Early Evolutionary History of Horses

Drawing upon the theoretical and empirical literature, this report describes the evidence obtained from the fossil record that supports 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. It is also associated with 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 can be observed from 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.

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 identifie

. . .
rst appearance. Hoofed animals began to occur in the Tertiary period. With two extinct families of the Perissodactyles, the earliest ancestral forms of the horses and tapirs of today can be observed. Fossil remains of 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 o
. . .

Some common words found in the essay are:
North America, Gaylord Simpson, North American, Summary Conclusions, Mesohippus Miohippus, Cope's Rule, Abstract Drawing, Cladogenesis Hulbert, World World, Northern Hemisphere, fossil record, hulbert 1996, macfadden 1992, north america, simpson 1951, cladogenetic branching, north american, macfadden 1994, species fossil, ridgeway 1972, hulbert 1996 macfadden, species fossil record, 1996 macfadden 1992, beginning late eocene, fossil record horses,
Approximate Word count = 2497
Approximate Pages = 10 (250 words per page)

More Essays on Early Evolutionary History of Horses