Pachycephalosaurus

Pachycephalosaurus (/ˌpækɪˌsɛfələˈsɔːrəs/; meaning "thick-headed lizard", from Greek pachys/παχύς "thickness", kephalon/κεφαλή "head" and sauros/σαῦρος "lizard") is a genus of pachycephalosaurid ornithischian dinosaur. The type species, P. wyomingensis, is the only known definitive species. The possibly synonymous taxon, Stygimoloch, might represent a distinct genus or a second species, P. spinifer. It lived during the Maastrichtian age of the Late Cretaceous period in what is now western North America. Remains have been excavated in Montana, South Dakota, Wyoming, and Alberta. Mainly known from a single skull and a few extremely thick skull roofs (at 22 cm (8.7 in) thick), Pachycephalosaurus is estimated to have reached 4.5 m (15 ft) long and weighed 370–450 kg (820–990 lb). More complete fossils would come to be found in the following years.

Like other pachycephalosaurids, Pachycephalosaurus was a bipedal herbivore, possessing long, strong legs and somewhat small arms with five-fingered hands. Pachycephalosaurus is the largest-known pachycephalosaur, known for having an extremely thick, slightly domed skull roof; visually, the structure of the skull suggests a "battering ram" function in life, evolved for use as a defensive mechanism or intra-species combat, similar to what is seen with today's bighorn sheep or muskoxen (with male animals routinely charging and head-butting each other for dominance). This hypothesis has been disputed in recent years.

Pachycephalosaurus was among the last species of non-avian dinosaurs on Earth before the Cretaceous–Paleogene extinction event. The genus Tylosteus has been synonymized with Pachycephalosaurus, as have the genera Stygimoloch and Dracorex, in recent studies.

71 years prior to the description and naming of Pachycephalosaurus, American paleontologist Joseph Leidy named the new taxon Tylosteus ornatus for a bone fragment interpreted as the armor of a reptile. The specimen named by Leidy in 1872 was indicated to have been collected by Ferdinand Vandeveer Hayden from the "head of the Missouri River", with a Cretaceous age indicated the next year. While Leidy first identified the bone as dermal armor of a reptile, it was doubtfully identified as a dinosaur instead by Oliver Perry Hay in 1902, and then more definitively as a dinosaur by Charles Whitney Gilmore in 1928. Tylosteus was not touched upon again until the study of American paleontologist Donald Baird in 1979, who attempted to reidentify the original place of discovery and the taxonomic identity of the genus. Baird and John R. Horner independently came across the Tylosteus holotype ANSP 8568 in the collections of the Academy of Natural Sciences of Philadelphia and concluded that it was the squamosal of the skull of Pachycephalosaurus, known only from the Lance and Hell Creek Formations of Montana, Wyoming, and South Dakota.

The original notes indicating the discovery of ANSP 8568 only describe the location as "Black Foot Country", with the indication of the head of the Missouri River presumably from interpersonal communication to Leidy. The specimen was likely collected from Hayden prior to 1867 as after that point he was employed by the United States Geological Survey with all specimens going to the Smithsonian Institution. Baird suggested that Leidy's identification of the head of the Missouri River, commonly accepted even at the time to refer to the confluence of the Jefferson and Madison Rivers, was the result of a misunderstanding, as such a location was well away from Blackfoot country. Instead, a provenance of the head of the Little Missouri River, which was a significant stop on Hayden's expedition of 1859-1860, would be a more probable location. Though the original locality cannot be determined, Hayden's route in the expedition led between the Little Missouri and Little Powder Rivers, across the sediments of the Hell Creek Formation, from where Tylosteus likely originated. This would be approximately 45 mi (72 km) south from the type locality of Pachycephalosaurus grangeri, and 140 mi (230 km) north from the type locality of P. wyomingensis, none of which can be distinguished anatomically. Because Tylosteus had remained essentially unused since first named, Baird advocated for conversing Pachycephalosaurus and rendering Tylosteus ornatus a nomen oblitum, which was affirmed by the International Commission on Zoological Nomenclature in 1986 where Tylosteus was made a nomen rejectum and Pachycephalosaurus a nomen conservandum. An ungual found alongside Tylosteus and tentatively referred by Leidy in 1873 was reidentified as from Edmontosaurus.

In 1930 American paleontologist George Fryer Sternberg discovered a partial skull within the Lance Formation of Niobrara County, Wyoming, around 9 mi (14 km) southwest of Warren Post Office along Buck Creek. This specimen, USNM 12031, was then described by Gilmore in 1931 as the new species Troödon wyomingensis, and suggested more definitively that the genus Troodon was present in the Lance Formation. Previously, isolated teeth from the region had been suggested to be from Troodon. Troodon itself had first been described by Leidy in 1856 as a lizard from the Judith River Formation of Montana, but with the 1924 description of a nearly complete skull and partial skeleton from the Belly River Group in Alberta Gilmore found Troodon to be the same as the existing ornithischian genus Stegoceras. T. wyomingensis could not be compared with the type species of Troodon, T. formosus, as the latter only preserved teeth, but significant differences in size, the supratemporal fenestra, and age could separate T. wyomingensis and T. validus (previously Stegoceras), potentially even to the point of requiring a new genus name for T. wyomingensis. As well as USNM 12031, Gilmore referred the specimens USNM 7806 and 8795 to T. wyomingensis.

In 1890, during the Bone Wars between Othniel Charles Marsh and Edward Drinker Cope, one of Marsh's collectors, John Bell Hatcher, collected a partial left squamosal (YPM VP 335) later referred to Stygimoloch spinifer near Lance Creek, Wyoming, in the Lance Formation. Marsh described the squamosal along with the dermal armor of Denversaurus as the body armor of Triceratops in 1892, believing that the squamosal was a spike akin to the plates on Stegosaurus. The squamosal spike was even featured in Charles Knight's painting of Cope's ceratopsid Agathaumas, likely based on Marsh's hypothesis. Marsh also named a species of now-dubious ankylosaur Palaeoscincus in 1892 based on a single tooth (YPM 4810), also collected by Hatcher from the Lance. The tooth was named Palaeoscinus latus, but in 1990, Coombs found the tooth to be from a pachycephalosaurid, possibly even Pachycephalosaurus itself. Hatcher also collected several additional teeth and skull fragments while working for Marsh, though these have yet to be described.

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 which was not corrected until 1945 by Charles M. Sternberg.