Remains attributable to "Pachycephalosaurus" may have been found as early as the 1850s. As determined by Donald Baird, in 1859 or 1860 Ferdinand Vandeveer Hayden, an early fossil collector in the North American West, collected a bone fragment in the vicinity of the head of the Missouri River, from what is now known to be the Lance Formation in southeastern Montana. This specimen, now ANSP 8568, was described by Joseph Leidy in 1872 as belonging to the dermal armor of a reptile or an armadillo-like animal. It became known as "Tylosteus". Its actual nature was not found until Baird restudied it over a century later and identified it as a squamosal (bone from the back of the skull) of "Pachycephalosaurus", including a set of bony knobs corresponding to those found on other specimens of "Pachycephalosaurus". Because the name "Tylosteus" predates "Pachycephalosaurus", according to the International Code of Zoological Nomenclature "Tylosteus" would normally be preferred. In 1985, Baird successfully petitioned to have "Pachycephalosaurus" used instead of "Tylosteus" because the latter name had not been used for over fifty years, was based on undiagnostic materials, and had poor geographic and stratigraphic information. This may not be the end of the story; Robert Sullivan suggested in 2006 that ANSP 8568 is more like the corresponding bone of "Dracorex" than that of "Pachycephalosaurus". The issue is of uncertain importance, though, if "Dracorex" actually represents a juvenile "Pachycephalosaurus", as has been recently proposed.