The Complete Dinosaur

25. Marginocephalia

527 Marginocephalia Peter Makovicky 25 The Marginocephalia, or margin-headed dinosaur group, comprises the thick-skulled pachycephalosaurs and the frill-bearing ceratopsians. Marginocephalians have a predominantly Cretaceous fossil record, although the earliest species of ceratopsians are now known from the Late Jurassic (Xu et al. 2006; Zhao et al. 1999; Zhao et al. 2006). Until recently, Marginocephalia was thought to constitute the sister group to all Ornithopoda (Sereno 1986, 1999), a group dating back to the Early Jurassic, which would imply a long, undiscovered history for Marginocephalia. Recently this view has been challenged by Butler et al. (2008), who found marginocephalians to nest inside the ornithopod radiation, which significantly shortens the implied missing stratigraphic range between marginocephalians and their closest relatives. It is noteworthy that Late Jurassic Yinlong downsi (Xu et al. 2006), the earliest member of the Ceratopsia, displays a mosaic of 25.2. A gallery of basal ceratopsian taxa: (A) Yinlong in lateral view; (B) Psittacosaurus in lateral view; (C) Liaoceratops in dorsal view; (D) Archaeoceratops in lateral view; (E) Leptoceratops in dorsal view; (F) Protoceratops in lateral view. (B, E, F) photographed by M. Ellison, other photos by the author. Not to scale. Peter Makovicky 528 traits that were previously considered diagnostic of either ceratopsians or marginocephalians, suggesting that Yinlong is temporally close to their last common ancestor. Although Cretaceous members of each clade are superficially very dif­ ferent from one another due to long separate evolutionary histories, cera­ top­ sians and pachycephalosaurs are united by a number of derived skeletal features. Chief among these is a backward extension of the rearmost skull bones (squamosals and parietals) to form a shelf or expanded margin at the back of the skull. In pachycephalosaurs, the frill is relatively short and shelflike, and most of this margin is composed of the two outer bones (squamosals) (Fig. 25.1A). It is adorned with tubercles or spikes along the edges and corners in most pachycephalosaur species. The skull margin shows the greatest development in derived ceratopsians, where it forms a prominent frill or shield extending from the back of the skull above the neck (Figs. 25.3D–F), sporting an array of diagnostic spikes or bumps in ceratopsids. In contrast to pachycephalosaurs, the ceratopsian frill margin is comprised mainly by the parietal in all Cretaceous ceratopsians. The basal ceratopsian Yinlong possesses a pachycephalosaur-like condition of the frill (Fig. 25.2A), however, thus muting some earlier concerns that the different makeup of the frill in the two groups could reflect separate origins of the structure (Galton and Sues 1983). Other evolutionary novelties that unite marginocephalians are exclusion of the premaxillae from the internal 25.3. A gallery of ceratopsid taxa: (A–D) The centrosaurine genera Centrosaurus, Styracosaurus, Einiosaurus, and Pachyrhinosaurus in lateral view, displaying the diverse nasal horn forms and frill spike configurations found in this group. (E–G) The chasmosaurine genera Triceratops and Pentaceratops, and Kosmoceratops in anterior and lateral views respectively. Chasmosaurines differ from centrosaurines in having a longer snout, pockets in the narial region, enlarged brow horns, and long frills. Triceratops is anomalous in having a secondarily shortened frill without fenestrae. (A–D) taken from Sampson et al. (1997); (F) taken from Lull (1933); (G) drawing by Lukas Panzarin, courtesy of Mark Loewen. Specimens not to scale. 8.118.226.105] Project MUSE (2024-04-26 04:21 GMT) Marginocephalia 529 nares and pubic bones that are not fused to the rest of the hip joint (Sereno 1986, 1999). Some of these features, such as the unfused pubes, also occur in ornithischians outside of Marginocephalia, however. Pachycephalosaurs range in size from the minute and possibly juvenile Wannanosaurus, with an estimated skull length of less than 10 cm (4 inches) to the large Pachycephalosaurus with a 65-cm (2-foot) skull. In general, pachycephalosaurs display a primitive, bipedal ornithischian body plan. Conversely, their skulls are highly modified and have vertically thickened skull-roof bones that form either a wide, flat table or a convex dome in more derived species. This dome is formed by the frontal and parietal bones that roof the brain cavity, and can be up to 8 inches thick in the largest pachycephalosaurs . In highly domed species, the bones around the edge of the orbit also fuse and contribute to the structure of the dome. The skull roof is always far thicker than the depth of the brain cavity, even in flat-headed species, and its possible function has been the focus of much paleobiological study (see...