Evolutionary history of the horse from Eocene to modern Equus

Evolutionary Development of the Horse

The horse is a hoofed mammal, a subspecies of the family Equidae, that has evolved over the past 45 to 55 million years from a small multi-toed creature into the large single-toed animal that we know today. The fossil record of the horse is extremely complex and detailed, probably due to the fact that paleozoologists have managed to put together the most complete picture of the horse's evolutionary history than that of any other animal. Another likely difficulty stems from the non-linear structure of the horse's evolutionary process. There are many branches of horse evolution, which makes daunting the prospect of conceiving an evolutionary diagram that accurately relates all the horse's evolutionary mechanisms. (Simpson, 1961) The horse as a vertebrae group is fairly well-known, and the modern view of evolution within this group is somewhat difficult to simplify and summarize.

The horse belongs to an order known as Perissodactyla, meaning 'odd-toed ungulates (mammals).' This order shares a number of physical traits, including hoofed feet and odd number of toes on each foot, along with the tendency to bear their weight on the central third toe, and mobile upper lips and a similar tooth structure. The majority of horse species arose in North America, including all the ancestors of the Equus. A sequence of their transitional fossils was first discovered in the 1870's by the paleontologist O.C. Marsh. This astonishing new find was a clear and precise depiction of evolution taking place in a single lineage. The fossil species Eohippus (the term was later changed to Hyracotherium) transformed into the recognizable form of its descendant, Equus, through a series of very clear intermediaries. This is what led the horse family to become the evolutionary standard used in museums, lectures, and textbooks. (McFadden, 1988)

The biological classification of the domestic horse, which is the animal that most people picture when the term 'horse' is used, is as follows: the horse belongs to the Animalia kingdom, the Chordata phylum, the Mammalia class, the Theria subclass, the Eutheria infraclass, the Perissodactyla order, the Equidae family, the Equus genus, the Equus Ferus species, and the Equus ferus caballus sub-species. The evolutionary process of the horse occurs within the categories of the family and the genus. (Hall and Olsen, 2007)