Archosaur - Archosaur Takeover in The Triassic

Archosaur Takeover in The Triassic

Synapsids (a group including mammals and their extinct relatives, which are often referred to as "mammal-like reptiles") were the dominant land vertebrates throughout the Permian, but most perished in the Permian-Triassic extinction event. Very few large synapsids survived the event, although one form, Lystrosaurus (an herbivorous dicynodont), attained a global distribution soon after the extinction.

But archosaurs quickly became the dominant land vertebrates in the early Triassic. The two most commonly suggested explanations for this are:

• Archosaurs made more rapid progress towards erect limbs than synapsids, and this gave them greater stamina by avoiding Carrier's constraint. An objection to this explanation is that archosaurs became dominant while they still had sprawling or semi-erect limbs, similar to those of Lystrosaurus and other synapsids.
• The early Triassic was predominantly arid, because most of the earth's land was concentrated in the supercontinent Pangaea. Archosaurs were probably better at conserving water than early synapsids because:
  • Modern diapsids (lizards, snakes, crocodilians, birds) excrete uric acid, which can be excreted as a paste. It is reasonable to suppose that archosaurs (the ancestors of crocodilians, dinosaurs and birds) also excreted uric acid, and therefore were good at conserving water. The aglandular (glandless) skins of diapsids would also have helped to conserve water.
  • Modern mammals excrete urea, which requires a lot of water to keep it dissolved. Their skins also contain many glands, which also lose water. Assuming that early synapsids had similar features, e.g., as argued by the authors of Palaeos, they were at a disadvantage in a mainly arid world. The same well-respected site points out that "for much of Australia's Plio-Pleistocene history, where conditions were probably similar, the largest terrestrial predators were not mammals but gigantic varanid lizards (Megalania) and land crocs."