One of the first things geologists learn about is the major extinction at the end of the Permian Period (and Paleozoic Era) 250 million years ago. 96 percent of all species died during that mass extinction.
Now geologists have discovered new evidence that may point to another extinction during the mid-Permian, 262 million years ago, just 12 million years before the major Permian-ending event. An April, 2015, Geologic Society of America (GSA) paper (Lead author David Bond) notes that the "Capitanian" saw the extinction of 87% of all brachiopod species.
The Capitanian was named 20 years ago based on research in tropical to sub-tropical areas in China and elsewhere and believed originally to be more localized. Newest research in northern Norway suggests that this extinction may actually be more global (though several dissenters have commented on the publication in the well-respected GSA paper).
Dr, Bond et al write that the Capitanian extinction was likely triggered by the eruption of the Emeishan Traps, located in the southwestern Chinese province of Sichuan. Volcanic eruptions release large amounts of carbon dioxide, causing ocean acidification. Seafloor oxygen depletion may also have played a role.
The major shift after the acidification and oxygen depletion was from domination of brachiopods to domination by bivalves after the Capitanian extinction:
Brachiopods and bivalves differ in the number of adducting muscles, the way they eat, in their very classification, and in the symmetry of their shells. The two shells of bivalves are mirror images of each other; they are not in brachiopods.
So, why were bivalves better suited than brachiopods for life after the Capitanian? Any guesses?
Shellfishly (and sleepily),