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 of horses and many fossil sites across the continent have been identified as containing abundant remains of ancient members of the family. MacFadden (1994) states that at any given fossil locality in North America from about 50-55 to 20-25 million years ago, it is usually possible to find 2-4 species of horses that seem to have lived side-by-side. However, from about 20-10 million years ago, the fossil record indicates that horses evolved rapidly, adapting to various environments and w...