Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs, or legs. An animal that usually moves in a bipedal manner is known as a biped, meaning "two feet" (from the Latin]] bi for "two" and ped for "foot"). Types of bipedal movement include walking, running, or hopping, on two appendages (typically legs). Bipedalism evolved more than once in archosaurs, the group that includes both dinosaurs and crocodilians. A few lizards can also move bipedally, but only in emergencies. There are no known living or fossil bipedal amphibians.