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Ceratosaurus /ˌsɛrətˈsɔːrəs/ (from Greek κέρας/κέρατος, keras/keratos meaning "horn" and σαῦρος/sauros meaning "lizard"), was a predatory theropod dinosaur in the Late Jurassic Period (Kimmeridgian to Tithonian). 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. This 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 it is possible that all three skeletons 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. Fragmentary remains have also been reported from Tanzania, Uruguay, and Switzerland, although their referral to Ceratosaurus is currently not accepted by most paleontologists.

Ceratosaurus
Temporal range: Late Jurassic, 153–148 Ma
Ceratosaurus mount utah museum 1.jpg
Cast of Ceratosaurus from the Cleveland Lloyd Quarry, on display at the Natural History Museum of Utah
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Order: Saurischia
Suborder: Theropoda
Family: Ceratosauridae
Genus: Ceratosaurus
Marsh, 1884
Species
Synonyms
  • Megalosaurus nasicornis (Marsh, 1884 [originally Ceratosaurus])

Ceratosaurus was a large theropod. The original specimen is estimated at 5.3 m (17 ft) in length, while the referred specimen described as Ceratosaurus dentisulcatus was substantially larger, at around 8.8 m (29 ft) in length. Ceratosaurus was characterized by deep jaws that supported proportionally very long, blade-like teeth, a prominent, ridge-like horn on the midline of the snout and a pair of hornlets over the eyes. The forelimbs were very short but remained fully functional; the hand possessed four fingers. The tail was deep in profile. A row of small osteoderms (bony plates) was present down the middle of the neck, back, and tail. Additional osteoderms were present at unknown positions on the animal's body.

Ceratosaurus is the eponymous genus of the Ceratosauria, a clade of theropod dinosaurs that diverged early from the evolutionary lineage leading to modern birds. Within Ceratosauria, some paleontologists proposed it to be most closely related to Genyodectes from Argentina, which shares the strongly elongated teeth. The geologically older genus Proceratosaurus from England, although originally described as a presumed antecedent of Ceratosaurus, is now found to be unrelated to the latter. Ceratosaurus shared its habitat with other large-sized theropod genera including Torvosaurus and Allosaurus, and it has been suggested that these theropods occupied different ecological niches in order to reduce competition. The characteristic nasal horn was probably not used as a weapon as was originally suggested by Marsh, but more likely served in display.

Contents

DescriptionEdit

 
Comparison of two specimens of Ceratosaurus nasicornis (red, pink) and other theropods from the Morrison Formation.

Ceratosaurus followed the bauplan typical for large theropod dinosaurs.[1] 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 about 5.3 m (17 ft) long; it is not clear whether this animal was fully grown.[2]:115[3]:66 Othniel Charles Marsh, in 1884, suggested that this specimen weighed about half as much as the contemporary Allosaurus.[4] In more recent accounts, it was estimated at 418 kilograms (922 lb), 524 kg (1,155 lb) and 670 kg (1,480 lb) by separate authors.[5] Three additional skeletons discovered in the latter half of the 20th century were substantially larger than USNM 4735.[3]:66[6]:21,36 The first of these, UMNH VP 5278, was informally estimated by James Madsen to have been around 8.8 m (29 ft) long.[7] American science writer Gregory S. Paul, in 1988, estimated this specimen at 980 kg (2,160 lb).[8] A considerably lower figure, 452 kg (996 lb), was proposed by John Foster in 2007. The second skeleton, MWC 1, was somewhat smaller than UMNH VP 5278 and estimated by Foster at 275 kg (606 lb).[9] A third, yet undescribed specimen (BYUVP 12893) was claimed to be the largest yet discovered, although estimates have not been published.[10]:192[6]:36

 
Artist's impression of Ceratosaurus 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 spine of the neck and back in front of the sacrum. The centra (bodies) of the cervical (neck) vertebrae were as long as tall in the middle portion of the neck but became shorter in both the front and rear portion. The upwards projecting spinous processes were comparatively large, and, in the dorsal (back) vertebrae, were as tall as the vertebral centra were long. The sacrum, consisting of 6 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).[4][2]:115 Characterized by high spinous processes and elongated chevrons (bones located below the vertebral centra), the tail was deep in profile. 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.[3]:55–58

 
Distinguishing skull features of Ceratosaurus: The co-ossified left and right nasal bones form a prominent nasal horn (top), and the teeth of the upper jaw are exceptionally long (bottom). 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.[3]:58 The Ceratosaurus nasicornis holotype was found with an articulated left forelimb 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 the holotype or other specimens, 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 (non-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.[11] In contrast to most more derived theropods, which showed only three digits on each manus, that of Ceratosaurus retained four digits. Digit IV was reduced in size and very similar to that of other basal theropods such as Dilophosaurus. The first and fourth metacarpal were reduced in length, 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 phalanx of digits II, III and IV is preserved in the holotype, respectively; the total number of phalanges and unguals is unknown. The anatomy of metacarpal I indicates that phalanges had been present originally on this digit as well. The pes (foot) consisted of three weight-bearing digits (digits II–IV). Digit I, which in theropods is usually reduced to a dewclaw and 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, while Charles Gilmore, in his 1920 monograph, noted an attachment area on the second metatarsal demonstrating the presence of this digit.[2]:112

Uniquely among theropods, Ceratosaurus possessed small, elongated and irregularly formed osteoderms (skin bones) along the midline of its back. Such osteoderms have been found above the neural processes of cervical vertebrae 4 and 5 as well as caudal vertebrae 4 to 10, and probably formed a continuous row which 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 reflects their exact position in the living animal. The osteoderms above the tail were found separated from the spinous processes by 25 mm (0.98 in) to 38 mm (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 mm (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.[2]: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 five meters apart from the skeleton, they were not, unlike in the Ceratosaurus nasicornis holotype, directly associated with any vertebrae; their original position on the body thus cannot be inferred from this specimen.[6]:32

SkullEdit

 
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.[1] It measures 55 cm (22 in) in length in the Ceratosaurus nasicornis holotype, measured from the tip of the snout to the occipital condyle, which connects to the first cervical vertebra.[2]:88 The width of this skull is difficult to reconstruct as it is heavily distorted, and the early reconstruction published by Charles Gilmore in 1920 was later found to be too wide.[12] 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.[6]:3 The back of the skull was very lightly built compared to other larger theropods due to extensive skull openings, yet the jaws were deep to support the proportionally large teeth.[8]:277 The lacrimal bone does not only form the back margin of the antorbital fenestra (a large opening between eye and nostril), but also part of its upper margin, unlike in members of the related Abelisauridae. The quadrate bone, which is connected to the lower jaw at its bottom end to form the jaw joint, is inclined so that the jaw joint is displaced backwards in relation to the occipital condyle. This also lead to a broadening of the base of the lateral temporal fenestra, a large opening behind the eyes.[3]:53 The most distinctive feature of the skull was a prominent horn situated on the skull midline behind the nostrils and formed from co-ossified protuberances of the left and right nasal bones.[2]: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 very rugose and lined with groves that would have contained blood vessels in life. 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.[2]:82 It is longer and lower in the skull of MWC 1.[6]:3 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.[10]:192 In addition to the large nasal horn, Ceratosaurus possessed smaller, semicircular, hornlike ridges in front of each eye, similar to those of Allosaurus. These ridges were formed by the lacrimal bones.[9] In juveniles, all three horns were smaller than in adults, and the two halves of the nasal horn core were not yet co-ossified.[13]

The paired premaxillary bone, which formed the tip of the snout, contained merely three teeth on each half, less than in most other theropods.[3]:52 The maxillary bones of the upper jaw were lined with 15 blade-like teeth on each side in the holotype of Ceratosaurus nasicornis. The first eight of these teeth were very long and robust, but from the ninth teeth onward they gradually decrease in size. As typical for theropods, they feature finely serrated edges, which in the holotype contain some 10 denticles per 5 mm (0.20 in).[2]:92 Specimen MWC 1 only shows eleven to twelve, and specimen UMNH VP 5278 twelve teeth in each maxilla, which in the latter are more massive and more recurved.[6]:3,27 In all specimens, the tooth crowns of the upper jaws were exceptionally long, measuring up to 9.3 cm (3.7 in) in length in specimen UMNH VP 5278, and therefore equal to the minimum height of the lower jaw. In the holotype, the upper tooth crowns are 7 cm (2.8 in) in length and 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.[14] In contrast, several members of the Abelisauridae feature very low tooth crowns.[3]:92 In the Ceratosaurus nasicornis 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 eleven teeth on each dentary, which were, as shown by the latter specimen, slightly straighter and less sturdy than those of the upper jaw.[6]:3,21

History of discoveryEdit

 
Ceratosaurus nasicornis skeleton restoration by Othniel Charles Marsh from 1892, depicted in an erroneous upright position and with excess vertebrae in the spine resulting in an overly elongated trunk

The first specimen, the holotype USNM 4735, was discovered and excavated by the farmer Marshall Parker Felch in 1883 and 1884.[15] Found in articulation (bones still connected to each other), it was nearly complete, including the skull. Significant missing parts include an unknown number of vertebrae; all but the last ribs of the trunk; the humerus (upper arm bone); parts of the fingers; and most of the foot.[2]:77 The specimen was found encased in hard sandstone; the skull and spine had been heavily distorted during fossilization.[2]:2,114 The site of discovery, located in the Garden Park area north of Cañon City, Colorado and known as the Felch Quarry 1, is regarded as one of the richest fossil sites of the Morrison Formation. Numerous dinosaur fossils had been recovered from this quarry even before the discovery of Ceratosaurus, most notably a nearly complete specimen of Allosaurus in 1883 and 1884. The specimen was shipped to the Peabody Museum of Natural History in New Haven, where it was studied by Marsh, who described it as the new genus and species Ceratosaurus nasicornis in 1884; the name Ceratosaurus may be translated as "Horn lizard" (from Greek κερας/κερατος, keras/keratos meaning "horn" and σαυρος/sauros meaning "lizard").[4][2]:7,114 Given the completeness of the specimen, the newly described genus was at the time the best-known theropod discovered in America. In 1898 and 1899, the specimen was transferred to the National Museum of Natural History in Washington, D.C., together with many other fossils originally described by Marsh. Only part of this material was fully prepared when it arrived in Washington; subsequent preparation lasted from 1911 to the end of 1918. Packaging and shipment from New Haven to Washington caused some damage to the Ceratosaurus specimen.[2]:2,114 In 1920, Charles Gilmore published an extensive re-description of this and the other theropod specimens received from New Haven, including the nearly complete Allosaurus specimen recovered from the same quarry.[2]:2

 
Ceratosaurus nasicornis holotype, as mounted by Charles Gilmore in 1910 and 1911, National Museum of Natural History

In a 1892 paper, Marsh published the first skeletal reconstruction of Ceratosaurus, which depicts the animal at 22 ft (6.7 m) in length and 12 ft (3.7 m) in height.[1] As noted by Gilmore in 1920, the trunk was depicted much too long in this reconstruction, incorporating at least six dorsal vertebrae too many. This error was repeated in several subsequent publications, including the first life reconstruction, which was drawn in 1899 by Frank Bond under the guidance of Charles R. Knight but not published until 1920. A more accurate life reconstruction, published in 1901, was produced by J. M. Gleeson, again under Knight's supervision. The holotype was mounted by Gilmore in 1910 and 1911 and since was on exhibit at the National Museum of Natural History. Most early reconstructions show Ceratosaurus in an upright posture, with the tail dragged over the ground.[2]:115–116 Gilmore's mount of the holotype, in contrast, was ahead of its time:[8]:276 Inspired by the upper thigh bones, which were found angled against the lower leg, he depicted the mount as a running animal with a horizontal rather than upright posture and a tail which did not make contact with the ground. Because of the strong flattening of the fossils, Gilmore mounted the specimen not as free-standing skeleton but as a basrelief within an artificial wall.[2]:114 With the bones being partly embedded in plaque, scientific access was limited. In the course of the renovation of the museum's dinosaur exhibition between 2014 and 2019, the specimen was dismantled and freed from encasing plaque.[16][17] In the new exhibition, which is set to open in 2019, the mount is planned to be replaced by a free-standing cast, and the original bones to be stored in the museum collection to allow for full access to scientists.[17]

 
The probably first life restoration of Ceratosaurus, drawn in 1899 by Frank Bond under the guidance of Charles R. Knight but not published until 1920.

After the discovery of the holotype of Ceratosaurus nasicornis, a significant Ceratosaurus find was not made until the early 1960s, when paleontologist James Madsen and his team unearthed a fragmentary, disarticulated skeleton including the skull (UMNH VP 5278) in the Cleveland-Lloyd Dinosaur Quarry in Utah. This find represents one of the largest known Ceratosaurus specimens.[6]:21 A second, articulated specimen including the skull (MWC 1) was discovered by Thor Erikson, the son of paleontologist Lance Erikson, in 1976 near Fruita, Colorado.[7] A fairly complete specimen, it lacks lower jaws, forearms and gastralia. The skull, although reasonably complete, was found disarticulated and is strongly flattened sidewards. Although a large individual, it had not yet reached adult size, as indicated by open sutures between the skull bones.[6]:2–3 Scientifically accurate three-dimensional reconstructions of the skull for use in museum exhibits were produced using a complicated process including molding and casting of the individual original bones, correction of deformed and reconstruction of missing parts, assembly of the bone casts into their proper position, and painting in order to match the original color of the bones.[18]

Both the Fruita and Cleveland-Lloyd specimens were described by Madsen and Samuel Paul Welles in a 2000 monograph, with the Utah specimen being referred to the new species Ceratosaurus dentisulcatus and the Colorado specimen to the new species Ceratosaurus magnicornis.[6] The name dentisulcatus refers to the parallel grooves present on the inner sides of the premaxillary teeth and the first three teeth of the lower jaw in that specimen; magnicornis points to the larger nasal horn.[6]:2,21 The validity of both species, however, was questioned in subsequent publications: Brooks Britt and colleagues, in 2000, claim that the Ceratosaurus nasicornis holotype was in fact a juvenile individual, with the two larger species representing the adult state of a single species.[19] Oliver Rauhut, in 2003, and Matthew Carrano and Scott Sampson, in 2008, consider the anatomical differences cited by Madsen and Welles to support these additional species to represent ontogenetic (age related) or individual variation.[20][10]:192 A further specimen (specimen number BYUVP 12893) was discovered in 1992 in the Agate Basin Quarry southeast of Moore, Utah, but still awaits description. The specimen, considered the largest known from the genus, includes the front half of a skull, seven fragmentary pelvic dorsal vertebrae, and an articulated pelvis and sacrum.[10]:192[6]:36 In 1999, Britt reported the discovery of a Ceratosaurus skeleton pertaining to a juvenile individual. Discovered in Bone Cabin Quarry in Wyoming, it is 34% smaller than the Ceratosaurus nasicornis holotype and consists of a complete skull as well as 30% of the remainder of the skeleton including a complete pelvis.[13]

Besides these five skeletal finds, fragmentary Ceratosaurus remains have been reported from various localities from stratigraphic zones 2 and 4-6 of the Morrison Formation,[21] including some of the major fossil sites of the formation. Dinosaur National Monument, Utah, yielded an isolated right premaxilla (specimen number DNM 972); a large shoulder blade (scapulocoracoid) was reported from Como Bluff in Wyoming. Another specimen stems from the Dry Mesa Quarry, Colorado, and includes a left scapulocoracoid as well as fragments of vertebrae and limb bones. In Mygatt Moore Quarry, Colorado, the genus is known from teeth.[6]:36

Finds outside of North AmericaEdit

 
Partial juvenile specimen, North American Museum of Ancient Life

In the years 1909 to 1913, German expeditions of the Berlin Museum für Naturkunde brought to light a diverse dinosaur fauna from the Tendaguru Formation in German East Africa, today Tanzania.[22] Although commonly considered the most important African dinosaur locality,[22] large theropod dinosaurs are only known through few and very fragmentary remains.[23] In 1920, German paleontologist Werner Janensch referred several dorsal vertebrae from the quarry "TL" to Ceratosaurus, as Ceratosaurus sp. (of uncertain species). In 1925, Janensch named a new species of Ceratosaurus, Ceratosaurus roechlingi, based on fragmentary remains from the quarry "Mw" encompassing a quadrate bone, a fibula, fragmentary caudal vertebrae and other fragments. This specimen stems from an individual substantially larger than the Ceratosaurus nasicornis holotype.[23] Madsen and Welles, in their 2000 monography, confirmed the referral of these finds to Ceratosaurus, and additionally ascribed several teeth to the genus which had originally been described by Janensch as Labrosaurus (?) stechowi.[6] Other authors questioned the referral of any of the Tendaguru finds to Ceratosaurus, noting that none of these specimens displays features diagnostic for that genus.[24][3]:66[10]:192[25] In 2011, Rauhut found both Ceratosaurus roechlingi and Labrosaurus (?) stechowi to be possible ceratosaurids, but found them to be not diagnostic at genus level and therefore designated them as nomina dubia (doubtful names).[25] In 1990, Timothy Rowe and Jacques Gauthier mentioned yet another Ceratosaurus species from Tendaguru, Ceratosaurus ingens, which purportedly was erected by Janensch in 1920 and was based on 25 isolated, very large teeth up to 15 cm (5.9 in) in length.[24][23] Janensch, however, did in fact not refer this species to Ceratosaurus but to Megalosaurus; this name therefore might be a simple copying error.[6]:37[23] Rauhut, in 2011, showed that Megalosaurus ingens was not closely related to neither Megalosaurus nor Ceratosaurus, but possibly represents a carcharodontosaurid instead.[25]

In 2000 and 2006, paleontologists led by Octávio Mateus described a find from the Lourinhã Formation in central-west Portugal (ML 352) as a new specimen of Ceratosaurus, consisting of a right femur (upper thigh bone), a left tibia (shin bone), and several isolated teeth recovered from the cliffs of Valmitão beach, between the municipalities Lourinhã and Torres Vedras.[26][27] The bones were found embedded in yellow to brown, fine-grained sandstones, which were deposited by rivers as floodplain deposits and belong to the lower levels of the Porto Novo Member, which is thought to be late Kimmeridgian in age. Additional bones of this individual (SHN(JJS)-65), including a left femur, a right tibia, and a partial left fibula (calf bone), were exposed since due to progressing cliff erosion. Although initially part of a private collection, these additional elements became officially curated after the private collection was donated to the Sociedade de História Natural in Torres Vedras, and were described in detail in 2015.[28] The specimen was ascribed to the species Ceratosaurus dentisulcatus by Mateus and colleagues in 2006.[27] A 2008 review by Carrano and Sampson confirmed the referral to Ceratosaurus, but concluded that the referral to any specific species is not possible at present.[10]:192 In 2015, Elisabete Malafaia and colleagues, who questioned the validity of C. dentisulcatus, assigned the specimen to Ceratosaurus aff. Ceratosaurus nasicornis.[28]

Other reports include a single tooth found in Moutier, Switzerland. Originally named by Janensch in 1920 as Labrosaurus meriani, the tooth was later referred Ceratosaurus sp. (of unknown species) by Madsen and Welles.[6]:35–36 In 2008, Matías Soto and Daniel Perea described teeth from the Tacuarembó Formation in Uruguay, including a presumed premaxillary tooth crown. This shows vertical striations on its inner side and lacks denticles on its front edge; these features are, in this combination, only known from Ceratosaurus. The authors, however, stress that a referral to Ceratosaurus is infeasible as the remains are scant, and furthermore note that the referral of the European and African material to Ceratosaurus has to be viewed with caution.[29]

ClassificationEdit

 
Restoration of Ceratosaurus nasicornis by J. M. Gleeson from 1901, made under supervision of Charles R. Knight.

In his original description of the Ceratosaurus nasicornis holotype and subsequent publications, Marsh noted a number of characteristics which were unknown in all other theropods known at the time.[10]:185 Two of these features, the fused pelvis and co-ossified metatarsus, were however known in from modern-day birds, and, according to Marsh, clearly demonstrate the close relationship between the latter and dinosaurs.[30] In order to set the genus apart from Allosaurus, Megalosaurus and coelurosaurs, Marsh made Ceratosaurus the eponymous and only member of both a new family, the Ceratosauridae, and a new infraorder, the Ceratosauria.[10]:185 This was questioned in 1892 by Edward Drinker Cope, Marsh's rival in the Bone Wars, who argued that distinctive features such as the nasal horn merely showed that Ceratosaurus nasicornis was a distinct species but were insufficient to justify a distinct genus. Consequently, he referred Ceratosaurus nasicornis to the genus Megalosaurus, creating the new combination Megalosaurus nasicornis.[31] Although Ceratosaurus was retained as a distinct genus in all subsequent analyses,[2]:76 its relationships remained controversial during the following century. Over the years, separate authors referred the genus to the Deinodontidae as a close relative of Allosaurus; the Megalosauridae; the Coelurosauria; the Carnosauria; and to the Deinodontoidea.[6]:2 In his 1920 revision, Gilmore argued that the genus was not closely related to any other contemporary theropod known at that time and thus warrants its own family, the Ceratosauridae. Rather, its convervative anatomical features suggest it to be the most basal ( theropod known from after the Triassic.[2]:76 Both the Ceratosauridae and Ceratosauria remained to be not widely accepted, with only few and poorly known additional members identified. It was not before the establishment of cladistic analysis in the 1980's that Marsh's original claim of the Ceratosauria as a distinct group gained ground. Gauthier, in 1986, recognized the Coelophysoidea to be closely related to Ceratosaurus, although this clade falls outside of Ceratosauria in most recent analyses. In 1985, the newly discovered South American genus Abelisaurus was found to be closely related to Ceratosaurus. A large number of additional members of Ceratosauria have been recognized since.[10]:185

The Ceratosauria split off early from the evolutionary line leading to modern birds, and thus is considered basal within theropods.[32] Ceratosauria itself contains a group of derived (non-basal) members, such as Carnotaurus and Noasaurus, bracketed within the clade Abelisauroidea, as well as a number of basal members, such as Elaphrosaurus, Deltadromeus, and Ceratosaurus. The position of Ceratosaurus within basal ceratosaurs is under debate. Some analyses consider Ceratosaurus as the most derived of these basal members, forming the sister taxon of Abelisauroidea.[10]:187[33] Oliver Rauhut, in 2004, proposes Genyodectes as the sister taxon of Ceratosaurus, as both genera are characterized by exceptionally long teeth in the upper jaw.[14] Rauhut grouped Ceratosaurus and Genyodectes within the family Ceratosauridae,[14] which was followed by several later accounts.[34][35][36] The Ceratosauridae was originally erected by Marsh in 1884, but contained Ceratosaurus as the only species and therefore was considered redundant and not used.[14]

 
Mounted cast of a juvenile skeleton, Dinosaur Discovery Museum

The following cladogram showing the relationships of Ceratosaurus is based on the phylogenetic analysis conducted by Diego Pol and Oliver Rauhut in 2012:[34]

Ceratosauria 

Berberosaurus



Deltadromeus





Spinostropheus




Limusaurus



Elaphrosaurus




 Neoceratosauria 
 Ceratosauridae 

Ceratosaurus



Genyodectes



 Abelisauroidea 

Noasauridae



Abelisauridae






A skull from the Middle Jurassic of England apparently displays a nasal horn similar to that of Ceratosaurus. In 1926, Friedrich von Huene described this skull as Proceratosaurus (meaning "before-Ceratosaurus"), assuming that it was an antecedent of the Late Jurassic Ceratosaurus.[37] Today, Proceratosaurus is considered a basal member of the Tyrannosauroidea, a much more derived clade of theropod dinosaurs;[38] the nasal horn therefore would have had evolved independently in both genera.[10]:185 Oliver Rauhut and colleagues, in 2010, grouped Proceratosaurus within its own family, Proceratosauridae. These authors also noted that the nasal horn is incompletely preserved, opening the possibility that it represented the foremost portion of a more extensive head crest, as seen in some other proceratosaurids such as Guanlong.[38]

PaleobiologyEdit

Ecology and feedingEdit

 
Skeletons of Allosaurus and Ceratosaurus nasicornis in fighting postures

All North American Ceratosaurus fossils stem from the Morrison Formation, one of the most important dinosaur-bearing rock units worldwide. Within the Morrison, Ceratosaurus fossils are frequently found in association with those of other large-sized theropods, including the megalosaurid Torvosaurus and the allosaurid Allosaurus. The Garden Park locality in Colorado contained, besides Ceratosaurus, also fossils attributed to Allosaurus. The Dry Mesa Quarry in Colorado as well as the Cleveland-Lloyd Quarry and the Dinosaur National Monument in Utah feature, respectively, the remains of at least three large-sized theropods: Ceratosaurus, Allosaurus and Torvosaurus.[12][39] Likewise, Como Bluff and nearby localities in Wyoming contained remains of Ceratosaurus, Allosaurus and at least one large megalosaurid.[40] Ceratosaurus was a rare element of the theropod fauna; in sites where it co-occurs with Allosaurus, it is outnumbered by the latter by 7.5 to 1 on average.[41]

Several studies attempt to explain how these sympatric species could have reduced direct competition. Donald Henderson, in 1998, analyzed Ceratosaurus as well as two morphs of Allosaurus – a morph with a shortened snout, a high and wide skull and short, backwards projecting teeth, and a morph characterized by a longer snout, lower skull, and long, vertical teeth. Generally speaking, the greater the similarity between sympatric species in regarding morphology, physiology and behavior, the more intense competition will be. Henderson came to the conclusion that the short-snouted Allosaurus morph occupied a different ecological niche than both the long-snouted morph and Ceratosaurus: The shorter skull in this morph would have reduced bending moments occurring during biting and thus increased bite force, comparable to the condition seen in cats. Ceratosaurus and the other Allosaurus morph, on the other hand, have long-snouted skulls, which are better compared to those of dogs: The longer teeth would have been used as fangs to deliver quick slashing bites, with the bite force concentrated at a smaller area due to the narrower skull. Henderson suggested that Ceratosaurus and the long-snouted Allosaurus morph must have engaged in direct competition with each other given the great similarities between their skulls. Thus, Ceratosaurus might had been pushed out of habitats dominated by the long-snouted morph. Indeed, Ceratosaurus is very rare in the Cleveland-Lloyd Quarry, which contains the long-snouted Allosaurus morph, but is much more common in the Dry Mesa Quarry, in which it co-occurs with the short-snouted morph. Furthermore, Ceratosaurus could have avoided competition by preferring different prey items; the evolution of its extremely elongated teeth could have represented a direct result of the competition with the long-snouted Allosaurus morph. Furthermore, both species could have preferred different parts of carcasses when acting as scavengers. The elongated teeth of Ceratosaurus could also have served as visual signals facilitating the recognition of members of the same species, or for other social functions. Furthermore, Henderson argues that the large size of these theropods would have tended to decrease competition, as the number of possible prey items increases with size.[12] In 2006, Foster and Daniel Chure concurred with Henderson that Ceratosaurus and Allosaurus likely had different feeding strategies to avoid competition, as is evidenced by different proportions of the skull, teeth, and forelimb.[41]

 
Restoration of a feeding Ceratosaurus nasicornis

In a 2004 study, Robert Bakker and Gary Bir suggested that Ceratosaurus was primarily specialized in aquatic prey such as lungfish, crocodiles and turtles. As indicated by a statistical analysis of shed teeth from 50 separate localities in and around Como Bluff, teeth of both Ceratosaurus and megalosaurids were most common in habitats in and around water sources such as wet floodplains, lake margins, and swamps. Ceratosaurus also occasionally occurred in terrestrial localities. Allosaurids, on the other hand, were equally common in terrestrial and aquatic habitats. From these results, Bakker and Bir concluded that Ceratosaurus and megalosaurids must have predominantly hunted at and inside the water, with Ceratosaurus also feeding upon carcasses of larger dinosaurs on occasion. The researchers furthermore noted the long, low and flexible body of Ceratosaurus and megalosaurids. Compared to other Morrison theropods, Ceratosaurus showed taller spinous processes on the foremost tail vertebrae, which were vertical rather than inclined towards the back. Together with the deep chevron bones on the underside of the tail, they indicate a deep, "crocodile-like" tail possibly adapted for swimming. On the contrary, allosaurids feature a shorter, taller and stiffer body with longer legs. They would have been adapted for rapid running in open terrain and for preying upon large herbivorous dinosaurs such as sauropods and stegosaurs, but, as speculated by Bakker and Bir, seasonally switched to aquatic prey items when the large herbivores were absent.[40]

Function of the nasal horn and osteodermsEdit

In 1884, Marsh considered the nasal horn of Ceratosaurus to be a "most powerful weapon" for both offensive and defensive purposes, and Gilmore, in 1920, concurred with this interpretation.[4]:331[2]:82 The use of the horn as a weapon is now generally considered unlikely, however.[7] In 1985, David Norman believed that the horn was "probably not for protection against other predators," but might instead have been used for intraspecific combat among male ceratosaurs contending for breeding rights.[42] Paul, in 1988, suggested a similar function, and illustrated two Ceratosaurus engaged in a non-lethal butting contest.[8] In 1990, Rowe and Gauthier went further, suggesting that the nasal horn of Ceratosaurus was "probably used for display purposes alone" and played no role in physical confrontations.[24] If used for display, it is likely that the horn would have been brightly colored.[9] A display function was also proposed for the row of osteoderms running down the body midline.[24]

Forelimb functionEdit

 
Cast of the hand of Ceratosaurus nasicornis (AMNH 27631). Most phalanges of the fingers are missing.

The strongly shortened metacarpals and phalanges of Ceratosaurus raise the question whether the manus retained the grasping function assumed for other basal theropods. Within Ceratosauria, an even more extreme manus reduction can be observed in abelisaurids, where the forelimb was vestigial,[43] and in Limusaurus. In a 2016 paper on the anatomy of the Ceratosaurus manus, Carrano and Jonah Choiniere stressed the great morphological similarity of the manus with those of other basal theropods, suggesting that it still fulfilled its original grasping function despite its shortening. Although only the first phalanges are preserved, the second phalanges would have been mobile as indicated by the well-developed articular surfaces, and the digits would likely have allowed a similar degree of motion as in other basal theropods. As in other theropods other than abelisaurids, digit I would have been slightly inturned when flexed.[11]

Brain and sensesEdit

 
Cast of the skull of the Ceratosaurus nasicornis holotype showing large nasal horn, American Museum of Natural History

A cast of the brain cavity of the holotype was made under supervision of Marsh probably during preparation of the skull, allowing Marsh to conclude that the brain "was of medium size, but comparatively much larger than in the herbivorous Dinosaurs". The skull bones, however, had been cemented together afterwards, so that the accuracy of this cast could not be verified by later studies.[4][2]:93 A second, well preserved braincase had been found with specimen MWC 1 in Fruita, Colorado, and was CT-scanned by paleontologists Kent Sanders and David Smith, allowing for reconstructions of the inner ear, gross regions of the brain, and cranial sinuses transporting blood away from the brain. In 2005, the researchers concluded that Ceratosaurus possessed a brain cavity typical for basal theropods, and similar to that of Allosaurus. The impressions for the olfactory bulbs, which house the sense of smell, are well-preserved. While similar to those of Allosaurus, they were smaller than in Tyrannosaurus, which is thought to have been equipped with a very keen sense of smell. The semicircular canals, which are responsible for the sense of balance and therefore allow for inferences on habitual head orientation and locomotion, are similar to those found in other theropods. In theropods, these structures are generally conservative, suggesting that functional requirements during locomotion have been similar across species. The foremost of the semicircular canals was enlarged, a feature generally found in bipedal animals. The lateral semicircular canal, when oriented horizontally, indicates that the head and neck were held horizontally in neutral position.[44]

Fusion of metatarsals and paleopathologyEdit

The holotype of Ceratosaurus nasicornis was found with its left metatarsals II to IV fused together.[45] Marsh, in 1884, dedicated a short article to this at the time unknown feature in dinosaurs, noting the close resemblance to the condition seen in modern birds.[30] The presence of this feature in Ceratosaurus became controversial in 1890, when Georg Baur speculated that the fusion in the holotype was the result of a healed fracture. This claim was repeated in 1892 by Cope while arguing that Ceratosaurus nasicornis should be classified as a species of Megalosaurus due to insufficient anatomical differences between these genera.[31] However, examples of fused metatarsii in dinosaurs that are not of pathological origin have been described since, including taxa more basal than Ceratosaurus.[45] Osborn, in 1920, explained that no abnormal bone growth is evident, and that the co-ossification is unusual but likely not pathological.[2]:112 Ronald Ratkevich, in 1976, argued that this co-ossification had limited the running ability of the animal, but this claim was rejected by Paul in 1988, who noted that the same feature occurs in many fast-moving animals of today, including ground birds and ungulates.[8] An analysis by Darren Tanke and Bruce Rothschild published in 1999 suggests that the fusion was indeed pathological, confirming the earlier claim of Baur.[45] Other reports of pathologies in Ceratosaurus fossils include a stress fracture in a referred foot bone[46] as well as a broken tooth of an unidentified species of Ceratosaurus that shows signs of further wear received after the break.[45]

Paleoenvironment and paleobiogeographyEdit

All North American Ceratosaurus finds come from the Morrison Formation, a sequence of shallow marine and alluvial sediments which, according to radiometric dating, ranges between 156.3 million years old (Ma) at its base,[47] and 146.8 million years old at the top,[48] which places it in the late Oxfordian, Kimmeridgian, and early Tithonian stages of the Late Jurassic period. Ceratosaurus is known from Kimmeridgian and Tithonian strata of the formation.[3]:49 The Morrison Formation is interpreted as a semiarid environment with distinct wet and dry seasons. The Morrison Basin stretched from New Mexico to Alberta and Saskatchewan, and was formed when the precursors to the Front Range of the Rocky Mountains started pushing up to the west. The deposits from their east-facing drainage basins were carried by streams and rivers and deposited in swampy lowlands, lakes, river channels, and floodplains.[49] This formation is similar in age to the Lourinhã Formation in Portugal and the Tendaguru Formation in Tanzania.[50]

 
Skeletons of Ceratosaurus nasicornis and Dryosaurus, Carnegie Museum

The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs.[51] Other dinosaurs known from the Morrison include the theropods Koparion, Stokesosaurus, Ornitholestes, Allosaurus and Torvosaurus, the sauropods Apatosaurus, Brachiosaurus, Camarasaurus, and Diplodocus, and the ornithischians Camptosaurus, Dryosaurus, Othnielia, Gargoyleosaurus, and Stegosaurus.[52] Diplodocus is commonly found at the same sites as Apatosaurus, Allosaurus, Camarasaurus, and Stegosaurus.[53] Allosaurus, which accounted for 70 to 75% of all theropod specimens, was at the top trophic level of the Morrison food web.[39] Other vertebrates that shared this paleoenvironment included ray-finned fishes, frogs, salamanders, turtles like Dorsetochelys, sphenodonts, lizards, terrestrial and aquatic crocodylomorphans such as Hoplosuchus, and several species of pterosaur like Harpactognathus and Mesadactylus. Shells of bivalves and aquatic snails are also common. The flora of the period has been revealed by fossils of green algae, fungi, mosses, horsetails, cycads, ginkgoes, and several families of conifers. Vegetation varied from river-lining forests of tree ferns and ferns (gallery forests) to fern savannas with occasional trees such as the Araucaria-like conifer Brachyphyllum.[54]

A partial Ceratosaurus specimen indicates the presence of the genus in the Portuguese Porto Novo Member of the Lourinhã Formation. Many of the dinosaurs of the Lourinhã Formation are the same genera as those seen in the Morrison Formation, or have a close counterpart.[50] Besides Ceratosaurus, the researchers also noted the presence of Allosaurus and Torvosaurus in the Portuguese rocks, which are primarily known from the Morrison, while Lourinhanosaurus has so far only been reported from Portugal. Herbivorous dinosaurs from the Porto Novo Member include, amongst others, the sauropods Dinheirosaurus and Zby as well as the stegosaur Miragaia.[55][26][27] During the Late Jurassic, Europe had just been separated from North America by the still narrow Atlantic Ocean, and Portugal, as part of the Iberian Peninsula, was still separated from other parts of Europe. According to Mateus and colleagues, the similarity between the Portuguese and North American theropod fauna indicates the presence of a temporary land bridge, allowing for faunal interchange.[26][27] Malafaia and colleagues, however, argue for a more complex scenario, as other groups, such as sauropods, turtles and crocodiles, show clearly different species compositions in Portugal and North America. Thus, the incipient separation of these faunas could have led to interchange in some but allopatric speciation in other groups.[28]

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