Tony Swain has called attention to the fact that during the Cretaceous period there were important changes in the composition of flora, including the relatively rapid rise of flowering angiosperms (see "Morphology and Biology of Reptiles" in Linnean Society Symposiums Series 3, 1976). He hypothesized that the ability of flowering plants to produce hydrolysable tañnins as deterrents to their consumption and the appearance of toxic alkaloid angiosperms may have had a deleterious effect on the large herbivorous dinosaurs.
As an extension of Swain's hypothesis, it is interesting to note that recent psychological research has documented the presence of a striking ability to form associations between novel flavors and gastrointestinal distress in a large cross section of mammalian, avian and reptilian species. Such taste aversions frequently can be learned in a single trial, with long delays separating ingestion and illness. These aversions are also immune to the effects of retrograde amnesia induced by electroconvulsive shock, and they can even develop under general anesthesia.
Perhaps it was this capacity to form conditioned food aversions, as an adaptation to the gradual appearance and dispersion of toxic plants, that was lacking among dinosaurs. The inability to associate and form appropriate discriminations between flavor and illness may not only have contributed to the extinction of the herbivorous dinosaurs, but to the demise of the large carnivorous dinosaurs that depended on the herbivorous ones for food.
Thus, the evolution of new, more ingestionally sophisticated herbivores, rather than creating competition with dinosaurs, may have reflected a unique psychological adaptation to emerging plant toxicity. Rather than being a byproduct of a biotic crisis, perhaps dinosaur extinction was the result of a biotic revenge.