Finally the herd reaches the river and spreads out through the shallows. It is hot today, and the flies are biting even through the tough rhino hides. Many of the adults go into deeper pools to roll and soak while the young drop their serious attitudes and frolic in the shallows.
As the day wears on, the herd leaves the river and feeds on the leaves and stems of scattered trees and willow thickets along the river. When twilight comes, the herd draws together, colts and females toward the center and bulls around the edges. After a period of milling and pushing, the herd finally beds down for the night, with only the perimeter guards moving around on the edges.
The daily routines of the other herds of grass- and leaf-eating animals generally follow somewhat different patterns from that of the rhinos. Of them, only the piglike oreodons wallow in the river. The others spend nearly all their time out on the savanna, coming to the river only at dawn or dusk to drink.
Oreodons were among the most abundant medium-sized animals of the Middle Tertiary. A strictly North American group, they have been described as looking like a cross between a sheep and a pig. As small as a house cat or as big as a domestic pig, these mammals reached a peak in abundance and variety between the Middle and Late Oligocene (though they are known from the Late Eocene through the Late Miocene). This peak probably has never been equalled by any other group of mammals in such a size range.
As we look out among the herds of animals dotting the plain, we can see only a few small bands of oreodons. There’s a group coming toward us now. These are some big, ugly ones with large triangular-shaped heads. The backs of their cheek bones flare out far to the sides, so that with their narrow snouts they are most peculiar looking. Their bodies are long and rather nondescript, and their legs are short but slender. This particular kind is known as Promerycochoerus (“before ruminant hog”) and is just about the largest of the oreodons. They are really a rare sight here at Agate. Perhaps the large herds of Menoceras fill their ecologic niche locally, and the oreodons have found they cannot successfully compete with the rhinos for food, water, and living space. After all, not everything can fit into Paradise.
Promerycochoerus
Look to the northeast: there’s a herd of Miohippus (“Miocene horse”) wading into the river to drink and browse in the willows along its banks. Let’s walk toward the herd slowly and quietly. We should take an especially good look at this herd—they are part of a doomed race! The genus Miohippus is making its last stand at this time. When conditions change, well adapted species may restrict their ranges to what is left of the old environment; they may adapt, if they are able, to the new conditions; or they may not survive if they cannot adapt.
Miohippus did all these things. Some species of the genus became extinct. Some evolved into something else. But the end result was the complete termination of the genus Miohippus as paleontologists recognize it. Much of the environmental pressure coming to bear on the genus Miohippus was a result of mountain building to the west. As the young Rockies rose, rain-bearing winds from the oceans far to the west were wrung of their moisture. This same circumstance makes the high plains a land of little rain today.
The scattered trees and groves we see from our vantage point of long ago will disappear and be replaced with a sea of drought-tolerant grasses. In effect, the savannas will give way to prairies. Miohippus will soon be yielding its place to descendants which can eat grass as a steady diet. Grass is much harsher on the teeth than the foliage that Miohippus eats. In eating grass, grazing animals pick up sand and silt enough to quickly wear away teeth designed for leaf-eating. The descendants of Miohippus will become better runners, too, with longer and more powerful legs. As the trees disappear there will no longer be friendly clumps of greenery to hide behind when hungry meat-eaters are on the prowl. From now on, fleetness of foot will be a most important factor in horse survival.