This timeline of pachycephalosaur research is a chronological listing of events in the history of paleontology focused on the pachycephalosaurs, a group of dome-skulled herbivorous marginocephalian dinosaurs. One of the first major events related to the history of pachycephalosaur research actually regards the discovery of an unrelated dinosaur called Troodon, reported from the western United States by Joseph Leidy in 1856. The type specimen of Troodon was simply an unusual tooth, but the close resemblance between Troodon teeth and pachycephalosaur teeth would cause taxonomic confusion for over a century. This was resolved by Phil Currie in 1987, who realized that Troodon belonged to a group of bird-like carnivores then known as saurornithoidids, but since renamed Troodontidae after Troodon itself. The first scientifically documented true pachycephalosaur remains were discovered in Early Cretaceous rocks from England and named Stenopelix not long after Troodon was named in America. Other notable early finds include the well-known pachycephalosaur Stegoceras validum. In 1924, Charles Whitney Gilmore named the family Troodontidae after Troodon, but most of its members would be recognizable today as pachycephalosaurs. Seven years later, Gilmore named the new species "Troodon" wyomingensis which would be formally reclassified as Pachycephalosaurus in 1943. Pachycephalosaurus was so unusual that Sternberg named a new family for it, the Pachycephalosauridae. From the time paleontologists identified the pachycephalosaurs as a distinct group of dinosaurs, the chief mystery surrounding their biology has been the function of their distinctive cranial domes. Edwin Colbert interpreted the structure as a biological battering ram in 1955, but never specified who or what may have been on its receiving end. The idea that it was used in head butting between members of the same pachycephalosaur species was first proposed by science fiction writer Sprague de Camp. From then it became a staple of both scientific and cultural reconstructions of these animals. Nevertheless, this perennial hypothesis would come to be criticized by researchers like Hans-Dieter Sues as less likely than "flank butting" where pachycephalosaurs' domed heads would be aimed at rivals' bodies rather than in head-to-head combat. Others, like Goodwin and others have thought the dome purely for display because its high density of internal blood vasculature may have rendered it too fragile for combat. Meanwhile, in 1998 Chapman and others found the biomechanics of pachycephalosaur domes consistent with the old head-butting hypothesis, suggesting that the idea retains scientific merit.
19th century
1857 Christian Erich Hermann von Meyer described the new genus and species Stenopelix valdensis.
1872 Joseph Leidy described the new genus and species Tylosteus ornatus.
20th century
1900s
1902 Lawrence Lambe described the new genus and species Stegoceras validum.
1910s
1918 Lawrence Lambe described the new species S. brevis.
1920s
1924 Charles Whitney Gilmore named the Troodontidae. At the time the name mainly applied to taxa now considered to be pachycephalosaurs because the groups have vaguely similar teeth.
1930s
1931 Gilmore described the new species Troodon wyomingensis.
1940s
1943 Barnum Brown and Erich Maren Schlaikjer described the new genus and species Pachycephalosaurus wyomingensis. He also named the species P. grangeri and P. reinhemeri. Additionally, they described the species Troodon sternbergi and Troodon edmontonensis. They also observed potential sexual dimorphism in Stegoceras validum. They noticed that there were two kinds of domes in the species. One variant was rougher and had a prominent "shelf" around it. The other was smoother and lacked the shelf. The researchers presumed that this more ornate morph was the male.
1945 Sternberg named the Pachycephalosauridae. He also named the species Stegoceras lambei.
1950s
1953 Birger Bohlin described the new species Troodon bexelli.
1955 Edwin Colbert first suggested that the domed heads of pachycephalosaurs were used as "battering ram[s]".
1956 Science fiction author Sprague de Camp was the first to specifically suggest that pachycephalosaurs used their domed heads to butt heads with other pachycephalosaurs.
1961
1961 Russian paleontologist Leo Shiovich Davitashvili suggested that pachycephalosaur skull domes were employed in mating displays or "combat" independently of Western science.
1969 Colbert reiterated his earlier suggestion that pachycephalosaurs used their domed heads as "battering ram[s]".
1970s
1971 Peter Malcolm Galton began fleshing out the hypothesis that pachycephalosaurs engaged in head-butting. Robert T. Bakker illustrated the hypothesis. Galton argued that Brown and Schlaikjer got the genders wrong in their 1943 hypothesis that Stegoceras validum exhibited sexual dimorphism. Under Galton's interpretation, as Stegoceras grew the dome expanded until there was no prominent shelf. The presence of a shelf was therefore more likely to suggest a less developed dome, as might be expected in a female morph. He continued to advocate for the idea that pachycephalosaurs butted heads. He observed that many aspects of pachycephalosaur anatomy seemed adapted to transmitting forces received from a blow to the head through the body. 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.
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