THE EVOLUTION OF ANIMAL LIFE 



quent to the Cretaceous. Remnants of another hne of descent from the 

 ancient ancestral stock persist to the present, however, represented by the 

 genus Nautilus, which is now hmited to the tropical Pacific and Indian Oceans 

 (see Fig. 13.16, p. 388). Thus, Nautilus has preserved the general organiza- 

 tion of its remote ancestors, without substantial change, through a span of 

 time vastly longer than that required for the evolution of the entire series of 

 vertebrate animals. This is an exceptional case; the great majority of animals 

 have become strongly modified during the passing ages, or, like the am- 

 monoids, have become extinct. 



Vertebrates: The Horse Family. The fossil record of horses is probably 

 more nearly complete than that of any other mammalian type. The largest 

 series of remains appears in fossil beds of the western United States, where 

 horses flourished for a long period before they became extinct during the 

 Pleistocene. In more recent times the members of the horse family (which 

 includes horses, wild asses, onagers, and zebras) have been found in their 

 original wild state only in Europe, Asia, and Africa. The "wild" horses of 

 the Americas in recent centuries were the descendants of horses introduced by 

 early settlers. The earliest native American horses, such as Hyracothenum 

 { = Eohippus), the dawn horse (Fig. 20.5), are believed to have originated in 

 some other region and migrated to North America; they appear in the fossil 

 record here without recognizable antecedent types. Central Asia is suspected 

 as the place of origin, although the ancestors of Hyracothenum have not yet 

 been discovered there. The fossil record shows that the horse family flourished 

 in North America for many millions of years; some of its members eventuallv 

 migrated to South America and became widelv distributed there. This south- 

 ern branch of the family became extinct in the Pleistocene, along with the 

 North American horses. The causes of this extinction, as those of the dis- 

 appearance of ammonoids, dinosaurs, and many other types of animals, 

 remain matters of speculation. One possibility is that all the horses of the 

 New World may have been swept away by some epidemic. We know onlv 

 that they disappeared in the New World, although a few species survived on 

 the continents of the Old World; these Old World lines were apparently 

 established by migrants from^ North America. 



The fossil record gives a clear indication of the probable course of evolution, 

 through which forms like Hyracothenum gradually evolved into horses of the 

 modern genus Equus (Fig. 20.6). The series begins with small animals, rang- 

 ing from the size of a small dog to half the size of a modern ponv, with rela- 

 tively short neck and limbs. There were four functional digits on each fore 

 foot, three on each hind foot, with "splint bones" representing the vestiges 

 of other toes no longer functioning to bear the weight of the body. The foot 

 was also supported behind by a cushion-like pad. The characteristics of the 

 feet and the teeth indicate that Hyracothenum was a forest-dwelling animal 

 which browsed the leaves of bushy plants and low trees; the vestigial digits 

 reveal that it had evolved from ancestors having five toes on each foot. The 

 dawn horse was markedly different in many respects from modern horses — so 



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