Certain African frogs, including the hairy frog, defend themselves by driving sharp toe bones through the skin of their hind feet, producing claws unlike any other vertebrate's because they have no keratin covering.

Harvard researchers showed these frogs have sectorial terminal phalanges that become functional by cutting through the skin, a claw design unique among living vertebrates.

Source: Concealed weapons: erectile claws in African frogs, Biology Letters, The Royal Society