REVOLUTIONS have unpredictable outcomes. Who is going to end up in power is rarely obvious at the start. That is as true in biology as it is in politics. Like political revolutions, though, biological ones often follow a predictable course. The old order is destroyed. There is a period of confusion. Then a new ecosystem emerges that looks surprisingly like the old one, but with different actors.

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.

Another one bites the dust

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.

Dr Whiteside used fossil evidence sandwiched between layers of lava from some of the earliest eruptions that accompanied the rift. Specifically, she located two sites in what is now eastern North America where a series of giant lakes had formed on the layers of cooled lava and plant matter had accumulated at the bottom of the lakes. Using detailed chemical analysis of waxy molecules extracted from the fossil plants, she examined the carbon isotopes they contained.

Non-radioactive carbon comes in two forms: 12C and the heavier (and much rarer form) 13C. The ratio varies in the atmosphere, depending on where carbon-rich gases, mainly carbon dioxide and methane, are coming from. Dr Whiteside found the ratio yawed around like a drunken sailor as the continents split apart and the lava started pouring out. First, the level of 13C plummeted. Then it shot up again, a long way over the recent historical average, before settling down.

Crucially, the period of plummeting coincides with a phenomenon called the late Triassic fern spike. This marks precisely—to within a few thousand years—the point of mass extinction on the land. What is believed to have happened is that something killed all the forests and with them the animals that depended on them. Freed from the competition for light (because the shade from the trees had gone), ferns flourished (their spores are ubiquitous in the rocks). Previous work has suggested that the oceans also became acidic at this time. Shelled creatures, whose calcium-carbonate-rich armour tends to dissolve in acid, suddenly became rare.

Put it all together and the probable course of events was this. 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 the 12C 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.