Paleontology
Leidy named the holotype specimen of Hadrosaurus foulkii, which was recovered from the marl pits of Haddonfield, New Jersey. It was notable for being the first nearly-complete fossilized skeleton of a dinosaur ever recovered. The specimen was originally discovered by William Parker Foulke. Leidy concluded, contrary to the view prevailing at the time, that this dinosaur could adopt a bipedal posture. He also described the holotype specimens of Arctodus (A. simus), the dire wolf (Canis dirus), and the American lion (Panthera leo atrox), among many others.
The noted American fossil collector and paleontologist E. D. Cope was a student of Leidy's, but the enmity and ruthless competition that developed between him and rival paleontologist O. C. Marsh eventually drove Leidy out of western American vertebrate paleontology, a field that Leidy had helped to found. Leidy inadvertently contributed to the falling out of the two by showing Cope in the presence of Marsh that Cope had mistakenly placed the head of a fossil Elasmosaurus on the tail, rather than on the neck, and then publishing a correction.
Leidy was an early American supporter of Darwin's theory of evolution, and lobbied successfully for Darwin's election to membership in the Academy of Natural Sciences of Philadelphia.
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