Tylocephale (meaning "swollen head") is a genus of pachycephalosaurid dinosaur, a group of dome-headed, herbivorous ornithischians, that lived during the Late Campanian stage (75-73 million years ago) of the Late Cretaceous in what is now Mongolia. It is known from a partial skull and associated mandible that were unearthed in 1971 by a Polish-Mongolian Expedition to the Barun Goyot Formation of the Gobi Desert. The specimen was described in 1974 by Polish paleontologists Teresa Maryańska and Halszka Osmólska as a new genus and species.
It was average-sized for a pachycephalosaur, reaching in length and in body mass. The skull is triangular in back view, the widest point being at the jugals with an apex at the top of the dome. Tylocephale's dome is the tallest known from a pachycephalosaur. This dome is also unusually thick and rugose on its exterior. Behind the dome, an array of spikes, nodes, and tubercules protrude posteriorly over the neck. The eyebrow ridge was ornamented with small, bony nodules and was thicker than in other genera. Tylocephale is closely related to other Asian pachycephalosaurs, like Homalocephale and Goyocephale, being part of the Old World branch of the group.
Being a pachycephalosaur it was herbivorous with small, ridged teeth adapted to break down fibrous plants. Its dentition also bears serrations, implying a potentially more heterogeneous diet of leaves, nuts, seeds, and insects. Fossils were found in the Khulsan locality, which preserves fossils of many other dinosaur groups like ceratopsians, ankylosaurs, oviraptorosaurs, dromaeosaurs, and a titanosaur. The Barun Goyot Formation was an alluvial plain, featuring large riverways in an arid to semi-arid climate.
Discovery and naming
During a joint Polish-Mongolian Expedition to the Khulsan outcrop of the Barun Goyot Formation in the Gobi Desert, a large skull and mandible of a pachycephalosaur (catalogue number ZPAL MgD-I/105) was unearthed in 1971. The rock layers of the Barun Goyot Formation derive from the Late Campanian stage of the Late Cretaceous, in numerical terms around 75 to 73 million years old. This was one of a series of expeditions carried out between 1963 and 1971 that were spearheaded by Polish paleontologist Zofia Kielan-Jaworowska, which collected scores of well-preserved dinosaur skeletons. Several other Polish scientists joined the venture, including Teresa Maryańska and Halszka Osmólska, who were aided by local Mongolian aides. The skull was incomplete, missing much of the anterior portions and the rest of the bony dome. The mandible and skull were preserved in articulation, laying loose in weathered sandstone blocks from a former river channel. The specimen was one of several dinosaur individuals discovered at Khulsan during the 1970s, with material of the ankylosaurs Tarchia and Saichania, ceratopsid Breviceratops, and theropod Hulsanpes found in the locale. All of the fossils unearthed during this expedition were then transported to the Polish Academy of Sciences in Warsaw, where they were described in the following years.
The pachycephalosaur skull found at Khulsan was one of several collected during the Polish-Mongolian Expeditions, with other specimens in the nearby Nemegt Formation gathered in addition to Barun Goyot. The pachycephalosaur material from both formations was described in scientific literature in the journal Palaeontologica Polonica in 1974 by Teresa Maryańska and Halszka Osmólska. The skull found at Khulsan was designated the type specimen of a new genus and species, Tylocephale gilmorei. The genus name Tylocephale comes from the Greek words tyle ("swollen") and cephale ("head") and refers to the skull's prominent cranial dome. The species name honors American paleontologist Charles Gilmore, who wrote the first detailed description of a pachycephalosaur. The 1974 paper also dubbed two new pachycephalosaurs based on the fossils from Nemegt, Homalocephale and Prenocephale. All of these taxa were grouped in a new order Maryańska and Osmólska named Pachycephalosauria, which contained North American genera like Stegoceras and Pachycephalosaurus as well.