MORPHOLOGY AND ONTOGENY OF THE CORNUAL SINUSES IN CHASMOSAURINE DINOSAURS (ORNITHISCHIA: CERATOPSIDAE)

Abstract

Ceratopsids, horned herbivorous ornithischians from the Cretaceous of North America, are unique among dinosaurs in the form and expression of their cranial ornamentation. All ceratopsid genera possess a bony parieto-squamosal frill that projects over the neck region and some degree of development of nasal and orbital horns. These horns vary in form and location, from low bosses (e.g., Pachyrhinosaurus Sternberg, 1950) to long, conical horns (e.g., Triceratops Marsh, 1889 postorbital horns). Internal structural changes occurred along with these extreme external modifications of the skull (Forster, 1996). A frontal sinus complex occupies space variably involving the postorbital, frontal, parietal, supraoccipital, and exoccipital bones of the skull roof in ceratopsid dinosaurs (Fig. 1.1; Forster, 1996). These sinuses are relatively small, simple chambers in most members of the two ceratopsid clades Chasmosaurinae and Centrosaurinae (Lehman, 1990). However, the frontal sinuses reach large proportions in some species, with a portion of the sinuses extending into the base of the postorbital horncores (Forster, 1996; Sampson et al., 1997; Lehman, 1998). Forster (1996) termed this part of the sinus complex the “cornual” sinus, analogous to the cornual diverticulum of the frontal sinuses in bovid mammals (e.g., domesticated goats; Nickel et al., 1973). Marsh (1887) was the first to note cornual sinuses in a large chasmosaurine (USNM 1871, probably a specimen of Triceratops or Torosaurus Marsh, 1891; personal observation). Sampson et al. (1997) described small cornual sinuses in the centrosaurines Achelousaurus Sampson, 1995, Einiosaurus Sampson, 1995, and Pachyrhinosaurus . Other authors noted similar cornual sinuses in the chasmosaurines Pentaceratops Osborn, 1923, Torosaurus , and Triceratops , but cornual sinus morphology has not been described fully for any chasmosaurine (Hatcher et al., 1907; Forster, 1996; Lehman, 1998). This note describes chasmosaurine cornual sinuses in detail, using undescribed …