Prior to the extinction of large terrestrial dinosaurs (birds survived), dinosaurs were having a great deal of trouble. When the inland sea in N. America dried, probably due to tectonic forces, there may have been a mixing of many species which probably caused parasites, diseases, and competition to drastically reduce the number of species. That information came from reading Robert Bakker's book, Hot Blooded Dinosaurs a long time ago (if memory serves). Bakker was a real genius in paleontology in my opinion. He wasn't afraid to go against the establishment and his ideas have largely been adopted by the mainstream. I think the movie Jurassic Park wouldn't have been the same without his influence.
Mammals were evolving with flowering plants which were taking over much of the world. Due to the relative long life of dinosaurs, it appears they were slower to evolve and mammals had an edge in that regard. It appears that the asteroid finished off the remaining dinosaurs with the exception of birds. Beyond the initial blast which may have caused basaltic eruptions on the opposite side of the earth and ejecta hurled all the way into space, the asteroid caused massive climate disruption from particles and aerosols that probably dramatically cooled the earth for a few years.
The volcano that erupted in Indonesia in the 13th century caused the year without a summer. It snowed in England in June. The sulfur compounds from that volcanic blast created cooling for at least a couple years so I figure the asteroid would likely be significantly longer and obviously much worse.