September 11. Rhynchocephalians

There were plenty of other reptile lineages evolving during the Triassic besides the archosaurs and their descendents, the dinosaurs. One group, the rhynchocephalians, became quite diverse soon after they emerged in middle Triassic time. They were reptiles covered with overlapping scales, in contrast to the other large group of reptiles, the archosaurs, which had more flexible skin and other coverings, including feathers in birds.  

Homoeosaurus, a rhynchocephalian. Photo by Haplochromis under GFDL 

The parent group to the rhynchocephalians also includes snakes and most lizards, but despite their abundance and diversity during the Triassic and later Mesozoic, only two species of rhynchocephalian survive today – two varieties of the tuatara of New Zealand, the only reptile native to New Zealand. It’s a large, 3-foot-long lizard-like reptile. As the only survivor of the rhynchocephalian order, its heritage goes back a long way and information about the tuatara may be significant to our understanding the evolution of snakes, dinosaurs, and birds.

The name rhynchocephalia means “beak-head” for various bony plates found on the skulls of the animal.

—Richard I. Gibson

Photo by Haplochromis under GFDL 

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  

September 9. Recovery from extinction

The extinction at the end of the Permian decimated marine invertebrates, with 90 to 95% of all species disappearing. The ammonites, the shelled cephalopods related to octopuses and squids and the modern chambered nautilus, were almost wiped out completely, but the few survivors – perhaps just a single lineage – adapted and increased in diversity almost explosively during the Triassic, but underwent another decline late in the period.  

Encrinus, Triassic crinoid from Germany. Photo by Ghedoghedo under GFDL. 

Rocks from the first few million years of the Triassic, right after the extinction, contain few marine invertebrate fossils, as you might expect. Some of the groups that had been prominent during the Paleozoic, including brachiopods and crinoids, never really rebounded from the extinction, although both survive to this day in small numbers and without much biodiversity.

Corals and clams seem to have adapted pretty well to the new conditions and their recovery from the extinction was accomplished within about 10 million years or so – but during the early Triassic, there may have been strong variations in the carbon cycle, affecting carbon dioxide in the atmosphere as well as in sea water, and that would have had a big impact on things like corals and clams that secrete calcium carbonate. Link (carbon cycle) Despite this, a new important branch of corals called the scleractinians evolved during the Triassic. They are also called stony corals and they represent the modern reef-building corals we have today. They probably evolved from relatives of the rugose corals that lived in the sea from Ordovician to Permian time, but rugose corals didn’t survive the end Permian extinction.

There are no known coal beds from the early Triassic, so much so that it’s called the global coal gap. Probably the best theory to explain this is the idea that land plants in enough abundance to make coal were severely affected by the Permian extinction, and they did not really recover for at least 6 and more like 20 million years – at least that’s how long it took for coal to appear in the rock record again in much volume. They would have had to evolve adaptations to the new conditions, and that takes time – probably more time for relatively long-lived things like trees than for short-lived ammonites. A further explanation for why it took so long for trees to recover is the suggestion that temperatures were “lethally hot” during the early Triassic, at least in tropical zones, maybe even too hot for trees to survive. The high temperatures may have extended to tropical sea water, with one report suggesting surface sea temperatures of 40°C, 15 degrees higher than today. That could certainly have affected sea life.  I have links below to several technical papers that address these issues if you are interested in more detail.

—Richard I. Gibson

Links:
Coal Gap 

Lethally hot temperatures (Sun et al. 2012) 

Early Triassic ocean life (Bottjer 2012)

Encrinus, Triassic crinoid from Germany. Photo by Ghedoghedo under GFDL.