When was the first triceratops fossil discovered?

In 1887, a skull roof with two horns belonging to Triceratops was found in Denver, Colorado. Paleontologist Othniel Charles Marsh originally misidentified as a prehistoric bison. In 1888, John Bell Hatcher found a much more complete skull of Triceratops in Wyoming, and then Marsh realized his mistake. Both of these specimens were assigned to the new genus Triceratops.