 Cope 's relations with Marsh turned into a competition for fossils between the two , known today as the Bone Wars . The conflict 's seeds began upon the men 's return to the United States in the 1860s . Cope introduced his colleague to the marl pit owner Albert Vorhees when the two visited the site . Marsh went behind Cope 's back and made a private agreement with Vorhees : any fossils that Vorhees 's men found were sent back to Marsh at New Haven . When Marsh was at Haddonfield examining one of Cope 's fossil finds — a complete skeleton of a large aquatic plesiosaur , Elasmosaurus , that had four flippers and a long neck — he commented that the fossil 's head was on the wrong end , evidently stating that Cope had put the skull at the end of the vertebrae of the tail . Cope was outraged and the two argued for some time until they agreed to have Leidy examine the bones and determine who was right . Leidy came , picked up the head of the fossil and put it on the other end . Cope was horrified since he had already published a paper on the fossil with the error at the American Philosophical Society . He immediately tried to buy back the copies , but some remained with their buyers ( Marsh and Leidy kept theirs ) . The whole ordeal might have passed easily enough had Leidy not exposed the cover @-@ up at the next society meeting , not to alienate Cope , but only in response to Cope 's brief statement where he never admitted he was wrong . Cope and Marsh would never talk to each other amicably again , and by 1873 , open hostility had broken out between them . 
