With the exception of snakes, all terrestrial vertebrates get around using their feet. Feet are truly remarkable organs. They can adapt to any sort of habitat and allow animals to move in all sorts of different ways: walking, running, jumping, climbing, digging, swimming. A lot can be said about an animal’s lifestyle and evolutionary history from looking at its feet. Vertebrates can be classified by their specific foot anatomy and locomotion into one of the following three categories, plantigrade, digitigrade, and unguligrade.
The most primitive kind of foot anatomy is known as plantigrade. Reptiles display plantigrade feet, as do the earliest mammals. Plantigrade animals walk with the entirety of the sole touching the ground; walking with this gait is sometimes known being flat-footed. Having the whole foot on the ground provides increased stability and greater weight-bearing capacity. However, having so many bones at end of the foot makes the leg short and heavy and as a result plantigrade animals are not typically built fir speed. Arboreal animals such as primates, raccoons, and red pandas are plantigrade, as are most rodents, mustelids, and marsupials. The largest plantigrade animals are bears, which are also the only megafaunal plantigrade species.
The form of foot anatomy that most people are familiar with is digitigrade. Rather than walking on the sole of the foot, digitigrade animals walk with their weight borne on their toes, or digits. Most mammals, as well as dinosaurs and birds, are digitigrade. What is usually mistaken for a backwards-bending knee is in reality the ankle joint. This is especially exaggerated in birds, whose actual knees are usually hidden by skin and feathers. Digitigrade animals have proportionally longer feet and shorter legs than plantigrade animals. Birds walk with their toes directly touching the ground where as mammals such as dogs and cats have paw pads. Having more of the limb raised off the ground increases stride length and coupled with less weight at the far end of leg allows digitigrade species to run with greater speed than plantigrade species.
The last variety of foot anatomy to evolve was the unguligrade foot. Unguligrade is just a fancy way for saying that an animal has hooves and indeed, the scientific term for hooded animals is “ungulate.” Where digitigrade animals walk on their toes, ungulates walk on just the very tips of their toes, or ungual bones. A hoof is a modified toenail composed of keratin and adapted for bearing weight. Ungulates can be further divided into the number of toes the possess. Artiodactyls are the even-toed ungulates and include the majority of hoofed mammals: deer, camels, cattle, giraffes, pigs, peccaries, hippos, goats, sheep, and antelope. Genetic research within the last few years has shown that whales and dolphins are also members of this group, having evolved from a hoofed ancestor distantly related to hippos. Odd-toed ungulates, or perissodactyls, are considerably less diverse and consist of only horses, rhinos, and tapirs. Being unguligrade is an even grater adaptation for cursorial locomotion than being digitigrade for much the same reasons that being digitigrade is better for running than being plantigrade is.
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