Double whammy (Image: Don Hammond/Design Pics Inc./Rex Features) Peter Ward fossil hunting on James Ross Island, Antarctica (Image: Don Hammond/Design Pics Inc./Rex Features) Joe Kirschvink (right) holds a gastropod fossil, Tom Tobin (Image: Don Hammond/Design Pics Inc./Rex Features) Joe Kirschvink and Peter Ward extracting a large ammonite (Image: Don Hammond/Design Pics Inc./Rex Features) Advertisement Ridge on James Ross Island, Antarctica (Image: Don Hammond/Design Pics Inc./Rex Features)

The mass extinction that wiped out the dinosaurs 65 million years ago was almost unprecedented in its size. There may be a simple reason why three-quarters of Earth’s species disappeared during the event – there were actually two extinctions at the end of the Cretaceous, each devastating species in distinct environments.

Famously, the dinosaurs met their end when a massive meteorite crashed into Mexico’s Yucatán Peninsula around 65 million years ago. The extinction paved the way for the rapid evolutionary diversification of mammals.

But sceptics have long questioned whether the meteorite was solely responsible for the extinction. They point out that there were massive volcanic eruptions in India more than 100,000 years earlier, which triggered global warming that might have contributed to the species fatalities. But convincing evidence for those claims has proved elusive, so the impact has taken most of the blame.

A key problem has been finding sedimentary rocks that were formed at exactly the right time to capture all of the events that might have contributed to the extinction. The rocks need to contain plenty of fossils too, to reveal exactly when the various species disappeared.

Thomas Tobin at the University of Washington in Seattle has just found rocks that fit the bill on Seymour Island, just off the Antarctic Peninsula. “It is really far south, so any climate changes are likely to be strongest there and have more biological effects,” he says.

Tobin found two layers in the rocks, which formed in a shallow sea, where several species of shelled animals went extinct. One of the layers dates to the time of the impact, but the other layer is 40 metres below. Dating showed that the lower extinction occurred some 150,000 years before the meteorite hit – at the peak of the Indian eruptions. Tobin’s team looked at isotopic ratios in the rock to work out the temperatures at the time: the first extinction followed a 7 °C rise in polar ocean temperatures – probably a result of global warming triggered by the Indian volcanism.

Comparable numbers of species in the region went extinct in each event. Surprisingly, though, the types of animals affected differed strikingly.

“The stuff living at the [ocean] bottom died out during the [volcanic extinction event],” says Peter Ward, Tobin’s thesis advisor and collaborator. That might be because the global warming triggered by the volcanic eruptions initially increased levels of biological activity in the oceans, but ultimately used up the oxygen dissolved in the water to create lethal anoxic conditions in deep water.

The later extinction, which is linked to the meteorite impact, wiped out creatures that lived in the surface waters.

The new data suggesting two distinct extinctions ties in with results of another new study. Gerta Keller of Princeton University and her team studied microfossils from the Bay of Bengal that lived during the end of the Cretaceous. The sea floor sediments in which they are preserved is interleaved with basalt from the massive Indian lava flows. Around half of the species went extinct during the initial volcanic eruptions, long before the meteorite impact. Here, however, it was the surface-dwelling organisms that were affected by the volcanism.

The case for multiple factors contributing to the extinction is adding up, says David Archibald, a vertebrate palaeontologist recently retired from San Diego State University, California, who was not involved in either study. “I’m not suggesting the [meteorite] impact didn’t have tremendous effects, and it probably was necessary for the extinctions, but there were other things leading up to it,” he says.

Journal reference: Tobin study: Palaeogeography, Palaeoclimatology, Palaeoecology DOI: 10.1016/j.palaeo.2012.06.029; Keller study: Earth and Planetary Science Letters, DOI: 10.1016/j.epsl.2012.06.021