Size of two specimens compared to a human, with the holotype of Ceratosaurus nasicornis
(USNM 4735) in orange and a larger specimen (UMNH VP 5278) in blue
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.:115 Whether this animal was fully grown is not clear.:66 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.:192 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.
Artist's impression of C. nasicornis
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.:55–58 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).:115 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.:55–58
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.
The scapula (shoulder blade) was fused with the coracoid, forming a single bone without any visible demarcation between the two original elements.:58 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, numbered II–IV. Digit I, which in theropods is usually reduced to a dewclaw that does not touch the ground, is not preserved in the holotype. Marsh, in his original 1884 description, assumed that this digit was lost in Ceratosaurus, but Charles Gilmore, in his 1920 monograph, noted an attachment area on the second metatarsal demonstrating the presence of this digit.:112
Uniquely among theropods, Ceratosaurus possessed small, elongated, and irregularly formed osteoderms (skin bones) along the midline of its body. Such osteoderms have been found above the neural spines of cervical vertebrae 4 and 5, as well as caudal vertebrae 4 to 10, and probably formed a continuous row that might have extended from the base of the skull to most of the tail. As suggested by Gilmore in 1920, their position in the rock matrix likely reflects their exact position in the living animal. The osteoderms above the tail were found separated from the neural spines by 25 millimetres (0.98 in) to 38 millimetres (1.5 in), possibly accounting for skin and muscles present in between, while those of the neck were much closer to the neural spines. Apart from the body midline, the skin contained additional osteoderms, as indicated by a 58 millimetres (2.3 in) by 70 mm (2.8 in) large, roughly quadrangular plate found together with the holotype; the position of this plate on the body is unknown.:113–114 Specimen UMNH VP 5278 was also found with a number of osteoderms, which have been described as amorphous in shape. Although most of these ossicles were found at most 5 m apart from the skeleton, they were not directly associated with any vertebrae, unlike in the C. nasicornis holotype, so their original position on the body cannot be inferred from this specimen.:32
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.
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.:88 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.:3 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.:277 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.:53
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.:82 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.:82 It is longer and lower in the skull of MWC 1.:3 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.:192 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.:52 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).:92 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.:3,27 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.:92 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.:3,21