HORSES

Overview

Horses (equids) are herbivore ungulates (hoofed mammals) belonging to order Perissodactyla, which also includes tapirs and rhinos. The fossil record of horses ranges from Paleocene?, and early Eocene to Recent. Their Cenozoic biodiversity (number of families, genera and species) was highest during the first half of the Cenozoic when they arose on northern continents and later spread southward into Africa and South America. During the second half of the Cenozoic, their biodiversity has gradually and significantly been reduced. 

Most of the early Cenozoic equids (like the other perissodactlys) lived in woodlands and were rooters. During the later Cenozoic, with the spread of grasses (tough to chew and digest), many perissodactyls adapted to living on prairie grasslands.

With the changes in where and how they lived, as well as in what they ate, horses adapted accordingly. Their bodies became larger with longer legs, and the number of toes on each foot decreased. These changes enabled horses to become stronger and faster runners, thus better fleeing their predators. Their skulls also got bigger, thereby allowing for bigger brains. Larger skulls allowed also for larger teeth that were more complex internally, with vertical layers (folds) of enamel growing down, deep inside the teeth. These kind of teeth are called hypsodont teeth (see one of my previous posts), and they are very effective in increasing the strength of the teeth, thereby resisting wear and tear when the horses eat tough grasses with a high content of abrasive silica. Like humans, horses have only two sets of teeth: milk and adult. The latter can last up to 15 years in modern horses.

 About 10,000 years ago, domestication of horses and other livestock began.

Earliest Horses

The earliest known perissodactyls are two genera (one species each) of early Eocene age. Both are small dog-size—see one of my previous posts: Eohippus in Wyoming, northwest New Mexico, and Colorado; and Hyracotherium in London, England and in Paris, France. The morphologic and biogeographic histories of horses are summarized in the following chart.

Horse Teeth

Like in most mammals, the teeth of a horse are differentiated into four types: incisors, canines, premolars, and molars. The premolars and molars are grouped together as the cheek teeth. The teeth are, furthermore, very characteristic of the orders of the mammals to which they belong, and the morphology of the teeth reflect the diets of the mammals.

The diagram above shows the right-side view of the skull/teeth of a modern horse.

Mammal-dental formulae typically designate the number of teeth (upper/lower) on just one side of both jaws. For example, the formula for many modern horses is 3/3, 0/0, 3/3, 3/3, and it refers to the presence of three incisors, no canines, three premolars, and three molars on one side of the upper and lower jaws = 18 teeth. In order to obtain the total number of 36 teeth (excluding the variations noted in the next two paragraphs), one has to include also the teeth on the other side of the skull. Thus, the horse has a total of 36 teeth (18 + 18), when both sides of the skull are counted. This dentition formula is the same for donkeys, zebras, and the Przewalski wild horse. It is important to mention that in all of these animals, their teeth continue to erupt (grow) during the life of the animal. This is because they eat coarse/tough vegetation which wears down the teeth. The cheek teeth of horses erupt in a tightly packed unit (called an "arcade") that acts like a single grinding tooth. Cheek teeth are a composite of three hard materials: enamel, dentin, and cementum.

Male horses typically have 40 teeth because between ages four and five years old, they develop upper and lower canine teeth. The canines are positioned nearer the incisors than the premolars (on both sides of the jaws). Only about 25% of female horses develop canines and are smaller in size than those found in males.  

There is an additional potential of dental variability: between 13 and 32% of both male and female horses can have small-sized vestigial (non-useful) premolar (referred to as a “wolf tooth”) located in front of the first premolar (typically only on the upper jaw). 

Reference Used:

Carson, D.M. and S. W. Ricketts. Equine dentistry. vcahospitals.com