Volcanic eruptions 201.4 million years ago cleared the stage for dinosaurs
At the end of the Triassic, colossal eruptions across what became the Atlantic pumped out carbon dioxide and mercury, heating the planet and souring the seas. Up to a third of marine genera vanished, and on land most reptile rivals disappeared. Dinosaurs, pterosaurs, crocodile relatives and mammals came through and dominated for the next 135 million years.
The Triassic-Jurassic extinction, dated to 201.4 million years ago, is one of the five great mass extinctions of the last half-billion years. In the oceans, between 23 and about 34 percent of genera died out. Corals, bivalves, brachiopods and radiolarians were hit hard, ammonites were nearly wiped out and conodonts, long-used index fossils, disappeared entirely. Fish, snails and bottom-dwelling foraminifera fared comparatively well. The crisis completed a long shift toward the kind of marine fauna that still dominates today.
Land suffered worse than sea. Apart from crocodylomorphs, dinosaurs and pterosaurs, every archosauromorph reptile lineage vanished, and Paul Olsen estimated in 1987, from rocks of eastern North America's Newark Supergroup, that 42 percent of land tetrapods died out. Unlike the later asteroid extinction, this one struck freshwater ecosystems heavily and did not favour small bodies. Large, crocodile-like temnospondyl amphibians dwindled, though one group, the brachyopoids, turned out to have lasted into the Cretaceous. Plant communities were thoroughly reorganised.
Explanations shifted with scientific fashion. Mid-20th-century researchers, wary of catastrophism for its old association with biblical creationism, preferred slow causes; Edwin Colbert blamed changing rainfall seasons and rising seas. In the 1980s Jack Sepkoski counted the event among the Big Five, and after an impact was shown to have ended the Cretaceous, some proposed an asteroid here too. Since the 1990s the consensus has pinned the blame on the Central Atlantic magmatic province, a vast outpouring of lava whose timing matched the boundary, causing warming and ocean acidification.
Recovery was comparatively quick. Seafloor ecosystems rebounded far faster than after the Permian-Triassic catastrophe, with British Early Jurassic sites showing renewal almost immediately despite recurring oxygen loss. Weathering of the exposed lava delivered so much silica to the sea that glassy siliceous sponges briefly took over. Some bivalves shrank after the crisis, an example of the so-called Lilliput effect, while others grew larger, the opposite pattern nicknamed the Brobdingnag effect.
Source: Triassic–Jurassic extinction
