I read a book about the Permian extinction by geologist who didn't think the extinction was caused by an asteroid because there was no iridium layer associated with it. He believed the extinction was associated with the Siberian Traps and the release of enough magma to cover Western Europe over the course of one million years.
I had heard the Permian extinction was caused by continental drift. All the shallow seas where Paleozoic Era life flourished were closed, pushed up and drained. Or so I heard.
There was and still is a very high resistance by the fossil academics to the impact extinction model. Iridium was anomalous because it is so rare on Earth, including in the junk barfed up by volcanoes, so that stood out to Walter Alvarez, whose father Luis Alvarez was a physicist and realized the significance of the find right away. *At one time* iridium wasn’t known from the PT boundary, now it is.
http://www.sciencedirect.com/science/article/pii/0031018293901286