September 10. Archosaurs

So we’re in the Mesozoic, the Age of Dinosaurs. Where are they? Well, hold on – we’re still pretty early in the Triassic Period, and true dinosaurs have not yet evolved. We do have things called archosaurs – the group that includes modern birds and crocodiles, extinct dinosaurs, and the ancestors of them all. “Archosaur” means “ruling lizard,” and they were reptiles that might have evolved in the very late Permian Period, but were definitely on the scene in early Triassic time. 

Archosaur drawing by Nobu Tamura under GFDL

Archosaurs succeeded the Permian synapsids, pre-mammals including dimetrodons and cynodonts, as the dominant land vertebrates in the early Triassic. Some cynodonts survived the Permian extinction, but archosaurs seem to have been able to adapt and use the vacated ecological niches to expand and diversify after the extinction event. They may have had better systems for managing water than their synapsid cousins, and that would have been an advantage in the hot, arid climate of the early Triassic. They might also have developed a more efficient, upright walking stance that was better suited to breathing. This could have led to the earliest upright, bipedal reptiles. In reconstructions, they look a lot like skinny lizards on tall legs.

Archosaurs are the ancestors of many diverse groups. By the late part of the middle Triassic, the group we know as dinosaurs had evolved from pre-dinosaur archosaurs. One of the oldest known animals that is definitively classed as a dinosaur is Eoraptor, whose name means dawn-raptor, discovered in northwestern Argentina in 1993. It was about a meter long – three feet – and was bipedal, standing upright and scurrying across the forested floodplains where it lived. Its teeth suggest that it was an omnivore – both carnivorous and herbivorous. The rocks it was found in are dated pretty accurately at about 231 million years ago, 20 million years after the Permian extinction, and near the end of the middle Triassic.

Archosaurs evolved from cold-blooded ancestors, but some evidence suggests that they, and their dinosaur descendents, were warm-blooded. This idea remains a focus of study, since the best evidence comes from anatomical studies of things like lungs and heart, which are seldom preserved. Inferences come from muscular structure, cavities in skeletons, and such, so the evidence is indirect. But it’s pretty generally accepted today that most dinosaurs were active, warm-blooded animals. Whether their ancestral archosaurs were or not is uncertain.

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Stephen Jay Gould was born September 10, 1941, in New York City. As an evolutionary paleontologist, he developed the concept of punctuated equilibrium, the idea that the history of life on earth was long periods of relative stability punctuated by occasions when rapid change took place. He was best known as one of the premier writers of popular science in the last quarter of the 20th century. Hundreds of essays and many books, including Wonderful Life, about the Cambrian Burgess Shale, fixed Gould firmly in the public eye as a popularizer of geological science.

—Richard I. Gibson

Link:
What’s an archosaur?

Drawing by Nobu Tamura under GFDL