fusion of the radius and ulna prevents an ungulate from rotating its forelimb. Since this skeletal structure has no specific function in ungulates, it is considered a homologous characteristic that ungulates share with other mammals. This trait would have been passed down from a common ancestor. While the two orders of ungulates colloquial names were based on the number of toes of their members ("odd-toed" for the perissodactyls and "even-toed" for the terrestrial artiodactyls), it is not an accurate reason they were grouped. Tapirs have four toes in the front, yet they were members of the "odd-toed" order; peccaries and modern cetaceans were members of the "even-toed" order, yet peccaries have three toes in the front and whales were an extreme example as they have flippers instead of hooves. Scientists had classified them according to the distribution of their weight to their toes. Perissodactyls have a mesaxonic foot meaning that the weight is distributed on the third toe on all legs thanks to the plane symmetry of their feet. There has been reduction of toes from the common ancestor, with the classic example being horses with their single hooves. In consequence, there was an alternative name for the perissodactyls the nearly obsolete Mesaxonia. Perissodactyls were not the only lineage of mammals to have evolved this trait; the meridiungulates have evolved mesaxonic feet numerous times. Terrestrial artiodactyls have a paraxonic foot meaning that the weight is distributed on the third and the fourth toe on all legs. The majority of these mammals have cloven hooves, with two smaller ones known as the dewclaws that were located further up on the leg. The earliest cetaceans (the archaeocetes), also have this characteristic in the addition of also having both an astragalus and cuboid bone in the ankle, which were further diagnostic traits of artiodactyls. In modern cetaceans, the front limbs have become pectoral fins and the hind parts were internal and reduced. Occasionally, the genes that code for longer extremities cause a modern cetacean to develop miniature legs (known as atavism). The main method of moving is an up-and-down motion with the tail fin, called the fluke, which is used for propulsion, while the pectoral fins together with the entire tail section provide directional control. All modern cetaceans still retain their digits despite the external appearance suggesting otherwise. Teeth Most ungulates have developed reduced canine teeth and specialized molars, including bunodont (low, rounded