The Triassic Period, new life in-between two major extinctions

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Article 26: During the Triassic, the survivors of the Permian extinction spread and recolonize. Coelophysis is an early dinosaur. Near the end of the period, the first mammals evolve.

The Triassic Period of the Earth’s history spans 50 million years, from 250 to 200 million years ago. The Triassic follows the largest extinction event in the history of life. It is the time when the survivors of that event spread and recolonize.

The organisms of the Triassic can be considered to belong to one of three groups: holdovers from the Permian-Triassic extinction, new groups which flourish briefly and new groups which go on to dominate the Mesozoic world. The holdovers include the lycophytes, a vascular plant, glossopteris, a seed fern, and dicynodonts, a herbivorous reptile, part of a group called therapsids.

Glossopteris leaves
Glossopteris leaves are a widespread fossil, but are difficult to assign by species, because of the wide variety of venation patterns and morphology. They are found throughout what was once Gondwana, another support for continental drift. The genus derives its name from the Greek words for tongue and fern. They favored a swampy habitat and some had leaves up to a meter in length. The leaves have only rarely ever been found attached to branches, but the restoration here is of one that had a tree-like habit. Few Glossopteris leaves have been found in strata younger than the Permian, a time that closed with the greatest of all mass extinctions on the planet. The deposits are infiltrated by iron minerals, whose staining results in dramatic specimens that are highly prized by collectors. The example seen here is replete with leaves on both sides.

Disynodont
Unnamed giant dicynodont from Late Triassic

Those that go on to dominate the Mesozoic world (Triassic, Jurassic and Cretaceous) include modern conifers, the gymnosperms cycadeoidales or bennettitales and the dinosaurs.

At the beginning of the Triassic Period, the land masses of the world are still bound together into the vast supercontinent, known as Pangea. Pangea begins to break apart in the Middle Triassic, forming Gondwana (South America, Africa, India, Antarctica, and Australia) in the south and Laurasia (North America and Eurasia) in the north.

Early Triassic
The supercontinent of Pangea, mostly assembled by the Triassic, allowed land animals to migrate from the South Pole to the North Pole. Life began to re-diversify after the great Perm-Triassic extinction and warm-water faunas spread across Tethys.

The movement of the two resulting supercontinents is caused by sea floor spreading at the mid-ocean ridge, lying at the bottom of the Tethys Sea, the body of water between Gondwana and Laurasia. Late in the Triassic, the seafloor spreading in the Tethys Sea leads to further rifting between the northern and southern portions of Pangea and to the separation of Pangea into the two continents, Laurasia and Gondwana, which will be completed in the Jurassic Period.

The climate of the Triassic Period is influenced by Pangea. The climate is generally very dry over much of Pangaea, with very hot summers and cold winters in the continental interior. A highly seasonal monsoon climate prevails nearer to the coastal regions. Much of the inland area is isolated from the cooling and moist effects of the ocean. Overall, it appears that the climate includes both arid dune environments and moist river and lake habitats with gymnosperm forests.

Although the climate is more moderate farther from the equator, it is generally warmer than today, with no polar ice caps. Plants and insects do not go through any extensive evolutionary advances during the Triassic. Due to the dry climate, the interior of Pangaea is mostly desert. In higher latitudes gymnosperms survive and conifer forests begin to recover from the Permian Extinction. Mosses and ferns survive in coastal regions. Spiders, scorpions, millipedes and centipedes survive as well as the newer groups of beetles. The only new insect of the Triassic is the grasshopper.

The oceans have been massively depopulated by the Permian Extinction, when as many as 95 percent of the existing marine genera are wiped out by high carbon dioxide levels. Fossil fishes 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 Pangea.

Early in the Triassic, a group of reptiles, the order Ichthyosauria, return to the ocean. Fossils of early ichthyosaurs are lizard-like and clearly show their tetrapod ancestry.

Ichthyosaurus
Ichthyosaurus. Pencil 2003, Credit: Raúl Martin

Later in the Triassic, ichthyosaurs evolve into purely marine forms with dolphin-shaped bodies and long-toothed snouts. Their vertebrae indicate they swim more like fish, using their tails for propulsion with strong fin-shaped forelimbs and vestigial hind limbs. These streamlined predators are air breathers and give birth to live young.

By the mid-Triassic, the ichthyosaurs are dominant in the oceans. One genus, Shonisaurus, measures more than 50 feet long (15 meters) and probably weighs close to 30 tons (27 metric tons). Plesiosaurs are also present, but not as large as those of the Jurassic Period.

The Mesozoic Era is divided into three time periods: the Triassic, the Jurassic and the Cretaceous. The Mesozoic Era is often known as the Age of Reptiles. Two groups of animals survive the Permian Extinction: Therapsids, which are mammal-like reptiles, and the more reptilian Archosaurs.

Dicynodont
Dicynodont therapsid. Credit: Anne Musser, Australian Museum

A lineage of Archosaurs evolve into true dinosaurs by the mid-Triassic. One genus, Coelophysis, is bipedal. They are probably fast as they have a flexibly jointed hip. Coelophysis also pick up speed by having lightweight hollow bones. They have 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.

Coelophysis
Coelophysis was a small, compact dinosaur that lived about 210 million years ago. It was a quick and agile hunter that had hollow bones and a hole-ridden skull, which helped to reduce its weight and increase its speed. Some fossils have been found with other small Coelophysis bones inside, and at first it was thought that Coelophysis might have given birth to live young. It is now believed, however, that it was probably a cannibal that occasionally devoured its own young.

The extinctions within the Triassic and at its end allow the dinosaurs to expand into many niches that have become unoccupied. Dinosaurs become increasingly dominant, abundant and diverse, and remain that way for the next 150 million years. However, the true Age of Dinosaurs is the Jurassic and Cretaceous rather than the Triassic.

Monte San Giorgio is a wooded mountain, 1.097 m above sea level, of the Lugano Pre-Alps, overlooking Lake Lugano in Ticino, Switzerland. Monte San Giorgio became a UNESCO World Heritage Site in 2003, because it “is the single best known record of marine life in the Triassic period and records important remains of life on land as well.”

In Meride you will find the Museum of Fossils Monte San Giorgio.

Fossil
Credit: Museum of Fossils Monte San Giorgio

Near the end of the Triassic Period, the first mammals evolve 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 are very small, rarely more than a few inches in length. They are mainly herbivores or insectivores and therefore they are not in direct competition with the Archosaurs or later dinosaurs. Many of them are probably at least partially arboreal or tree dwelling and nocturnal as well.

Most, such as the shrew-like Eozostrodon, are egg layers, although they clearly have fur and suckle their young. They have three ear bones like mammals and a jaw with both mammalian and reptilian characteristics.

Eostrodon
Eozostrodon

Morganucodon
Morganucodon, a close relative of Eozostrodon

The Triassic-Jurassic boundary is similar to the Permian-Triassic boundary in that the global climate does not radically alter, though a major extinction of terrestrial vertebrates occurs.

What caused this Late Triassic extinction is not known with certainty. With the end of the Triassic and the beginning of the Jurassic, Pangea continues to break apart in Laurasia and Gondwana, inevitably affecting the climate, though not as radically as it did during the Triassic.

Thanks to the University of California, Berkeley, Museum of Paleontology, to Christopher R. Scotese, and to LiveScience

ShantiShanti is a regular contributor to Osho News
All essays of this series can be found in: At Home in the Universe
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