The femur, or thigh bone, is the most proximal (closest to the body) bone of the leg in vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs, the femur is found only in the rear legs. Some species of whales^[1], snakes^[2], and other non-walking vertebrates have vestigial femurs. One of the earliest known vertebrates to have a femur is the Eusthenopteron, a prehistoric lobe-finned fish from the Late Devonian period. In invertebrates, the name femur is also given to the most proximal full-length jointed segment of the legs of some arthropods such as spiders.

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