The past is catching up to the present...
But at least now we have a situation of that we're familiar with--
mammalian herbivores--and plants. In the later Tertiary, further changes occur that bring us even closer to the modern day. Climatic changes occur, with a major global cooling in the latest Eocene (about 30 million years ago) . It warms up slightly into the Miocene and then it cools off down to the present day, for we are really present in an interglacial period. As a result, by the Early Miocene we begin to see appearance of some other vegetation units that are more familiar to us including coniferous forests. But even these coniferous forests have a lot of broad leafed, deciduous, trees. But there is still no tundra and there are no ice caps.
We come closer to the present day and we see that one of the other features in this climactic cooling in general is the appearance of grasslands . Grasslands--let me back up on that one for just a moment--during the early Tertiary, during the Paleocene and Eocene, most of the world is subtropical in nature. Way up north where I'm pointing, there was a deciduous forest. Not so much because of temperature but because of the light; there wouldn't be enough light in the wintertime for an evergreen tree to photosynthesize and support itself. When we cross over this latest Eocene boundary and the world cools off, not only do we see the appearance of more cool adapted plants in the northern polar region, but we also see the spreading of grasslands in the mid-continental regions that had been subtropical forest. The appearance of grassland s elicited an evolutionary response from herbivores, and grazers appeared, many with a "hypsodont" condition, long,
erodable teeth that allow these herbivores to cope with the erosive silica so common in grass leaves. We see a radiation that brings us yet closer to the present day.
Some of these mammalian herbivores were huge. We still have
elephants. Indrichotherium was a very large creature, perhaps on the order of ten tons in size, is probably the largest terrestrial mammal that has ever lived. It was an early resident on some of these grasslands but it disappeared and we don't know why. But still, elephants are small potatoes in the realm of terrestrial dinosaurs, and only one mammal as large as
Indrichotherium has ever lived. We simply don't have huge mammalian herbivores. We're only left with things like elephants, which can still be tremendously destructive. You go to Africa and look what elephants can find in a small national park, can do, they can turn a forest into a grassland in a matter of years.