Comment by adastra22

I should qualify by stating that I'm only slightly above amateur status here. I've had a life-long interest in study of the ancient Earth, and spent my early career supporting the closely related field of astrobiology (which studies evolution of life and mass extinctions to draw inferences about possible life elsewhere). But I am not a paleontologist myself.

To your question, my understanding is that if you ignore the K-T asteroid impact and look at the fossil record up to but not including the iridium layer, it looks very similar to the P-T die off. Same early indicators, same die-off patterns (e.g. starting with single celled marine organisms, changeup of specialized niche species, acidification, ecosystem collapse, more fungi etc.) Then of course the impact happened and reshaped everything. BUT, the argument goes, the impact had such drastic effects on Earth life because basically all the existing ecosystem slack had been taken up by the downstream effects of the Deccan Trap eruptions.

IMHO the researchers like Gerta Keller who push the Deccan Trap only theory are mainly butthurt about Alvarez. But if I were to steelman their position, it is that a P-T like extinction event was already clearly underway, and if the asteroid impact hadn't happened we would have seen basically the same die off over a slightly longer period (or if the impact had happened without the Deccan Trap eruptions, there'd have been enough resilience that maybe the impact wouldn't have caused extinctions?), and so we should assign causality to the volcanos. Hard to prove a hypothetical though, which is why I think the one-two punch explanation is better science.