A Lesser-Known Dying: The End-Triassic Extinction.

Greetings. The End-Triassic extinction, which occurred approximately 201.4 million years ago, marks the boundary between the Triassic and Jurassic periods. It is one of the major extinction events in the history of life on Earth, profoundly affecting life on land and in the oceans. 

In the Earth's seas, about 23–34% of marine genera disappeared. On the land, all archosauromorph reptiles other than crocodylomorphs, the lineage leading to modern crocodilians, the majority of dinosaurs, and a somewhat smaller number of pterosaurs went extinct. Some of the groups that died out completely were previously diverse, abundant and highly successful, such as aetosaurs, phytosaurs, and rauisuchids. Plants, crocodylomorphs, dinosaurs, pterosaurs and mammals all suffered substantial losses, but the survivors allowed the dinosaurs, pterosaurs, and crocodylomorphs to become the dominant land animals for the next 135 million years.

The specific cause of the End-Triassic extinction event may have been extensive volcanic eruptions in the Central Atlantic Magmatic Province, which released massive amounts of carbon dioxide directly into the Earth's atmosphere, causing profound global warming along with a high level of ocean acidification. Other hypotheses have been proposed that posit that gradual climate or sea level change may be the culprit, or perhaps one or more asteroid strikes. No definitive answer has been found to explain the event.

The End-Triassic extinction completed the transition from the Palaeozoic evolutionary fauna to the modern evolutionary fauna, a change that began just prior to the Permian-Triassic extinction event, often referred to as "The Great Dying." Ammonites were affected substantially by the End-Triassic extinction and were nearly wiped out. Ceratitidans, the most prominent group of ammonites in the Triassic, became extinct at the end of the period. Bivalves suffered heavy losses, although the extinction was highly selective, with some bivalve clades escaping substantial diversity losses. The diversity of gastropods was barely affected at the End-Triassic boundary, although gastropods gradually suffered numerous losses during the subsequent Jurassic Period. Brachiopods declined in diversity at the end of the Triassic before fully rediversifying. Around 96% of coral genera died out, with integrated corals being especially devastated. Corals practically disappeared from the Tethys Ocean at the end of the Triassic except for its northernmost reaches, resulting in an Early Jurassic "coral gap."

Fish were not affected by the End-Triassic extinction event, continuing on as if nothing was happening to the biosphere. Unlike fish, marine reptiles experienced a substantial drop in diversity between the Middle Triassic and the Jurassic. However, their extinction rate at the End-Triassic boundary was not elevated, which remains unexplained as of this writing. The only marine reptile families which became extinct at or slightly before the End-Triassic boundary were the placochelyids, the last family of placodonts, making plesiosaurs the only surviving sauropterygians. 

Terrestrial fauna was affected by the End-Triassic extinction far more severely than marine ecosystems. One of the earliest pieces of evidence for a Late Triassic extinction was a massive turnover in terrestrial tetrapods such as amphibians, reptiles, and synapsids. Scientific studies have debated whether the turnover in Triassic tetrapods was abrupt at the end of the Triassic, or instead a more gradual process. 

During the majority of the Triassic, amphibians were mainly represented by large, crocodile-like members of the order temnospondyli. Although the earliest frogs and salamanders did appear during the Triassic, they would become more common in the Jurassic while the temnospondyls diminished in diversity after the End-Triassic extinction. Several temnospondyl groups did go extinct near the end of the Triassic despite earlier abundance, but it is uncertain how close their extinctions were to the end of the Triassic, meaning that their extinction may have been a gradual process, as opposed to a sudden dying at the End-Triassic. 

Terrestrial reptile faunas were dominated by archosauromorphs during the Triassic, particularly phytosaurs and members of pseudosuchia, the reptile lineage which led to modern crocodilians. In the Early Jurassic and onwards, dinosaurs and pterosaurs became the most dominant land reptiles, while small reptiles were mostly represented by lepidosauromorphs, such as lizards and relatives of the extant tuatara. Among pseudosuchians, only small crocodylomorphs did not become extinct by the end of the Triassic. Phytosaurs, drepanosaurs, trilophosaurids, tanystropheids, and procolophonids, which were common reptiles in the Late Triassic, had also become extinct by the start of the Jurassic. However, pinpointing the extinction of these different land reptile groups is extremely difficult, as the fossil record is woefully incomplete. 

In the aftermath of the End-Triassic extinction, dinosaurs experienced a major radiation, filling many of the ecological niches vacated by the victims of the extinction. Crocodylomorphs likewise underwent a very rapid and major adaptive radiation. Surviving non-mammalian synapsid clades similarly played a role in the post extinction adaptive radiation during the Early Jurassic. 

The End-Triassic extinction event, far less known than "The Great Dying" or the Cretaceous/Paleogene extinction event that extinguished the non-avian dinosaurs. The End-Triassic extinction event, an natural milestone that remains unexplained and worthy of continuing scientific exploration. 

Thank you for your time and consideration.