The Triassic Period was the first period of the Mesozoic Era and occurred between 251 million and 199 million years ago. It followed the great mass extinction at the end of the Permian Period and was a time when life outside of the oceans began to diversify.
At the beginning of the Triassic, most of the continents were concentrated in the giant C-shaped supercontinent known as Pangaea. Climate was generally very dry over much of Pangaea with very hot summers and cold winters in the continental interior. A highly seasonal monsoon climate prevailed nearer to the coastal regions. Although the climate was more moderate farther from the equator, it was generally warmer than today with no polar ice caps. Late in the Triassic, seafloor spreading in the Tethys Sea led to rifting between the northern and southern portions of Pangaea, which began the separation of Pangaea into two continents, Laurasia and Gondwana, which would be completed in the Jurassic Period.
The oceans had been massively depopulated by the Permian Extinction when as many as 95 percent of extant marine genera were wiped out by high carbon dioxide levels. Fossil fish from the Triassic Period are very uniform, which indicates that few families survived the extinction. The mid- to late Triassic Period shows the first development of modern stony corals and a time of modest reef building activity in the shallower waters of the Tethys near the coasts of Pangaea.
Early in the Triassic, a group of reptiles, the order Ichthyosauria, returned to the ocean. Fossils of early ichthyosaurs are lizard-like and clearly show their tetrapod ancestry. Their vertebrae indicate they probably swam by moving their entire bodies side to side, like modern eels. Later in the Triassic, ichthyosaurs evolved into purely marine forms with dolphin-shaped bodies and long-toothed snouts. Their vertebrae indicate they swam more like fish, using their tails for propulsion with strong fin-shaped forelimbs and vestigial hind limbs. These streamlined predators were air breathers and gave birth to live young. By the mid-Triassic, the ichthyosaurs were dominant in the oceans. One genus, Shonisaurus, measured more than 50 feet long (15 meters) and probably weighed close to 30 tons (27 metric tons). Plesiosaurs were also present but not as large as those of the Jurassic Period.
Plants and insects
Plants and insects did not go through any extensive evolutionary advances during the Triassic. Due to the dry climate, the interior of Pangaea was mostly desert. In higher latitudes, gymnosperms survived and conifer forests began to recover from the Permian Extinction. Mosses and ferns survived in coastal regions. Spiders, scorpions, millipedes and centipedes survived, as well as the newer groups of beetles. The only new insect group of the Triassic was the grasshoppers.
The Mesozoic Era is often known as the Age of Reptiles. Two groups of animals survived the Permian Extinction: Therapsids, which were mammal-like reptiles, and the more reptilian Archosaurs. In the early Triassic, it appeared that the Therapsids would dominate the new era. One genus, Lystrosaurus, has been called the Permian/Triassic “Noah,” as fossils of this animal predate the mass extinction but are also commonly found in early Triassic strata. However, by the mid-Triassic, most of the Therapsids had become extinct and the more reptilian Archosaurs were clearly dominant.
Archosaurs had two temporal openings in the skull and teeth that were more firmly set in the jaw than those of their Therapsid contemporaries. The terrestrial apex predators of the Triassic were the Rauisuchians, an extinct group of Archosaurs. In 2010, the fossilized skeleton of a newly discovered species, Prestosuchus chiniquensis, measured more than 20 feet (6 meters) in length. Unlike their close relatives the crocodilians, Rauisuchians had an upright stance but are differentiated from true dinosaurs by the way that the pelvis and femur were arranged.
Another lineage of Archosaurs evolved into true dinosaurs by the mid-Triassic. One genus, Coelophysis, was bipedal. Although smaller than the Rauisuchians, they were probably faster as they had a more flexibly jointed hip. Coelophysis also picked up speed by having lightweight hollow bones. They had long sinuous necks, sharp teeth, clawed hands and a long bony tail. Coelophysis fossils found in large numbers in New Mexico indicate the animal hunted in packs. Some of the individuals found had remains of smaller members of the species inside the larger animals. Scientists are unclear as to whether this indicates internal gestation or possibly cannibalistic behavior.
By the late Triassic, a third group of Archosaurs had branched into the first pterosaurs. Sharovipteryx was a glider about the size of a modern crow with wing membranes attached to long hind legs. It was obviously bipedal with tiny, clawed front limbs that were probably used to grasp prey as it jumped and glided from tree to tree. Another flying reptile, Icarosaurus, was much smaller, only the size of a hummingbird, with wing membranes sprouting from modified ribs.
The first mammals evolved near the end of the Triassic Period from the nearly extinct Therapsids. Scientists have some difficulty in distinguishing where exactly the dividing line between Therapsids and early mammals should be drawn. Early mammals of the late Triassic and early Jurassic were very small, rarely more than a few inches in length. They were mainly herbivores or insectivores and therefore were not in direct competition with the Archosaurs or later dinosaurs. Many of them were probably at least partially arboreal and nocturnal as well. Most, such as the shrew-like Eozostrodon, were egg layers although they clearly had fur and suckled their young. They had three ear bones like modern mammals and a jaw with both mammalian and reptilian characteristics.