"P. wyomingensis", the type and currently only valid species of "Pachycephalosaurus", was named by Charles W. Gilmore in 1931. He coined it for the partial skull USNM 12031, from the Lance Formation of Niobrara County, Wyoming. Gilmore assigned his new species to "Troodon" as "T. wyomingensis". At the time, paleontologists thought that "Troodon", then known only from teeth, was the same as "Stegoceras", which had similar teeth. Accordingly, what are now known as pachycephalosaurids were assigned to the family Troodontidae, a misconception not corrected until 1945, by Charles M. Sternberg.