Wallaby Illustrating Syndactyly

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Having two or more digits (fingers or toes) wholly or partly joined together. The extent of the join may vary from a membrane of skin to a full fusion of bone.
Syndactyly sometimes occurs naturally in some species. For example, the bones of toes II and III of the hind feet of kangaroos are fused up to the claw. (Syndactyly is a characteristic feature of diprotodonts, which also includes wallabies, koalas and possums.)
Syndactyly may also be a congenital abnormality in, for example, cats, dogs and humans.
This is known as split foot in domestic cats.
Cf. polydactyl