The term Toarcian turnover, alternatively the Toarcian extinction, the Pliensbachian-Toarcian extinction, or the Early Jurassic extinction, refers to the wave of extinctions that marked the end of the Pliensbachian

Pliensbachian

The Pliensbachian is an age of the geologic timescale or stage in the stratigraphic column. It is part of the Early or Lower Jurassic epoch or series and spans the time between 189.6 ± 1.5 Ma and 183 ± 1.5 Ma . The Pliensbachian is preceded by the Sinemurian and followed by the Toarcian.The...

 stage and the start of the Toarcian

Toarcian

The Toarcian is, in the ICS' geologic timescale, an age or stage in the Early or Lower Jurassic. It spans the time between 183.0 Ma and 175.6 Ma...

 stage of the Early Jurassic

Early Jurassic

The Early Jurassic epoch is the earliest of three epochs of the Jurassic period...

 period, c. 183 million years ago.

The Toarcian turnover was most strongly manifested in aquatic lifeforms, notably in mollusk groups like ammonites; its reach was global in extent, as evidenced by research in Japanese waters, the Andean basin, and the floor of the former Tethys Sea. Evidence points to anoxic

Hypoxia (environmental)

Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...

 bottom waters as the probable cause of these marine extinctions, linked in turn to the massive volcanism of the Karoo-Ferrar

Karoo-Ferrar

Karoo and Ferrar denote a major geologic province consisting of flood basalt, which mostly covers South Africa and Antarctica, although portions extend further into southern Africa and into South America, India, Australia and New Zealand...

eruptions in the relevant era.