page_title: Ceratosaurus
section_title: Skull
hierarchical_section_title: Ceratosaurus / Description / Skull
caption_attribution_description: English: Skull diagram of Ceratosaurus nasicornis, from Gilmore, C.W. (1920). "Osteology of the carnivorous Dinosauria in the United States National Museum, with special reference to the genera Antrodemus (Allosaurus) and Ceratosaurus". Bulletin of the United States National Museum. 110 (110): 1–154. doi:10.5479/si.03629236.110.i. Osteology of the carnivorous Dinosauria in the United States National museum&#160;: with special reference to the genera Antrodemus (Allosaurus) and Ceratosaurus /
context_page_description: Ceratosaurus was a carnivorous theropod dinosaur in the Late Jurassic period. This genus was first described in 1884 by American paleontologist Othniel Charles Marsh based on a nearly complete skeleton discovered in Garden Park, Colorado, in rocks belonging to the Morrison Formation. The type species is Ceratosaurus nasicornis.
  The Garden Park specimen remains the most complete skeleton known from the genus, and only a handful of additional specimens have been described since. Two additional species, Ceratosaurus dentisulcatus and Ceratosaurus magnicornis, have been described in 2000 from two fragmentary skeletons from the Cleveland-Lloyd Quarry of Utah and from the vicinity of Fruita, Colorado. The validity of these additional species has been questioned, however, and all three skeletons possibly represent different growth stages of the same species. In 1999, the discovery of the first juvenile specimen was reported. Since 2000, a partial specimen was excavated and described from the Lourinhã Formation of Portugal, providing evidence for the presence of the genus outside of North America.
context_section_description: The skull was quite large in proportion to the rest of its body. It measures 55 cm (22 in) in length in the C. nasicornis holotype, measured from the tip of the snout to the occipital condyle, which connects to the first cervical vertebra. The width of this skull is difficult to reconstruct, as it is heavily distorted, and Gilmore's 1920 reconstruction was later found to be too wide. The fairly complete skull of specimen MWC 1 was estimated to have been 60 cm (24 in) in length and 16 cm (6.3 in) in width; this skull was somewhat more elongated than that of the holotype. The back of the skull was more lightly built than in some other larger theropods due to extensive skull openings, yet the jaws were deep to support the proportionally large teeth. The lacrimal bone formed not only the back margin of the antorbital fenestra, a large opening between eye and bony nostril, but also part of its upper margin, unlike in members of the related Abelisauridae. The quadrate bone, which was connected to the lower jaw at its bottom end to form the jaw joint, was inclined so that the jaw joint was displaced backwards in relation to the occipital condyle. This also led to a broadening of the base of the lateral temporal fenestra, a large opening behind the eyes.
  The most distinctive feature was a prominent horn situated on the skull midline behind the bony nostrils, which was formed from fused protuberances of the left and right nasal bones. Only the bony horn core is known from fossils—in the living animal, this core would have supported a keratinous sheath. While the base of the horn core was smooth, its upper two-thirds were wrinkled and lined with groves that would have contained blood vessels when alive. In the holotype, the horn core is 13 cm (5.1 in) long and 2 cm (0.79 in) wide at its base, but quickly narrows to only 1.2 cm (0.47 in) further up; it is 7 cm (2.8 in) in height. It is longer and lower in the skull of MWC 1. In the living animal, the horn would likely have been more elongated due to its keratinous sheath. Behind the nasal horn, the nasal bones formed an oval groove; both this groove and the nasal horn serve as features to distinguish Ceratosaurus from related genera. In addition to the large nasal horn, Ceratosaurus possessed smaller, semicircular, bony ridges in front of each eye, similar to those of Allosaurus. These ridges were formed by the lacrimal bones. In juveniles, all three horns were smaller than in adults, and the two halves of the nasal horn core were not yet fused.
  The premaxillary bones, which formed the tip of the snout, contained merely three teeth on each side, less than in most other theropods. The maxillary bones of the upper jaw were lined with 15 blade-like teeth on each side in the holotype. The first eight of these teeth were very long and robust, but from the ninth tooth onward they gradually decrease in size. As typical for theropods, they featured finely serrated edges, which in the holotype contained some 10 denticles per 5 mm (0.20 in). Specimen MWC 1 merely showed 11 to 12, and specimen UMNH VP 5278 12 teeth in each maxilla; the teeth were more robust and more recurved in the latter specimen. In all specimens, the tooth crowns of the upper jaws were exceptionally long. In specimen UMNH VP 5278, they measured up to 9.3 cm (3.7 in) in length, which is equal to the minimum height of the lower jaw. In the holotype, they are 7 cm (2.8 in) in length, which even surpasses the minimum height of the lower jaw. In other theropods, a comparable tooth length is only known from the possibly closely related Genyodectes. In contrast, several members of the Abelisauridae feature very short tooth crowns. In the holotype, each half of the dentary, the tooth-bearing bone of the mandible, was equipped with 15 teeth, which are, however, poorly preserved. Both specimens MWC 1 and UMNH VP 5278 show only 11 teeth in each dentary, which were, as shown by the latter specimen, slightly straighter and less sturdy than those of the upper jaw.
caption: Diagram of the Ceratosaurus nasicornis holotype skull in top and side view by Charles Gilmore, 1920: This reconstruction is now thought to be too wide in top view.
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