The Elements of Evolution (18) The Triassic

The Triassic

The Triassic (248–206 MYA) opened with lethally hot temperatures. The sea surface at the equator exceeded 39 ºC at times. Shallow ocean waters were both anoxic and acidic.

After the early intense hothouse, Triassic climates eased into greater equability. The extreme aridity relented somewhat. Massive swamps again formed.

The Triassic was a transitional period for terrestrials; a time of abundant amniotes. Frogs, turtles, lizards, dinosaurs, and mammals all radiatively evolved during the Triassic.

At the close of the Triassic, amphibians were not wiped out, but the stage was set for the reign of reptiles to come. Meanwhile, mammals bided their time.

Amniotes are egg layers: either carried by females or laid on the ground. 2 main amniote lines evolved: synapsids and sauropsids. Both persist into modern times.

Sauropsids led the lineage from which all reptiles and birds emerged, including dinosaurs, the immediate ancestor of birds. From synapsids evolved mammals.

The earliest synapsids were small-brained, ectothermic, lizard-like creatures; some up to 3 meters or more, though most species were much smaller. These synapsids, informally called pelycosaurs, were the first successful group of amniotes. They became the predominant large land animals in the Late Carboniferous and early Permian periods. But they did not last. All but a few pelycosaurs went extinct before the end of the Permian.

A later, more diverse line of synapsid supplanted the earlier. These appeared during the first half of the Permian and became the dominant large terrestrials in the back half.

Only a few synapsid species survived the Great Dying. Those that did became successful in the early Triassic.

3 groups of synapsids lived during the Triassic: therocephalians, which were large-skulled carnivores; dicynodonts, a beaked herbivore; and cynodonts, from which mammals descended. All were amniotes.

Both therocephalians and dicynodonts were large-bodied and stayed that way until their demise. Therocephalians only lasted into the early Triassic.

Herbivorous dicynodonts were the most abundant and successful land vertebrate of the Late Permian. Most were wiped out in the Great Dying. Those that survived lasted into the Late Triassic, when climate change, especially increasing aridity, caused a drastic decline. Dicynodonts died out by the end of the Triassic.

The first mammal was a cynodont, about the size of a housecat, though cynodont means “dog teeth,” owing to the canine jaw and powerful bite of fossils found. Cynodonts made their debut 260 MYA.

Cynodonts diverged from earlier synapsids in becoming endothermic and being covered in hair. Skull changes included a secondary palate: a separation of between the oral cavity and the nasal cavity (which therocephalians also had), and a distinctive jaw structure, which included a cheek bone (zygomatic arch). Cynodonts were omnivorous. Having a flexible diet would serve them well.

Unlike contemporaneous therapsids, who maintained their girth, cynodonts shrank. By the onset of the Jurassic, cynodonts were somewhere between shrew small and badger big. These odd, toothy creatures bid their time during the dominion of sauropsids, awaiting the demise of the dinosaurs.

Despite the devastation, Permian flora persisted into the Triassic. Ferns were especially successful, evolving toward their modern form.

The continents continued to dance about the world during the Triassic. Early on, sea levels rose. Glaciation subsided as Pangea moved away from the South Pole.

Marine life became more abundant near the North Pole. The continental shelves in the middle of the continent were narrow, restricting opportunities for life there.

Toward the end of the Triassic, as continental rifts appeared in Pangea, sea levels fell 50–100 meters. The waters on the continental shelves became oxygen poor. Marine life suffered. The reef systems in the Paleo-Tethys Ocean were decimated. It was even worse on land.