Rauisuchia

From Tokaia

Rauisuchians are crurotarsian reptiles that resembled the theropod dinosaurs. It has been suggested that they are a polyphyletic group rather than a valid order, but for reasons of convenience they are treated as an actual clade here.

[edit] Evolutionary History

The first rauisuchians appeared in the Triassic period; given the fact that the previous dominant predators, the gorgonopsids, had been wiped out in the Permian extinction, they soonly became the dominant land predators, and not before long the five families presented here would be established. However, both in our world and in What if Dinosaurs' one, the ascension of predatory dinosaurs caused the extinction of this earlier predators, which could not stand their more advanced competitors.

This, obviously, didn't occur in Terra Alternativa, in which the absence of dinosaurs allowed crurotarsians to dominate the land ecosystems, and therefore the rauisuchians remained as the dominant predators. They were divided in five families:

• Prestosuchidae: mostly crocodile like predators, these were the most basal of rauisuchians. As a whole, they were never very diverse, and eventually became extinct in the middle Jurassic, due to competition with other rauisuchians.

• Rauisuchidae: larger, more efficient predators than the previous, these were the tyrannosaurs of the Triassic and Jurassic periods. Some species reached huge sizes, competing with the larger theropods for the title of the largest terrestrial predators that ever lived.

• Poposauridae: while the primitive Poposaurus was a typical rauisuchian, later species were quite different from any other crurotarsian that ever lived; bipedal and tooth-less, these occupied a niche similar to that of ornithomimid dinosaurs. Among the last and most diverse families of this group (see below).

• Ctenosauriscidae: bizarre, sail-backed rauisuchians, not unlike dinosaurs like Spinosaurus or synapsids like Dimetrodon. Like the prestosuchids, they were too specialized for a long term survival (although some species endured as much as the Cretaceous).

• Ornithosuchidae: bipedal, carnivorous rauisuchians; the ones that most resembled theropods (after poposaurids).

As mentioned before, the basal prestosuchids and ctenosauriscids were replaced as the dominant predatory group as soon as the Jurassic began, and therefore rauisuchids and ornithosuchids rised in dominance, and remained in that position for most of the period; the first, in particular, gave rise to the largest land predators that ever lived. However, as the Jurassic ended, vegetation and climate changes caused the decline of the dominant herbivores, and as such both groups declined, with ornithosuchids reduced to a few Gondwannan forms that endured until the K-T event. This gave a chance for poposaurids to rise in dominance, although they were soonly replaced by crocodiles, which had toothed jaws. Some poposaurids developed flight, giving rise to the largest non-pterosaurian fliers ever; others became specialized ant eaters. Another linage gave rise to fast herbivorous forms, and were the only known plant eating rauisuchians ever.

By the time of the K-T event, the only rauisuchians that remained were a few surviving ornithosuchids, the dominant predators of South America (elsewhere, crocodiles had replaced them), and the still diverse poposaurids. None of this forms survived to the K-T event...

[edit] See Also

Rauisuchia on wikipedia