Early Evolutionary History of Horses

A Cladogenetic View of the Early Evolutionary History of Horses

Drawing upon the theoretical and empirical literature, this report describes evidence obtained from the fossil record supporting the anagenetic view of the horse phylogeny and its early evolutionary development. The anagenetic perspective advances the notion that a gradual microevolutionary process has occurred within a species. This view is also associated with a phyletic transformation from ancestral to descendant species. The fossil record for horses presents, in general, a progressive replacement of one genus with another without a strong indication of temporary overlapping. However, the literature also suggests that anagenetic development was dominant within the Eocene and Oligocene, while genetic diversity (branching) suggesting a cladogenetic developmental pattern is observed in the fossil record from the second half of horse development. The report focuses, therefore, on the North American Eocene and Oligocene fossil records. Both periods are considered to be a time of low equine diversity, depicted as a simple anagenetic progression of genera from Mesohippus to Miohippus representing a single trunk of the phylogenetic tree. From this time on, a cladogenetic developmental process is seen to be at work.

The fossil record of horses (family Equidae) over the past 55 million years is a classic example of macroevolution (MacFadden, 1999). North America is the ancestral home

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horses are found abundantly in deposits of the most recent geological age in almost every part of America and Ridgeway (1972) believes that in pre-glacial times, North America possessed at least nine perfectly distinct wild species of Equidae. These species varied significantly in size, but disappeared from America at the time of the Spanish conquest. Few satisfactory explanations for this disappearance have been offered, though Ridgeway (1972) makes reference to the possibility that after coming to the end of their evolutionary tether in the attainment of speed, these horses fell prey to one or more predators. Another theory is that an infestation of parasites of various types may have led to the extinction of the North American Equidae, which may most probably have crossed into Asia prior to such an infestation. Hulbert (1996) states that at the beginning of the Eocene epoch, about 57 million years ago, tiny "dawn horses" known as hyracotheres appeared in the faunas of North America and Eurasia. Evidence obtained from the fossil record at Costillo Pocket in south-central Colorado provides evidence of two disparate species, one larger and one smaller. At the end of the early Eocene, the continental drift severed the direct

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