In each order, a progression of elongation can be seen among families. In horses , the 3rd metapodial is very long, similar in length to the other main limb elements. The other toes are lost or reduced to the point of being slivers of bones that fuse with the 3rd. Among the much more diverse artiodactyls, pigs , peccaries and hippos have moderately long metapodials, which are unfused. In camels , the third and fourth metapodials are very long and fused for most of their length, although the distal ends remain separate.
Fusion is complete in bovids and cervids , and the resulting bone is called a cannon bone.
Other digits are much reduced or lost. Lengthening and fusion of metapodials is also associated with ricochetal locomotion. Kangaroos , for example, have a very long fourth digit, especially the metatarsal. The fifth digit is also important, but smaller than the fourth. The first toe is lost.