their infancy. The great theropod carnivores will not evolve for many millions of years.
Our slayer is none other than Postosuchus – the name means “Crocodile from Post”, a reference to the Post Quarry where so many of their fossils have been uncovered. Postosuchus was not itself a dinosaur at all but an Archosaur, a member of a reptilian group that gave rise to Crocodiles living and extinct, the Pterosaurs, the dinosaurs and by extension, birds. Postosuchus belonged to the Rauisuchian order of the Archosaur group.
Postosuchus lived in the mid to late Triassic in what is now the southern portion of the United States. For most of its existence it sat at the top of the food chain. Up to 18 feet in length and weighing as much as a ton, when in bipedal mode this creature bore a superficial resemblance to the theropod meat eaters like the Allosaurs. A large, powerful wedge shaped head bore numerous sharp and hooked tearing teeth. Postosuchus had large forward looking eyes implying keen vision, and based on examinations of its skull cavities, a highly developed sense of smell. The animal was well equipped for predatory behavior.
That Postosuchus was a voracious hunter is beyond dispute; a fossil found in South Carolina in 1985 fortuitously preserved the gut contents of the predator, which include portions of an aetosaurid, a cynodont, a dicynodont and a temnospondyl. What it was that killed this particular animal is not immiediately apparent but indigestion can certainly not be ruled out.
Two legs or four?
Rauisuchians are an oddity among reptiles, as they walked with their legs held beneath them like columns much like a modern wolf or a tiger, without the lizard sprawl so typical of modern day crocodiles. They were almost certainly fleet footed and agile and again, adapted to a predatory life style. Most Rauisuchians were quadrupeds and a number of paleontologists believe that Postosuchus may have been one as well, at least part of the time. And yet, the fact that the forelimbs are only sixty percent the length of the hind limb coupled with the fact that the “hands” were adapted for grasping and bore an enlarged, deadly claw argue for at least partial bipedalism.
Perhaps a fossil or fossils yet to be discovered will shed more light on this topic, but for the time being it is probably safe to assume that Postosuchus could have assumed both methods of locomotion as desired.
Passing the torch:
The Rauisuchians did not survive the Triassic. The mass extinction that took place at the Triassic – Jurassic boundary swept them from the board, and paved the way for the more familiar dinosaurs whose life style they foreshadowed.