252 million years ago
In More Detail:
The Permian-Triassic Boundary (PTB) event of 252.25 million years ago marks the most severe biological mass extinction of all time. It has been estimated that 90% of all marine fauna disappeared abruptly. Many kinds of land animals were also lost and the extinction of plants resulted in complete cessation of coal formation (the ‘coal gap’).
This important timescale boundary is defined by a sequence of rocks in southern China. Here, paleontologists have mapped the decline of the fauna (trilobites, brachipods etc) that characterize the Paleozoic Era and the first appearances of those that typify the Mesozoic Era. In what is formally known as the global body-stratotype section and point (or GSSP) in an old limestone quarry at Meishan, a golden spike has been placed, not where the extinction happened but precisely at the point where geologists can find the first fossil of the new (ie Triassic) fauna. A little later, the first dinosaurs appeared on land.
The PTB event is also notable for the number and diversity of theories for its cause. These theories range from severe climate change, rapid changes in sea-level, toxic gases released by massive volcanic eruptions and a toxic ocean driven by eutrophication. Given that we see evidence for all of these phenomena, one hypothesis is that it was a combination of all of them (‘Tangled Web of Causality”). Although some people claim there was a bolide impact, little solid evidence has been produced for this.
There is a wide consensus that the PT extinction coincided with a particularly extreme oceanic anoxic event (OAE). The loss of O2 from the deep ocean made it difficult for any air-breathing organisms to survive there. Even more damaging were the high concentrations of toxic carbon dioxide and hydrogen sulfide that made the shallow oceans uninhabitable. This event is a stark reminder of the importance of healthy oceans for all life on Earth.
The head of the hammer marks the extinction horizon in China