The age of the dinosaurs ended 66 million years ago, when an asteroid six miles in diameter crashed into what is now southeastern Mexico. The world went up in flames. Dinosaurs, along with the massive reptiles that ruled the sea and the sky, perished as forest fires raged across the globe, dust blotted out the sun, and Earth experienced intense heat, frigid cooling, and then more heat. Conventional wisdom states that mammalian diversity emerged from the ashes of the mass extinction, ultimately giving rise to our own humble species. But according to a study in the Journal of Evolutionary Biology,...