Box Horse evolution the most famous example of an evolutionary trend

Fossil horses were often found by early 19th century paleontologists, such as Cuvier (see p. 12). By 1875, a convincing evolutionary story had been worked out by Marsh, based on his studies of sequences of mammalian fossils through the North American Tertiary: he could document evolution from the Eocene Hyracotherium, the size of a terrier, to the modern large horse, through a series of intermediates. Here was a perfect, single-line, progressive, evolutionary trend, showing how horses became ever larger and larger, and faster and faster through time.

The reality is, however, more complex. There was no single one-way pattern of change. The evolutionary tree of horses (Fig. 20.6) branches many times, and small, medium and large horse species coexisted in North America in the Miocene. The lengthening of the legs, reduction in the numbers of toes from four or five to one, and deepening of the teeth happened haphazardly across the diversity of Oligocene and Miocene horses as great prairie grasslands spread over North America. The height and single hooves were adaptations for fast running, and the deep teeth were necessary to permit horses to deal with the tough grasses. The survival of similar large species of Equus today - horse, donkey, zebra - is the result of chance events, such as the catastrophe in the Late Pliocene when the diverse North American native species died out. Equus is good at running fast on open grassy plains, and Hyracotherium was good at camouflaging itself in leafy woods. Is Equus better, or merely different?

Read more about horse evolution in MacFadden (1992), and at web sites listed on http://www. blackwellpublishing.com/paleobiology/.

Phylogeny Horses

Figure 20.6 The evolution of the horses has been interpreted as a simple one-way trend towards large size, single toes and deep teeth. The reality is more complex: horse evolution has followed a branching pattern, and the line to the modern horses, Equus, was not preordained: notice the diversity of North American horses in the late Miocene and Pliocene. The evolutionary steps did not all occur in parallel: Merychippus was a grazing horse, with deep-rooted teeth, but retained a three-toed foot. (Based on MacFadden 1992, and other sources.)

Figure 20.6 The evolution of the horses has been interpreted as a simple one-way trend towards large size, single toes and deep teeth. The reality is more complex: horse evolution has followed a branching pattern, and the line to the modern horses, Equus, was not preordained: notice the diversity of North American horses in the late Miocene and Pliocene. The evolutionary steps did not all occur in parallel: Merychippus was a grazing horse, with deep-rooted teeth, but retained a three-toed foot. (Based on MacFadden 1992, and other sources.)

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