If you ask a person what elephants, pigs, elk, deer, hippos, giraffes, tapirs, horses, zebras, manatees, goats, sheep, antelope and camels all have in common, there is a pretty good chance that the person would only be able to say that they are all animals and perhaps that they are all mammals. They are indeed animals and they are all mammals; however, they have another connection. All of these are also ungulates.
Members of the ungulata order are mammals that have hooves. As simple as that may sound, it encompasses species that might not be thought of as belonging to the group. For instance, an elephant's hooves appear more like fingernails, and yet they are true hooves. A hoof is really a modified toenail. People might not even consider manatees and dugongs since these creatures live their entire lives in the water and don't have legs, but they have the hoof structure, so they are ungulates. The hooves (or hoofs; both spellings are technically correct) amount to an enlarged toenail. Thus, elephants are indeed ungulates.
Number of toes
Normally, the ungulate order is divided into the artiodactyla and perissodactyla suborders. The primary difference between the two is that the latter usually has an odd number of toes, and the former normally has an even number of toes. Thus, a horse is a member of the perissodactyla suborder, since it has a single hoof on each foot. There are exceptions, however. There are some peccaries (a kind of wild pig) that are considered to be in the artiodactyla suborder, despite having four toes on their front feet and three on their rear.
The reason for this is that, rather than the number of toes, it is actually the toe arrangement that makes the difference. More specifically, in artiodactyls, the plane of the foot passes between the third and fourth toe. In perissodactyls, it passes directly through the third toe. Horses are perissodactyls not because they have a single hoof on each foot, but because that hoof happens to be on the third toe and the plane of the foot passes through it.
Living and extinct
Many members of the ungulates are extinct and scientists know about them primarily from fossil records. This includes such creatures as the Baluchitherium or Indricotherium, which was sort of like a huge rhinoceros without a horn. This also includes the Eohippus, a miniature horse that is believed to have died out about 50 million years ago.
In some instances, many or most of the genus and species in the family are extinct, but the group is still marginally represented. This is the case of the proboscideans (order proboscidea). This was once an order that had a large number of species, and the order was quite successful. Today, only two species survive (or three, depending on the sources used to define the species): the African Elephant and the Asian Elephant. Notable extinct members of the proboscideans include the mammoths and the mastodons.
Still other groups are currently well represented, such as the bovidae (cattle, bison and oxen) and the suidae, which includes pigs, peccaries, warthogs and boars. Members of the deer family (cervidae), goats and sheep (both members of the bovidae family) and many others are also well represented and populous today.
Confusion and clarification
Though all of this sounds pretty straightforward, in more recent years it has become more complex and confusing. Due to anatomical similarities, some taxonomists now class the cetaceans (whales) in with the ungulates. Others feel that cetacea should remain as its own order.
At the same time, looking at anatomical similarities has clarified some relationships within the ungulates. For example, for some time, many people felt that elephants, hyraxes and tapirs were more distantly related to other ungulates and barely fit within the order. Anatomically, it has been found that they are actually a lot more related than once believed, making these creatures more solidly ungulates.
It is hard to find an order that is more diverse than the ungulates. They are wide-spread and survive from the arctic to the deserts, mountains to oceans, land to water. Many are plant eaters, others are omnivores and yet others eat insects. Many species are extinct and still others live on. The ungulate order is one that is quite interesting, while also being a challenge to taxonomists, who've been forced to reconsider some of their classification scheme.