At one time, Cope and Marsh were amicable. They met in Berlin in 1864 and spent several days together. They even named species after each other. Over time their relations soured, due in part to their strong personalities. Cope was known to be pugnacious and possessed a quick temper; Marsh was slower, more methodical, and introverted. Both were quarrelsome and distrustful. Their differences also extended into the scientific realm, as Cope was a firm supporter of Neo-Lamarckism while Marsh supported Charles Darwin's theory of evolution by natural selection. Even at the best of times, both men were inclined to look down on each other subtly. As one observer put it, "The patrician Edward may have considered Marsh not quite a gentleman. The academic Othniel probably regarded Cope as not quite a professional."

Cope and Marsh came from very different backgrounds. Cope was born into a wealthy and influential Quaker family based in Philadelphia. Although his father wanted his son to work as a farmer, Cope distinguished himself as a naturalist. In 1864, already a member of the Academy of Natural Sciences, Cope became a professor of zoology at Haverford College and joined Ferdinand Hayden on his expeditions west. In contrast, Marsh would have grown up poor, the son of a struggling family in Lockport, New York, had it not been for the benefaction of his uncle, philanthropist George Peabody. Marsh persuaded his uncle to build the Peabody Museum of Natural History, placing Marsh as head of the museum. Combined with the inheritance he received from Peabody upon his death in 1869, Marsh was financially comfortable (although, partly because of Peabody's stern views on marriage, Marsh would remain a lifelong bachelor).

On one occasion, the two scientists had gone on a fossil-collecting expedition to Cope's marl pits in New Jersey, where William Parker Foulke had discovered the holotype specimen of Hadrosaurus foulkii, described by the paleontologist Joseph Leidy (under whom Cope had studied comparative anatomy); this was one of the first American dinosaur finds, and the pits were still rich with fossils. Though the two parted amicably, Marsh secretly bribed the pit operators to divert future fossil finds to him, instead of Cope. The two began attacking each other in papers and publications, and their personal relations deteriorated. Marsh humiliated Cope by pointing out that his reconstruction of the plesiosaur Elasmosaurus was flawed, with the head placed where the tail should have been (or so he claimed, 20 years later; it was Leidy who published the correction shortly afterwards). Cope, in turn, began collecting in what Marsh considered his private bone-hunting turf in Kansas and Wyoming, further damaging their relationship.