Whatever happened at that time must have been very dramatic indeed. We don't know what did happen. Perhaps we will know, with continued study. Consider the relationships between the mass extinctions and the Eras in the geological timetable. Eras are recognized by their distinctive assemblages of faunas.
Extinction events shape the fossil record, more perhaps that any other contributing factor.
Only after the extinction of dinosaurs did mammals finally have the opportunity to radiate. R. L. Carroll Mammals didn't arise at the beginning of the age of mammals. But with the elimination of Mesozoic fauna including dinosaurs, mammals diversified explosively. The origin of the modern mammalian Orders is the best known radiation shown by the fossil record for a major group of vertebrates. Mammals have evolved heterodont teeth, teeth preserve marvelously, and individual fossil teeth can be identified as to species, as, for example, was done by Archibald and colleagues, studying sediments in Montana.