Absence of an ossified calcaneum has been suggested as a synapomorphy of Diplodocidae and used to construct sauropod phylogenetic hypotheses. Restudy of sauropod material at the Carnegie Museum of Natural History revealed a diplodocid pes with an unknown ossified pedal element. The relatively complete pes, CM 30767, has been identified as Diplodocus. Comparisons with known sauropod calcanea, carpals, and phalanges, indicate that this element is a calcaneum. The presence of an ossified calcaneum in a diplodocid has important functional and systematic implications. Ossified calcanea may have helped to stabilize the pes during locomotion and assisted in the proper alignment and insertion of lateral plantar musculature. The unusual morphology and unreliable preservation of the sauropod calcaneum suggest that its presence or absence should not be used as a character state, nor should the absence of the calcaneum be used as a synapomorphy of Diplodocidae.