Asteroids or volcanoes: What's responsible for mass-extinctions?

Scientists believe a massive explosion (not an eruption) of a volcano in present-day China wiped out half the life on earth about 260 million years ago. (Discover) The finding is the latest in a string of evidence suggesting volcanoes, not asteroids are responsible for mass-extinctions in Earth's history.

The explosion of the China volcano would have created a "massive cloud formation… cooling the planet and producing acid rain."

Late last year, a group of scientists suggested "the Deccan Traps", an area in India that "convulsed with volcanic activity around 65 million years ago," was responsible for killing of the dinosaurs. (Discover)

What's wrong with the asteroid-that-slammed-into-the-Yucatan theory of how the dinosaurs died? For one, the impact seems to have occurred 300,000 years before the dinosaurs started dying off. Also, "an asteroid impact wouldn’t kick up enough dust and sulfur dioxide to alter the climate around the planet."

Super-volcanoes, on the other hand, "may have spewed 10 billion to 150 billion tons of sulfur dioxide into the air with each pulse of eruptions."