In 1877, Marsh received a letter from Arthur Lakes, a schoolteacher in Golden, Colorado. Lakes reported that he had been hiking in the mountains near the town of Morrison, when he and his friend, H. C. Beckwith, discovered massive bones embedded in the rock. Lakes further advised that the bones were "apparently a vertebra and a humerus bone of some gigantic saurian." While awaiting Marsh's reply, Lakes dug up more "colossal" bones and sent them to New Haven. As Marsh was slow to respond, Lakes also sent a shipment of bones to Cope.
When Marsh responded to Lakes, he paid the prospector $100, urging him to keep the finds a secret. Learning that Lakes had corresponded with Cope, Marsh sent his field collector Benjamin Mudge to Morrison to secure Lakes' services. Marsh published a description of Lakes' discoveries in the American Journal of Science on July 1, and before Cope could publish his own interpretation of the finds, Lakes wrote to him that the bones should be shipped to Marsh, a severe insult to Cope.
A second letter arrived from the west, this time addressed to Cope. The writer, O. W. Lucas, was a naturalist who had been collecting plants near Cañon City, Colorado when he came upon an assortment of fossil bones. After receiving more samples from Lucas, Cope concluded the dinosaurs were large herbivores, gleefully noting that the specimen was larger than any other previously described, including Lakes' discovery. Hearing of Lucas' finds, Marsh instructed Mudge and a former student, Samuel Wendell Williston, to set up a quarry on his behalf near Cañon. Unfortunately for Marsh, he learned from Williston that Lucas was finding the best bones and refused to quit Cope to come work for Marsh. Marsh ordered Williston back to Morrison, where Marsh's small quarry collapsed and nearly killed his assistants. This setback would have dried up Marsh's bone supply from the west, if not for receipt of a third letter.
At the time of Lakes' discoveries, the Transcontinental Railroad was being built through a remote area of Wyoming. Marsh's letter was from two men identifying themselves as Harlow and Edwards (their real names were Carlin and Reed), workers on the Union Pacific Railroad. The two men claimed they had found large numbers of fossils in Como Bluff, and warned that there were others in the area "looking for such things", which Marsh took to mean Cope. Williston, who had just wearily arrived in Kansas after the collapse of the Morrison mine, was quickly dispatched to Como Bluff by Marsh. His former student sent back a message, confirming the large quantities of bones and that it was Cope's men snooping around the area. Wary of repeating the same mistakes he had made with Lakes, Marsh quickly sent money to the two new bone hunters and urged them to send additional fossils. Williston struck a preliminary bargain with Carlin and Reed (who had been unable to cash Marsh's check due to it being made out to their pseudonyms), but Carlin decided he would head to New Haven to deal with Marsh directly. Marsh drew up a contract calling for a set monthly fee, with additional cash bonuses to Carlin and Reed possible, depending on the importance of the finds. Marsh also reserved the right to send his own "superintendents" to supervise the digging if needed, and advised the men to try to keep Cope out of the region. Despite a face-to-face meeting, Carlin failed to negotiate better terms from Marsh. The paleontologist procured Carlin's and Reed's services, but seeds of resentment were sown as the bone hunters felt Marsh had bullied them into the deal. Marsh's investment in the Como Bluff region soon produced rich results. While Marsh's own collectors headed east for the winter, Reed sent carloads of bones by rail to Marsh throughout 1877. Marsh described and named dinosaurs such as Stegosaurus, Allosaurus, and Apatosaurus in the December 1877 issue of the American Journal of Science.
Despite Marsh's precautions against alerting his rival to Como Bluff's rich bone beds, word of the discoveries rapidly spread. This was at least partly due to Carlin and Reed helping spread the rumors. They leaked information to the Laramie Daily Sentinel, which published an article about the finds in April 1878 that exaggerated the price Marsh had paid for the bones, possibly to raise prices and demand for more bones. Marsh, attempting to cover the leak, learned from Williston that Carlin and Reed had been visited by a man ostensibly working for Cope by the name of "Haines". After learning of the Como Bluff discoveries, Cope sent "dinosaur rustlers" to the area in an attempt to quietly steal fossils from under Marsh's nose. During the winter of 1878, Carlin's dissatisfaction with Marsh's sporadic sending of payment reached a head, and he began working for Cope instead.
Cope and Marsh used their personal wealth to fund expeditions each summer, then spent the winter publishing their discoveries. Small armies of fossil hunters in mule-drawn wagons or on trains were soon sending literally tons of fossils back east. The paleontological digs lasted fifteen years, from 1877 to 1892. The workers for both Cope and Marsh suffered hardships related to the weather, as well as sabotage and obstruction by the other scientist's workers. Reed was locked out of the Como train station by Carlin, and was forced to haul the bones down the bluff and crate the specimens on the train platform in the bitter cold. Cope directed Carlin to set up his own quarry in Como Bluff, while Marsh sent Reed to spy on his former friend. As Reed's Quarry #4 dried up, Marsh ordered Reed to clear out the bone fragments from the other quarries. Reed reported he had destroyed all the remaining bones to keep them away from Cope. Concerned that strangers were encroaching on Reed's quarries, Marsh sent Lakes to Como to assist in excavations, and in June 1879 visited Como himself. Cope likewise toured his own quarries in August. Although Marsh's men continued to open new quarries and discover more fossils, relations between Lakes and Reed soured, with each offering his resignation in August. Marsh attempted to placate the two by sending each to opposite ends of the quarries, but after being forced to abandon one bone quarry in a freezing blizzard, Lakes submitted his resignation and returned to teaching in 1880. The departure of Lakes did not ease tensions among Marsh's men; Lake's replacement, a railroad man named Kennedy, felt he did not have to report to Reed, and the fighting between the two caused Marsh's other workers to quit. Marsh tried separating Kennedy and Reed, and sent Williston's brother Frank to Como in an effort to keep the peace. Frank Williston ended up leaving Marsh's employ and taking up residence with Carlin. Cope's own digging in Como began faltering, and Carlin's replacements soon quit work altogether.
As the 1880s progressed, Cope's and Marsh's men faced stiff competition from each other and from third parties interested in bones. Professor Alexander Emanuel Agassiz of Harvard sent his own representatives west, while Carlin and Frank Williston formed a bone company to sell fossils to the highest bidder. Reed left and became a sheep herder in 1884, and Marsh's Como quarries yielded little after his departure. Despite these setbacks, Marsh had more operational quarries than Cope at this point of time; Cope, who at the early 1880s had more bones than he could fit in a single house, had fallen behind in the race for dinosaurs.
Cope's and Marsh's discoveries were accompanied by sensational accusations of spying, stealing workers and fossils, and bribery. The two men were so protective of their digging sites that they would destroy smaller or damaged fossils to prevent them from falling into their rival's hands, or fill in their excavations with dirt and rock; while surveying his Como quarries in 1879, Marsh examined recent finds and marked several for destruction. On one occasion the scientists' rival teams fought each other by throwing stones.