and species Gravitholus albertae. Sues and Philippe Taquet described Majungatholus atopus as a Gondwanan pachycephalosaur. 1980s 1981 Ralph Chapman and others performed a detailed statistical analysis on the relative size of dome to braincase in 23 Stegoceras validum domes and domes from 6 other pachycephalosaur taxa. They were able to account for the age and development of the specimens using the size of the braincase as an indicator. The analysis uncovered two distinct variations in dome size probably denoting dimorphism between males and females. The form with the larger dome achieved the size of this feature by having a slightly higher rate of growth relative to the braincase. 1982 Altangerel Perle, Maryańska and Osmólska described the new genus and species Goyocephale lattimorei. Perle and others followed Dong's 1978 suggestion that the Pachycephalosauria contained two families, the Homalocephalidae, which had flat heads, and the Pachycephalosauridae, which were characterized by the famous domed skulls. 1983 Galton and Sues described the new genus Ornatotholus to house the species Stegoceras browni. They also described the new genus and species Stygimoloch spinifer. They noted that the bone composing its skull dome was rich in blood vessels. Peter Dodson argued that the rarity of pachycephalosaurs in the fossil record suggests that they preferred dryer habitats farther from coasts and rivers where ceratopsians and ornithopods lived and where fossilization is more likely to occur. He also observed that many pachycephalosaur fossils show signs of damage by transport over significant distances after death. He hypothesized that this explains why skeletal remains of Stegoceras validum are so rare compared to the isolated remains of their skull domes in the Judith River Formation. The sediments he attributed to the Judith River Formation are now considered part of the Dinosaur Park Formation. 1986 Paul Sereno conducted the first comprehensive study of pachycephalosaur evolutionary relationships. He also named the Goyocephala and Homalocephaloidea. 1987 Emily Buchholtz Giffin, Diane Gabriel, and Rolf Johnson described the new genus and species Stenotholus kohleri. Sues and Galton conducted the second comprehensive study of pachycephalosaur evolutionary relationships. Unlike Sereno, they found that the flat-headed pachycephalosaur taxa formed a natural family called the Homalocephalidae. They further elaborated on their support for pachycephalosaurs using their skull domes for flank-butting rather than head-to-head butting. Currie noted traits in the teeth of Troodon that distinguished them from those of pachycephalosaurs. He split the Troodontidae off from the latter group and noted that the family Saurornithoididae, which then included many dinosaurs now considered troodontids, was actually its junior synonym. 1989 Robert McNeill Alexander argued that since the forces generated by head-butting in pachycephalosaurs were probably smaller than previously expected and "glancing blows" sustained on impact presented little risk of harm to the animals involved, that head-butting was the likely function of the pachycephalosaur dome after all. He also contended that the flatter domes of primitive pachycephalosaurs were actually better suited for flank-butting than the rounded domes that supposedly put their owners at risk of injury in the case of a "glancing blow" at impact. 1990s 1990 Maryańska followed Dong's 1978 suggestion, which had also been supported by Sues and Galton in 1987, that the Pachycephalosauria contained two families, the Homalocephalidae, which had flat heads, and the Pachycephalosauridae, which were characterized by the famous domed skulls. Chapman published a morphometric analysis of pachycephalosaur skulls. Mark Goodwin published a morphometric analysis of pachycephalosaur skulls. 1996 David Fastovsky and David Weishampel examined the paleobiology and paleoecology of pachycephalosaurs in detail. They observed that pachycephalosaurs had differing teeth in different regions of the jaw. The front teeth were peg-like, sometimes the last of these front teeth was