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In the year 2006 paleontologists will celebrate the 150th anniversary of the first description of a dinosaur fossil from North America. In March 1856 Joseph Leidy of Philadelphia wrote a brief paper describing and naming four kinds of 75-million-year-old reptilian teeth that had been discovered the previous year in the Cretaceous beds of the Judith River region of Montana. Without any of the media hoopla that accompanies many dinosaur discoveries today, he showed that two of the new reptiles were unquestionably dinosaurs, as judged from comparison with discoveries made in England 30 years before. These were the first American dinosaurs. (Leidy thought that the other two came from "lacertilians"—lizards—but they eventually turned out to be dinosaurs too.) Or perhaps they weren't. Who gets credit for the first North American dinosaur is a matter that depends on what constitutes a description, what counts as a fossil and whether its collector or describer knew what he had, and when.