Volume 2 | Issue 2 Article 10

1940 Evolution of the Horse M. R. Benson Iowa State College

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Recommended Citation Benson, M. R. (1940) "Evolution of the Horse," Iowa State University Veterinarian: Vol. 2 : Iss. 2 , Article 10. Available at: https://lib.dr.iastate.edu/iowastate_veterinarian/vol2/iss2/10

This Article is brought to you for free and open access by the Journals at Iowa State University Digital Repository. It has been accepted for inclusion in Iowa State University Veterinarian by an authorized editor of Iowa State University Digital Repository. For more information, please contact [email protected]. Evolution of the Horse M. R. BENSON Class of 1940

HE phylogeny of the horse family digits (c) on both fore and rear feet. The T is perhaps the most complete record animal was more adapted for speed. of organic evolution that modern science Many representatives of this group have has discovered. This article is an at­ been discovered, ranging from 18 inches tempt to show briefly the outstanding to 24 inches, or 6 hands in height. This steps in the development of the modern animal is usually considered as a tran­ horse. sitional type between the earlier forest­ Most investigators agree that the horse dwelling animal and the later plains­ tribe originated during the Eocene epoch. inhabiting creature. Luxuriant forests grew in the warm, The Miocene era was a time of great humid climate which characterized this topographical change during which our era in North America. Numerous streams highest mountain ranges were formed. and lakes gave rise to sedgy meadows Great increases of prairie lands and a and extensive grass lands. decrease of forest lands resulted from these physiographical alterations. As a Eohippus result of these land changes, many brows­ Eohippus (A), the four-toed horse, ing animals perished, but the grazing represents the first stage in equine evo­ types, horses, camels, deer and antelope, lution. It was a slender, lithe creature, adapted themselves to the new condi­ averaging about 12 inches or 3 hands in tions and became the dominent forms height at the withers. In fact, the gen­ of mammalian life. This period saw a eral proportions were similar to those wide dissemination of the horse tribe, of a dog of the fox terrier type. The spreading to the European continent by fore foot bore four complete toes, each means of newly formed land bridges. terminating in a hoof-like nail (a). The rear foot had three similar toes with Merychippus remnants of the first and fifth. Perhaps the best example of the horses of this period is Merychippus (D), which Orohippus marks the transition from horse-like Orohippus (B), the next stage, showed forms of the past with short-crowned, an advance in adaptation by the loss of uncemented teeth to the true horses, the splint of the fifth digit in the rear whose long-crowned, fully cemented mo­ foot. The digits of the fore foot still lars and premolars are more suited to numbered four, but the middle digit the harsh vegetation of plains areas. became more prominent. The animal's Merychippus was three-toed, but the skeleton, as mounted at Yale University, lateral toes did not reach the ground so measures 13% inches in height. the animal was functionally one-toed. The Oligocene period was a time of increased aridity due to changes in con­ Pliohippus tinental contour, which gave impetus to Pliohippus (E) was the first one-toed the development of broad meadows and horse. It is the direct ancestor of the true prairie lands. prehistoric wild horses of Eurasia from which modern breeds are descended. Mesohippus This animal had a shoulder height of Mesohippus (C) attained the size of some 40 inches, or 10 hands. The skeletal a collie dog and had three functional remains of this animal show an adapta-

66 T he Veterinary Stlldent &",,;son '¥

Winter-I 94° 6""I tion of the one-toed structure (e) as Symptoms: The symptoms are marked found in modern horses, although the weakness, loss of weight, rough coat, terminal phalange retains a cleft in its diarrhea, and occasionally convulsions. anterior border, as was common in pre­ In outbreaks that occur during the winter vious genera. C\ pus-like discharge is noticed from the eyes and nostrils. In summer outbreaks Prejvalski horse this symptom has never been noticed. Several species of horse-like animals Post Mortem Lesions: The eyes are are yet alive in their wild condition in markedly sunken, the spleen is much Asia and Africa, all of those of Europe enlarged and usually a very dark color. and the Americas being either domesti­ The intestines are greatly inflamed. In cated or of domestic ancestry. Of the cases showing a nasal or eye discharge true wild horses but one remains: the the upper respiratory tract is inflamed Mongolian or Prejvalski horse, which in­ and frequently pneumonia is present. In habits the Gobi desert of central Asia those cases that fail to show an eye or and the neighboring regions. "It is a nasal. discharge, the lungs and respira­ small animal standing but 12 hands, of tory tract appear to be normal. a yellow dun or 'buckskin' color, with The paratyphoid organism can be iso­ black mane, tail, and legs and a white lated from the spleen of fatal cases and muzzle. There is no forelock, the mane when injected into healthy foxes will is short and upright and there is a de­ produce the disease with the same symp­ cided beard beneath a relatively large toms as found in ranch cases. The or­ head." (2) ganism can be passed serially from fox Many other genera and species of ex­ to fox-usually killing them in eighteen tinct equidae have been discovered and days. described by various authors, and sev­ Vaccination with a product prepared eral extinct examples of the genus from the specific organisms causing the Equus have been found in North Amer­ disease appears to be effective in con­ ica. However, the modern horse, as we trolling the disease in some cases. This, know it, is descended from European along with proper sanitation, hygiene and Asian forms. and isolation of infected cases, tends to control the infection. Bibliography (1) Loomis, F. B., The Evolution of the Horse, Fox Encephalitis 1926. This disease became very prevalent in (2) Lull, R. S., Organic Evolution, 1929. 1926-1927 and was known to have been (3) Lydekker, R., The Horse and Its Relatives, 1929. introduced on a number of farms in several states by foxes that had been ex­ hibited at fox shows. Fox encephalitis FOXES- is due to a filtrable virus which especial­ (Continued from Page 64) ly effects the nervous system. The dis­ ease appears to attack young and old Paratyphoid Infection animals in a bout the same proportion, This is a disease which was very the mortality rate ranging from fifteen prevalent in Iowa in 1925 and again in to forty per cent. Death usually occurs 1938. It is an infection especially of early in the disease and for this reason young animals. The course of the disease the dead fox is usually in good flesh. varies greatly, the sick animal living Symptoms: Most animals dying dur­ from one week to eighteen days. The ing an outbreak will be found dead, and mortality rate is extremely high in some usually it is reported that the fox was cases, reaching sixty per cent of all seen some hours before and appeared puppies born during the year. The dis­ perfectly healthy. When symptoms are ease usually occurs as an epidemic. observed on ranches there are usually

68 The Veterillary Studen t