The new bit is the Triassic transition, but an Economist article is a handy summary of current thoughts on the Permian and later extinctions (emphases mine):
… This cycle has happened five times in the history of modern life. The most famous occasion was 65m years ago, when the dinosaurs were wiped out and the mammals emerged victorious from the wreckage. A bigger mass extinction, at the end of the Permian period 251m years ago, killed 70% of the world’s land vertebrates (and 96% of all marine animals) and paved the way for the age of reptiles.
Exactly which sort of reptile would come out on top, however, was not something that was decided until later—201.4m years ago, to be precise. This was towards the end of the Triassic period. Then, the ranks of aetosaurs, phytosaurs, shuvosaurs and many other uncrocodile-like relatives of the crocodiles were suddenly thinned, and a previously obscure group came to the fore. The result, once natural selection had done its work over the course of millions of years, was the now familiar cast of Allosaurus, Diplodocus,Triceratops and Tyrannosaurus rex…
… The dinosaurs were done for, as everybody knows, by a collision with an asteroid. The Permian was curtailed by massive volcanism. But what exactly happened towards the end of the Triassic has been much debated. A study just published in the Proceedings of the National Academy of Sciences, by Jessica Whiteside of Brown University in Rhode Island and her colleagues, pretty well nails it down. It was the geological chaos that created the North Atlantic Ocean.
… The initial volcanism as North America split from Europe released carbon dioxide from deep inside the Earth. That produced a greenhouse effect which, in turn, melted seabed structures known as methane clathrates, which trap that gas in ice. This caused a massive release of 12C-rich methane into the atmosphere, explaining the initial drop in 13C concentrations. The methane, being a much more potent greenhouse gas than carbon dioxide, exacerbated things, while the carbon dioxide acidified the oceans, killing most of the animal shellmakers and fertilising the photosynthesis of planktonic plants. The subsequent plankton bloom sucked up the12C and the isotope ratio veered off in the opposite direction.
The greenhouse warming and the acid rain also did for the forests and many of the reptiles. Only once things had settled down could the survivors regroup. New species of trees took over. The forests grew back. And a bunch of hitherto not-so-terrible lizards began their long march.
So a spike in CO2 from deep sources led to a methane spike. Together the two baked and acid burned the planet. A plankton bloom sucked down the CO2 and things settled down again.
We, of course, are on track to repeat history.
CO2 or not, we are in the midst of a mass “holocene” extinction anyway. What comes from that remains to be seen, but if humans last a bit longer it might be retrospectively labeled the transition to the age of the machines.