This is another mystery surrounding the whole question of dinosaur extinction at the end of the Cretaceous. If indeed the dinosaurs were suddenly killed off by a catastrophe, there should be a thick layer of bones—or a “bone spike”—in the fossil record. However, no such bone spike has been found to date at the boundary between the Cretaceous and Tertiary periods. Equally puzzling, few dinosaur bones have been found within a foot or so below this boundary.

One possible theory to explain these “missing” dinosaur bones (if they truly are missing) involves acid rain. Models have shown that one consequence of a large asteroid impact would be highly acidic rainfall over the planet. This acidic water could have dissolved most of the dinosaur bones lying on the surface, and would have also penetrated below the surface into the upper soil zones. Combined with bacteria, the water would become even more acidic, dissolving any bones found there. Only already fossilized bones would have resisted the acidic water. Since the fossilization process takes a very long time, none of the more recent dinosaur bones would have been spared.

This theory—and it is only a theory at present—neatly explains why there are so few dinosaur bones found in rock below the boundary, and none at the boundary itself. Supporting evidence comes from the boundary layer: in many places around the world, a relatively thin layer of clay exists that could have formed from the erosion of rocks due to acid rain.