Tylocephale (meaning "swollen head") is a genus of pachycephalosaurid dinosaur, a group of dome-headed, herbivorous ornithischians, that lived during the Maastrichtian stage 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 2 m (6.6 ft) in length and 40 kg (88 lb) 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 Early Maastrichtian stage of the Late Cretaceous, in numerical terms around 72 to 71 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.
Description
Tylocephale was a medium-sized pachycephalosaur, with estimates of 2 m (6.6 ft) in length and 40 kg (88 lb) in body mass. No postcranial fossils belonging to Tylocephale have been found, though there are well preserved skeletons of the related Stegoceras, Homalocephale, and Prenocephale. Based on these taxa, Tylocephale had a short neck, tiny forelimbs, long hindlimbs, and a thick, rodlike tail for balance. The neck was slender and U-shaped, and held in a curved posture, attaching at the occipital condyle on the back of the skull. Its spinal column bore firm connections between the vertebrae that were reinforced by ossified tendons. Its arms were lightly built and slender ending in a hand with five fingers. The limbs terminated in a pes with three toes, the middle being the longest, all of which had unguals.
Skull and ornamentation The only known specimen consists of a partial skull lacking the braincase, palate, and anterior parts of the skull. Its posterior half of the mandible is preserved as well. Most dinosaurs have three fenestrae (hollow spaces) in their skulls, but Tylocephale bears only two. The infratemporal fenestra is angled vertically and is the same width for much of its length. The skull is very tall and narrow posteriorly with a stubby postorbital portion. Notably, the skull roof is tall with an apex very close to the posterior margin of the cranium. This roof is also thicker and bears a peak further posteriorly than observed in other members of the group, a distinguishing feature of the taxon. The dorsal part of the squamosal is the densest element of the skull, as well as being sharpened and not smoothened or swollen. However, the ventral surface of the squamosal is thinner and contacts the exoccipital.
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