The phylogeny of the horse family is perhaps the most complete record of organic evolution that modern science has discovered. This article is an attempt to show briefly the outstanding steps in the development of the modern horse

Benson, M. R.

English

The phylogeny of the horse family is perhaps the most complete record of organic evolution that modern science has discovered. This article is an attempt to show briefly the outstanding steps in the development of the modern horse.</p

Benson, M.

Digital Repository @ Iowa State University (ISU)

Evolution of the Horse M. R. BENSON Class of 1940 THE phylogeny of the horse family is perhaps the most complete record of organic evolution that modern science has discovered. This article is an at-tempt to show briefly the outstanding steps in the development of the modern horse. Most investigators agree that the horse tribe originated during the Eocene epoch. Luxuriant forests grew in the warm, humid climate which characterized this era in North America. Numerous streams and lakes gave rise to sedgy meadows and extensive grass lands. Eohippus Eohippus (A), the four-toed horse, represents the first stage in equine evo-lution. It was a slender, lithe creature, averaging about 12 inches or 3 hands in height at the withers. In fact, the gen-eral proportions were similar to those of a dog of the fox terrier type. The fore foot bore four complete toes, each terminating in a hoof-like nail (a). The rear foot had three similar toes with remnants of the first and fifth. Orohippus Orohippus (B), the next stage, showed an advance in adaptation by the loss of the splint of the fifth digit in the rear foot. The digits of the fore foot still numbered four, but the middle digit became more prominent. The animal's skeleton, as mounted at Yale University, measures 13% inches in height. The Oligocene period was a time of increased aridity due to changes in con-tinental contour, which gave impetus to the development of broad meadows and true prairie lands. Mesohippus Mesohippus (C) attained the size of a collie dog and had three functional 66 digits (c) on both fore and rear feet. The animal was more adapted for speed. Many representatives of this group have been discovered, ranging from 18 inches to 24 inches, or 6 hands in height. This animal is usually considered as a tran-sitional type between the earlier forest-dwelling animal and the later plains-inhabiting creature. The Miocene era was a time of great topographical change during which our highest mountain ranges were formed. Great increases of prairie lands and a decrease of forest lands resulted from these physiographical alterations. As a result of these land changes, many brows-ing animals perished, but the grazing types, horses, camels, deer and antelope, adapted themselves to the new condi-tions and became the dominent forms of mammalian life. This period saw a wide dissemination of the horse tribe, spreading to the European continent by means of newly formed land bridges. Merychippus Perhaps the best example of the horses of this period is Merychippus (D), which marks the transition from horse-like forms of the past with short-crowned, uncemented teeth to the true horses, whose long-crowned, fully cemented mo-lars and premolars are more suited to the harsh vegetation of plains areas. Merychippus was three-toed, but the lateral toes did not reach the ground so the animal was functionally one-toed. Pliohippus Pliohippus (E) was the first one-toed horse. It is the direct ancestor of the prehistoric wild horses of Eurasia from which modern breeds are descended. This animal had a shoulder height of some 40 inches, or 10 hands. The skeletal remains of this animal show an adapta-T he Veterinary Stlldent &",,;son '¥ Winter- I 94° 6"" I tion of the one-toed structure (e) as found in modern horses, although the terminal phalange retains a cleft in its anterior border, as was common in pre-vious genera. Prejvalski horse Several species of horse-like animals are yet alive in their wild condition in Asia and Africa, all of those of Europe and the Americas being either domesti-cated or of domestic ancestry. Of the true wild horses but one remains: the Mongolian or Prejvalski horse, which in-habits the Gobi desert of central Asia and the neighboring regions. "It is a small animal standing but 12 hands, of a yellow dun or 'buckskin' color, with black mane, tail, and legs and a white muzzle. There is no forelock, the mane is short and upright and there is a de-cided beard beneath a relatively large head." (2) Many other genera and species of ex-tinct equidae have been discovered and described by various authors, and sev-eral extinct examples of the genus Equus have been found in North Amer-ica. However, the modern horse, as we know it, is descended from European and Asian forms. Bibliography (1) Loomis, F. B., The Evolution of the Horse, 1926. (2) Lull, R. S., Organic Evolution, 1929. (3) Lydekker, R., The Horse and Its Relatives, 1929. FOXES-(Continued from Page 64) Paratyphoid Infection This is a disease which was very prevalent in Iowa in 1925 and again in 1938. It is an infection especially of young animals. The course of the disease varies greatly, the sick animal living from one week to eighteen days. The mortality rate is extremely high in some cases, reaching sixty per cent of all puppies born during the year. The dis-ease usually occurs as an epidemic. 68 Symptoms: The symptoms are marked weakness, loss of weight, rough coat, diarrhea, and occasionally convulsions. In outbreaks that occur during the winter C\ pus-like discharge is noticed from the eyes and nostrils. In summer outbreaks this symptom has never been noticed. Post Mortem Lesions: The eyes are markedly sunken, the spleen is much enlarged and usually a very dark color. The intestines are greatly inflamed. In cases showing a nasal or eye discharge the upper respiratory tract is inflamed and frequently pneumonia is present. In those cases that fail to show an eye or nasal. discharge, the lungs and respira-tory tract appear to be normal. The paratyphoid organism can be iso-lated from the spleen of fatal cases and when injected into healthy foxes will produce the disease with the same symp-toms as found in ranch cases. The or-ganism can be passed serially from fox to fox-usually killing them in eighteen days. Vaccination with a product prepared from the specific organisms causing the disease appears to be effective in con-trolling the disease in some cases. This, along with proper sanitation, hygiene and isolation of infected cases, tends to control the infection. Fox Encephalitis This disease became very prevalent in 1926-1927 and was known to have been introduced on a number of farms in several states by foxes that had been ex-hibited at fox shows. Fox encephalitis is due to a filtrable virus which especial-ly effects the nervous system. The dis-ease appears to attack young and old animals in a bout the same proportion, the mortality rate ranging from fifteen to forty per cent. Death usually occurs early in the disease and for this reason the dead fox is usually in good flesh. Symptoms: Most animals dying dur-ing an outbreak will be found dead, and usually it is reported that the fox was seen some hours before and appeared perfectly healthy. When symptoms are observed on ranches there are usually The Veterillary Studen t 

Evolution of the Horse

https://dr.lib.iastate.edu/handle/20.500.12876/46007

Evolution of the Horse

Abstract

Similar works

Full text

Available Versions

Digital Repository @ Iowa State University (ISU)