c. 200 Million BCE

Triassic Extinction

The second-most recent of Earth’s five known large mass extinction events occurred at the end of the Triassic geologic time period, just over 200 million years ago. Across a relatively short span of geologic time (tens of thousands of years), at least half the species known to be living on Earth became extinct. Particularly hard-hit on land were the archosaurs, a group of sauropsid vertebrate amniotes that had evolved to become the dominant land vertebrates in the Triassic period.

In the seas, over a third of the genera (the taxonomic class above species) went extinct, including many of the large amphibians and the entire class of eel-like vertebrates known as conodonts. The rapid wiping out of the conodonts, precursors of which had been around for hundreds of millions of years and which had survived the even more catastrophic End Permian mass extinction, is puzzling. Most all that is left of those sea animals are abundant and sharp fossilized teeth (among the first biostructures built from hydroxyapatite, a calcium-rich mineral that remains a key component of our own bones and teeth today).

The event(s) or circumstances responsible for the End Triassic mass extinction remain as controversial as the explanation for The Great Dying and several of the other large extinction events in the history of life on Earth. A large asteroid impact event is an obvious candidate to explain the rather sudden loss of species globally, but no “smoking gun” impact crater or other obvious geologic signal has been found for this event. Massive volcanic eruptions and ejection of large amounts of greenhouse gases into the atmosphere has also been suggested, perhaps associated with large-scale mantle upwelling that also led to the hypothesized start of the breakup of the supercontinent of Pangea around the same time. Other climate-change ideas have been floated, but many act too slowly to fit this scenario.

Surviving classes of archosaurs included precursors of the dinosaurs that became the dominant land vertebrates for more than 130 million years, from the Jurassic until the next mass extinction event at the end of the Cretaceous. Another species that survived the End Triassic extinction, the mammals, continued to evolve traits and behaviors that would ultimately enable them, unlike the non-avian dinosaurs, to survive that next great catastrophe.

High-resolution microscope view of a millimeter-sized fossilized tooth from a jawless, eel-like Triassic vertebrate called a conodont.