260 million years ago: the Capitanian mass extinction
A capsule summary of the evolution of life on Earth goes like this: There is steady progress in adaptation, driven especially by arms races, sometimes involving competitors, sometimes predators and prey. But this progress is interrupted from time to time by catastrophes – mass extinctions resulting from extrinsic causes, sometimes astronomical, but more often geological. (We’ll see much later in the year that a similar summary of human history goes like this: steady progress in the scale of cooperation driven by arms races, with occasional catastrophic interruptions, often associated with the spread of epidemic diseases.)
The geological causes of mass extinctions have been coming into focus lately. Many mass extinctions co-occur with the formation of Large Igneous Provinces (LIPS), regions where vast amounts of lava have flowed out of the earth, triggering a whole cascade of changes: the destruction of the ozone layer by halogen gases, global warming induced by CO2 and methane, and anoxic seas.
Large Igneous Provinces aren’t always associated with mass extinctions. What makes some episodes of massive lava flow particularly destructive is that they produce short circuits in the “planetary fuel cell.” The development of complex life has depended on the separation between an oxygen-rich, electron-hungry atmosphere and a reducing, electron-stuffed planetary interior. Some of the biggest setbacks to complex life have happened when lava flows from deep in the Earth’s interior punch through carbon deposits on their way up, and bridge this chemical gap between surface and interior.
The mass extinction 260 million years ago, the Capitanian, is not one of the classic five greatest mass extinctions, and has been overshadowed by the mother of all mass extinctions, the end-Permian, which happened just 8 million years later. But it took a major toll on living things, from marine organisms to dinocephalians. (The dinocephalians – more closely related to mammals than to dinosaurs, ranging up to hippo sized, and including both herbivores and carnivores – went entirely extinct with the Capitanian. See picture.) The Capitanian extinctions coincide with, and were probably caused by, the formation of the Emeishan LIP, now in southwest China.
A book published recently, The Worst of Times, pulls together the latest evidence that the Capitanian was the beginning of an 80 million year period in which mass extinctions were exceptionally common. Apparently the formation of the supercontinent of Pangaea and the Panthalassic superocean made living things particularly vulnerable to volcanically induced extinctions. Once Pangaea breaks up, mass extinctions are less frequent, and generally have different causes. The death of the dinosaurs had an extra-terrestrial cause, and the mass extinction we’re in the middle of results from the activities of one very unusual species.