We cross the Cretaceous/Tertiary boundary and for whatever of many possible hypotheses, the terrestrial dinosaurs drop out of the mix. Birds and mammals radiate. We've undergone another major transition. We've gone from an ecosystem dominated by very large herbivores back to one much more like the dicynodonts and therapsids of the Late Permian /Early Triassic situation, to a world dominated by relatively small herbivores. But the dicynodont radiation of the Permian was done in a vegetation that was not particularly diverse in the individual plant morphologies that were available.
The radiation that takes place at the beginning of the Tertiary, however, occurs in a morphologically and taxonomically extremely diverse vegetation. got dinosaurs as plant predators that are, in essence, predating upon the entire plant; they're eating the whole plant. Talk about a plant's worst nightmare, it's going to be a herd of diplodici coming through and just literally mowing it off.
One of the features about that radiation is that we go from a world where we've This might force the Diplodocus to migrate on a continental scale, in order to always find fresh fodder. And indeed, we know some of these organisms did migrate on a continental scale. We can hypothesize that this may have been partially a function of running out of food. When we come into the Tertiary, we're doing a totally different relationship between plants and animals. It becomes a matter of organ level predation rather than whole plant predation. That allows us to set up things like specific animal/plant interactions, by example, a pollinator/pollen relationship. Mammals and birds both can act as pollinators; this can influence gene flow and underwrite the potential for new speciation.
The plant communities that radiate at this time, at the same time that the mammals and birds radiate, are basically modern in aspect but they don't include all the communities of the present-day world. Communities that first appeared in the early Tertiary are largely subtropical. We've finally come to a point where the present and the past are beginning to converge. I can look here at a mountainside from China and say that much of this vegetation and many of the taxa in this vegetation were present in the early Tertiary. We can actually plot the distribution of these vegetational units around the globe in the Eocene. But you notice some things are missing. There are no ice caps, there's no boreal forest, there's no tundra. There is a polar broad leaved deciduous forest, members of which are largely allied with communities that today grow in northern New England or Tibet. South of this is a broad leaf evergreen forest that grades into a semi-deciduous, drought-tolerant forest the south. We're beginning to establish a pattern of the biomes that becomes familiar to us, but we're missing a few.