Triassic–Jurassic extinction

The Triassic–Jurassic extinction event, often called the TJME, marks the boundary between the Triassic and Jurassic periods. This event is recognized as one of the five major mass extinction events during the Phanerozoic eon. It was a period of profound biological change that affected life across the entire planet. The crisis fundamentally reshaped ecosystems in both the deep oceans and on land.

By removing many established species, the event cleared the way for new groups to dominate the Earth for millions of years.

Scientists generally agree that massive volcanic activity caused this extinction. This activity took place in the Central Atlantic magmatic province, known as CAMP. This was a large igneous province where immense amounts of lava were released. The eruptions discharged vast quantities of carbon dioxide into the Earth's atmosphere. This gas caused significant global warming and led to ocean acidification, which is the lowering of the ocean's pH. Additionally, the volcanoes released immense amounts of toxic mercury into the environment. This combination of heat, acid, and toxins created a lethal world for many species.

The marine environment experienced a massive loss of biodiversity. Between 23% and 34.1% of all marine genera disappeared during this time. Corals were devastated, with approximately 96% of coral genera dying out. This led to a "coral gap" in the Tethys Ocean where reefs almost entirely vanished. Other groups like bivalves and brachiopods also suffered heavy losses. Specifically, conodonts, which were important vertebrate groups, were completely wiped out.

However, some groups like gastropods and certain fish were relatively unaffected by the crisis.

Life on land was also heavily impacted, though the patterns were different. It is estimated that 42% of all terrestrial tetrapods, or four-limbed vertebrates, went extinct. Many large amphibians, such as the Temnospondyli, saw a major decline in diversity. Specifically, groups like the capitosaurs and plagiosaurids were among the major victims.

Many archosauromorph reptiles also vanished. However, certain groups like dinosaurs, pterosaurs, and mammals were left largely untouched. This survival allowed these animals to become the dominant land creatures for the next 135 million years.

Our understanding of this event has changed as scientific methods improved. In the mid-20th century, most researchers followed the paradigm of uniformitarianism. This meant they believed Earth's history was shaped by gradual changes rather than sudden catastrophes. Paleontologist Edwin H. Colbert suggested that changes in rainfall and sea levels caused the extinction. In the 1980s, Jack Sepkoski identified the TJME as one of the "Big 5" mass extinctions. During the 1980s and 1990s, some scientists even proposed an asteroid impact as the cause.

Today, the volcanic theory involving CAMP is the scientific consensus.

Following the extinction, different types of marine life began to recover at different speeds. In some regions, such as the British Early Jurassic environments, benthic ecosystems recovered very rapidly. In the Neuquén Basin, large bivalves began to reappear shortly after the event.

In other areas, like the northern Tethys, recovery was much slower. In those places, microbial carbonate producers dominated the seafloor for a long time. The influx of silica from weathered volcanic rocks also allowed siliceous sponges to dominate the immediate aftermath.

The Triassic–Jurassic extinction was a pivotal moment in the history of life. It completed the transition from the Paleozoic evolutionary fauna to the Modern evolutionary fauna. This shift determined which types of organisms would rule the oceans in the modern era. On land, the extinction facilitated a massive biological turnover. It moved the world from an era of diverse ancient reptiles and amphibians into the age of dinosaurs. This event shows how sudden geological changes can completely rewrite the story of life on Earth.