The impactor theory for the P-Tr extinction is finding a hard sell these days in favor of climate change due to volcanic out gassing of the Siberian traps. Nova on PBS blamed the extinction on global warming caused by volcanic CO2. Never once did Nova mention attendant cooling caused by volcanic out gassing.
It's quite a coincidence that there is evidence for both impact and volcanism as the cause for at least two of the major mass extinctions (K-T and P-Tr), or is it?
Nova on PBS blamed the extinction on global warming caused by volcanic CO2.
Of course -- the global warming demagogues and "Sixth Extinction" demagogues are trying to sell it because they're trying to sell their political agenda. I appreciate what you're saying though:
It's quite a coincidence that there is evidence for both impact and volcanism as the cause for at least two of the major mass extinctions (K-T and P-Tr), or is it?
There really isn't any evidence for massive volcanism at the boundaries, but if there were (and it is often said that there is, such as on that NOVA program), what could have caused it just at those times when mass extinction occurs?
When the iridium abundance in the boundary layers were found to be too high, the previously gradualists started claiming (falsely) that the iridium would be enriched if all the volcanoes on Earth erupted simultaneously and strongly.
The question then, obviously, was what would cause that? :') After that, someone claimed that even a large impact wouldn't cause eruptions anyway.
So that left the impact model standing alone, which is where it still stands. There isn't a viable alternative to it, and the efforts to discredit it is the result of something analogous to what Judge Bork warned of, the political seduction of the sciences.
I think it was the former young turk Robert T. Bakker (of "The Hot-Blooded Dinosaurs" fame) who joined in the non-chorus, claiming that a large impact not only never happened, but also wouldn't cause extinction, and furthermore, that there was no evidence that the mass extinction was even sudden. So there. :')
The defense against the impact model seems to be summed up as, the extinction was already inevitable, and the impact may have happened, but was just a huge coincidence and didn't do anything to the species which went extinct.