The end-of-the-Cretaceous extinction killed 75% of all species living on Earth 66 million years ago, including all dinosaurs that were not ancestors of today’s birds. Scientists have recently blamed a one-two punch of massive volcanism followed by an asteroid impact off the Yucatan Peninsula. Now, new research has identified two substantial spikes of global warming that support this notion. To determine global temperatures at the time of each event, scientists looked at the amounts of carbon and oxygen in well-preserved fossil shells from Seymour Island, Antarctica. The shells, from five clamlike relatives that survived the die-off (above), represent a 3.5-million-year-long interval that straddles the onset of the volcanism and the end of the dinosaur era. In particular, they looked at the ratio of carbonate minerals in the shells that included two forms—or isotopes—of oxygen and carbon: oxygen-18 and carbon-13, which—when together in the same molecule—are a keen indicator of the water temperature at which the shells formed. They found that local water temperatures rose, on average, about 1.1°C (2°F) in the wake of the asteroid impact. But water temperatures jumped a whopping 7.8°C (14°F) around the same time the volcanoes, which erupted in what is now India, began spewing planet-warming carbon dioxide some 150,000 years before the impact, the researchers report online today in Nature Communications. Previous studies of fossils from the site suggest that, of the 24 species that died out during that time, 10 went extinct due to the volcanism, whereas the rest died out after the asteroid impact, the researchers note. It’s not clear how the warming spikes killed the marine species, but the ultimate cause of each set of die-offs may have been distinct: Almost all of the species that disappeared in the first wave of extinctions were bottom-dwellers, but eight of the 14 that died out after the asteroid impact were living in surface or mid-depth waters.