page_title: Ceratosaurus
section_title: Description
hierarchical_section_title: Ceratosaurus / Description
caption_attribution_description: English: Fossils of Ceratosaurus in the Dinosaur Journey Museum, Fruita, showing characteristic features of the genus. Top: The co-ossificated nasal bones show the prominent nasal horn. Bottom: Left maxilla, showing the proportionally long teeth. The specimens were found at the Fruita Paleontological Area near the Dinosaur Journey museum, and have been referred to Ceratosaurus magnicornis.
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: Ceratosaurus followed the body plan typical for large theropod dinosaurs. A biped, it moved on powerful hind legs, while its arms were reduced in size. Specimen USNM 4735, the first discovered skeleton and holotype of Ceratosaurus nasicornis, was an individual 5.3 m (17 ft) or 5.69 m (18.7 ft) in length according to separate sources. Whether this animal was fully grown is not clear. Othniel Charles Marsh, in 1884, suggested that this specimen weighed about half as much as the contemporary Allosaurus. In more recent accounts, this was revised to 418 kilograms (922 lb), 524 kg (1,155 lb), or 670 kg (1,480 lb). Three additional skeletons discovered in the latter half of the 20th century were substantially larger. The first of these, UMNH VP 5278, was informally estimated by James Madsen to have been around 8.8 m (29 ft) long, but was later estimated at 7 m (23 ft) in length. Its weight was calculated at 980 kg (2,160 lb), 452 kg (996 lb), and 700 kg (1,540 lb) in separate works. The second skeleton, MWC 1, was somewhat smaller than UMNH VP 5278 and might have been 275 kg (606 lb) in weight. The third, yet undescribed, specimen BYUVP 12893 was claimed to be the largest yet discovered, although estimates have not been published. Another specimen (ML 352), discovered in Portugal in 2000, was estimated at 6 m (20 ft) in length and 600 kg (1,320 lb) in weight.
  The exact number of vertebrae is unknown due to several gaps in the spine of the Ceratosaurus nasicornis holotype. At least 20 vertebrae formed the neck and back in front of the sacrum. In the middle portion of the neck, the centra (bodies) of the vertebrae were as long as they were tall, while in the front and rear portions of the neck, the centra were shorter than their height. The upwards projecting neural spines were comparatively large, and in the dorsal (back) vertebrae, were as tall as the vertebral centra were long. The sacrum, consisting of six fused sacral vertebrae, was arched upwards, with its vertebral centra strongly reduced in height in its middle portion, as is the case in some other ceratosaurians. The tail comprised around 50 caudal vertebrae and was about half of the animal's total length; in the holotype, it was estimated at 9.33 ft (2.84 m). The tail was deep from top to bottom due to its high neural spines and elongated chevrons, bones located below the vertebral centra. As in other dinosaurs, it counterbalanced the body and contained the massive caudofemoralis muscle, which was responsible for forward thrust during locomotion, pulling the upper thigh backwards when contracted.
  The scapula (shoulder blade) was fused with the coracoid, forming a single bone without any visible demarcation between the two original elements. The C. nasicornis holotype was found with an articulated left fore limb including an incomplete manus (hand). Although disarticulated during preparation, a cast had been made of the fossil beforehand to document the original relative positions of the bones. Carpal bones were not known from any specimen, leading some authors to suggest that they were lost in the genus. In a 2016 paper, Matthew Carrano and Jonah Choiniere suggested that one or more cartilaginous (not bony) carpals were probably present, as indicated by a gap present between the forearm bones and the metacarpals, as well as by the surface texture within this gap seen in the cast. In contrast to most more-derived theropods, which showed only three digits on each manus (digits I–III), Ceratosaurus retained four digits, with digit IV reduced in size. The first and fourth metacarpals were short, while the second was slightly longer than the third. The metacarpus and especially the first phalanges were proportionally very short, unlike in most other basal theropods. Only the first phalanges of digits II, III, and IV are preserved in the holotype; the total number of phalanges and unguals (claw bones) is unknown. The anatomy of metacarpal I indicates that phalanges had originally been present on this digit, as well. The pes (foot) consisted of three weight-bearing digits, num
caption: Distinguishing skull features of Ceratosaurus: The fused left and right nasal bones form a prominent nasal horn (top), and the teeth of the upper jaw are exceptionally long (bottom). These fossils are part of specimen MWC 1 from Fruita, Colorado, and are on display at the local Dinosaur Journey Museum.
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