Many paleontologists considered the case of the dinosaur extinction closed as of 65 million years ago, when a large meteor slammed into Earth. Dirt and dust tossed up by the impact blotted the sun, and the resulting chill shoved the dinosaurs into evolutionary oblivion. They even found the gunshot wound - a huge crater off Mexico's Yucatan peninsula. Except there were a few nagging details that didn't quite fit the picture. Many believe dinosaurs were already in decline for millions of years before the supposed impact.
Volcanoes can also be a source of iridium.
That explanation doesn't work, either. Radiometric dating of the lava flows indicate they started long before the meteor impact. So maybe dinosaurs were just unlucky. The volcanic eruptions triggered climactic changes that caused their decline, and the meteor impact was just the coup de grace that finished them off. Now Zikui Zhao of the Institute of Vertebrate Paleoanthropology in Beijing suggests the meteor didn't even do that.