@High Seas,
I Disagree HS, the Geological SOciety of AMerica is split on this. Both sides have evidence and the position that dicos were extinct before chixclub is based upon a maybe imperfect stratigraphic continuity.
The facts are that the end of the K happened very abruptly (the KT boundary is, to me , a more important markr than the extinction of dinosaurs). This end of the K may have been a few thousand years or a few months. It is another fact that reduced solar radiation accompanied this event and a mass extinction was occuirng. This mass extinction was second to all others and accounted for 90% extinction of planktonic forms, half of all the other macro-invertebrates and 75% of angiosperms. Species diversity didnt restore itself for about another 10 million years.
The reduced solar radiation destroyed most of the existing photosynthetic plants at the bottom of the food chain. The first plants to spread out at the base of the tertiary were ferns , this always has indicated a fire boundary had occured (The fire s occured in vast areas where vulcanism was NOT evidenced, nor were ther evidences of pyroclastics or ashfalls).
The meteoritic impactor has lots of data to support it as THE major force. We include the multicontinental spread of stishovite (shocked quartz), iridium and a minor worldwide boundary layer, beneath which all dinosaurs lie, is pretty good evidence. That, and the fact that there is a paucity of C13 after the bolide hit (indicating planetary wide fires)
The argument for the vulcanism is based upon an idea thatmy own team published in 1988 in which we were doing a survey of SO4 caprocks and we did a quick mass balance of all anhydrite deposits and came to an approximation that Sulphate in K sediments,(derived from Sulfur and Sulfoxides) had accumulated quickly during the late K so, we speculated that vulcanism was quite active and could be a factor in the terminal mass extinction. We never had any really hard data like the present authors (who are published in reputable juried journals) and besides , we were being paid to work on other things. So the accumulation of anhydrites, a bolide impactor, several areas of vulcanism, and changing climates due to drifting continents were all suspected. NO one of them are compelling individually but one of them has to be a keystone . My own feeling is that the keystone event is probably the chixclub bolide. It finished off the dinosaurs that were already in severe decline due to other events, which were more routine in our planets history. I often change my mind as new data arise , but the event is undoubtedly one of the baddest days in our geologic past and cannot be dismissed as "pop" speculation.