Horse
by Amy S.
A horse is a domesticated, four-legged, hoofed mammal that ranges in size from a small dog to a small elephant. In the wild, horses live mostly in herds on grasslands or deserts. Uniquely, horses are measured in hands, or units of four inches. If a horse is shorter than 14.2 hands, it is considered a pony. Female horses above the age of four are called mares. Males above the age of four are called stallions, but if they have been gelded, or neutered, they are referred to as geldings. If a horse is under one year old, it is referred to as a foal no matter the gender. Females between one and four years old are called fillies; males of the same age are called colts. You can roughly tell how old a horse is by looking at its teeth. This is because a horse’s teeth grow as it gets older. Therefore, the longer and thicker the teeth are, the older the horse is. Horses crossed the North Pacific Bridge to Asia twenty-four million years ago, while they were still evolving.
Horses evolved through eight different traits according to our cladogram. First, horses became vertebrates, evolving backbones and skulls. Next, horses became Gnathostomes, Tetrapods, and Amniotes, developing jaws, four limbs, and watertight eggs. They later evolved to be Synapsids and Mammals, acquiring holes behind their eye sockets and three middle ear bones. Subsequently, horses became Placentals, or animals that give live birth. Finally, horses developed hooves and became Ungulates, becoming the equines that we know today.
The Whale, Mammoth, Irish elk, and Hylochoerus Meinertzhageni are the animals most closely related to the horse on our cladogram. Hoofed feet are the most advanced trait that they share. I used our cladogram to figure these two things out. I traced the path of the horse backwards to the most recent node. Then I looked to see what other animals also branched off from that node to find the closest relatives of the horse. Once I had found the closest relatives, I followed those branches backwards on the cladogram to find the most advanced node that they all shared. Interestingly, whales are considered Ungulates. However, they don’t have hoofs. Believe it or not, they used to, when they walked on land. Now they live in water and have abandoned their hoofs, but they are still considered Ungulates. This is the most important difference between horses and whales – horses live on land and whales live in the ocean. Mammoths and Irish elk are extinct, distinguishing them from still existent horses. Hylochoerus Meinertzhageni is of the order Artiodactyla, or even-toed ungulates, and horses are of the order Perissodactyla, or odd-toed ungulates.
Chalicotheres and Brontotheres, or large, hoofed mammals, are the closest relatives of the horse on the cladogram at the American Museum of Natural History. Two advanced traits that they share are hoofs and three toes with the middle being the longest. I got this information by using the cladogram at the museum the same way that I used our cladogram to figure out the closest relatives and most advanced traits of the horse.
Last updated April 7, 2007.