The Rauisuchians

The Middle Triassic Ticinosuchus goes for a stroll

The rauisuchians were a group of Triassic archosaurs that usually grew between three and thirty feet in length and stood between two and a half and nine feet tall. Nowadays the group “Rauisuchia” is considered an evolutionary grade or ‘wastebin taxon’ in that it includes most of the large, predatory carnivorous pseudosuchians that lived during the Triassic Period and which didn’t fall into the crocodylomorpha. Modern studies indicate that the rauisuchians weren’t a natural group but is a ‘catch-all’ that includes a number of smaller clades, such as the proper Rauisuchidae, the Ctenosauriscidae, and the Shuvosauridae. Thus the ‘rauisuchians’ become a group of pseudosuchian archosaurs that are more closely related to one another than they are to other pseudosuchians. Many rauisuchians are well-known, such as the Middle Triassic Ticinosuchus of Europe, the Late Triassic Saurosuchus of Argentina, and the Late Triassic Postosuchus of North America. 

a pair of Postosuchus attack the aetosaur Desmatosuchus

The rauisuchians didn’t blow up to epic proportions until the Late Triassic. At their genesis earlier in the Triassic, they were relatively small and would’ve fed on small lizards and proto-mammals. As they grew larger as the millions of years eked by, they became better suited for taking down large prey, such as the tusked dicynodont therapsids that flourished during the Triassic. Towards the Middle and Late Triassic they became the top predators, the terror of Triassic nightmares. Rauisuchians, along with all their pseudosuchian brethren except for the crocodylomorphs, were wiped out during the Triassic-Jurassic extinction. Their disappearance meant that their niches as top predators could be filled by entrepreneurial critters, and theropod dinosaurs moved into the vacuum: it’s likely that their ability to grow larger is linked, at the very least, with the disappearance of the rauisuchians, which had preyed on the smaller theropod dinosaurs in the twilight of the Late Triassic. Some paleontologists speculate that a lineage or two of rauisuchians survived into the Early Jurassic before being outdone by the opportunistic theropods, but this is mere speculation based on questionable bone fragments from South Africa. 

a panoply of larger-sized rauisuchians

the tyrant rauisuchian Fasolasuchus attacks a prosauropod 

Rauisuchians in the Middle and late Triassic grew to enormous sizes, and scientists have speculated that the largest of them all – Fasolasuchus – evolved to specifically prey on the larger Late Triassic prosauropods such as Plateosaurus and Riojasaurus. The carnivorous rauisuchians generally had thin, sharp, serrated teeth used to tear chunks out of their prey. The prey, wound down and missing good amounts of flesh, would’ve collapsed due to shock or blood loss to find a new home in a rauisuchian stomach. Rauisuchians could rear up on their hind legs for short amounts of time; by doing this they could scout for prey or enemies, and when hunting they could raise up and come crashing back down to intensify the lethality of their blows. Some rauisuchians were capable of a semi-bipedal lifestyle; rather than being able to simply rise up and down on their back legs, they could actually run on them. Examples of semi-bipedal rauisuchians include Postosuchus and Sillosuchus. Rauisuchian legs were positioned vertically beneath the body rather than sprawling outward. Though this type of gait is seen in dinosaurs, it evolved independently between the two groups. Rauisuchians had a hip socket that faced downward to form a shelf of bone under which the femur connects (a ‘pillar-erect’ posture), whereas dinosaurs have a hip socket that faces outward with the femur connecting to the side of the hip (an ‘erect’ posture). When covered in flesh, both hip arrangements give the same appearance; it’s only when muscle and tendon are removed that the differences become evident. 

Arizonasaurus of Middle Triassic Arizona

Two clades of “rauisuchians” – the Ctenosauridae and Shuvosaurids – deserve honorable mention, since they vary so greatly from the general rauisuchian morphology. The ctenosaurs had long spines on their backbones that probably supported a big sheet of skin. Other prehistoric creatures had such ‘sails’: the mammal-like Dimetrodon of the Permian, for example, and the sail-backed dinosaur Spinosaurus. The evolution of sails in these creatures and the ctenosaurs is an example of convergent evolution, wherein different species unrelated to each other evolve similar traits independently. Scientists speculate about the purpose of these sails; though it was once thought that they helped with thermoregulation, this theory has become outdated due to the fact that all the creatures mentioned above were already self-regulating their body temperatures (i.e. they were warm-blooded). The sail’s function was most likely for species recognition or sexual displays; the male with the biggest and most colorful sail, for instance, may have captured the most prized females. Ctenosaur fossils have been found throughout the world in North America, Europe, Africa, and China. The North American Arizonasaurus (discovered, I’m sure you guessed, in Arizona) lived during the Middle Triassic and was found as a complete skeleton, sail included. It had long jaws full of sharp teeth. However, not all ctenosaurs were necessarily carnivores: the Chinese Lotosaurus is an unusual ctenosaur because it had a short head and toothless beak, indicating that it may have been a plant-eating variant (though turtles also have beaks and are omnivorous). 

the pseudosuchian Effigia; despite an obvious resemblance
to later theropod dinosaurs, this creature was more closely
related to crocodiles than to birds

The shuvosaurs include Shuvosaurus and Effigia from the American Southwest and Sillosuchus from Argentina. These pseudosuchians had long back legs and short front legs, indicating they were bipedal. Their necks were long, and in place of jaws of sharp teeth they had ostrich-like beaks. Their resemblance to the ostrich-like theropods during the Jurassic and Cretaceous Periods are instances of convergent evolution, and for a while they puzzled paleontologists who thought they may have been earlier dinosaurs of the same type. The shuvosaurs may be related to the herbivorous ctenosaur Lotosaurus, carrying on its beak but losing its sail. The Triassic shuvosaurs are considered plant-eaters.