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Ceratosaurus /ˌsɛrətˈsɔːrəs/ (from Greek κερας/κερατος, keras/keratos meaning "horn" and σαυρος/sauros meaning "lizard"), was a large predatory theropod dinosaur from the Late Jurassic Period (Kimmeridgian to Tithonian), found in the Morrison Formation of North America, and the Lourinhã Formation of Portugal (and possibly the Tendaguru Formation in Tanzania).[1] It was characterized by large jaws with blade-like teeth, a large, blade-like horn on the snout and a pair of hornlets over the eyes. The forelimbs were powerfully built but very short. The bones of the sacrum were fused (synsacrum) and the pelvic bones were fused together and to this structure[2] (i.e. similar to modern birds). A row of small osteoderms was present down the middle of the back.

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])

Contents

DescriptionEdit

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

Ceratosaurus followed the bauplan typical for large theropod dinosaurs.[3] A biped, it moved on powerful hind legs, while its arms were reduced in size. The holotype specimen was an individual about 5.3 metres (17 ft) long; it is not clear whether this animal was fully grown.[4][5] Marsh (1884) suggested that the holotype individual weighed about half as much as Allosaurus.[6] In more recent accounts, it was estimated at 418 kilograms (922 lb), 524 kilograms (1,155 lb) and 670 kilograms (1,480 lb) by separate authors.[7] Two skeletons, assigned to the new species C. magnicornis and C. dentisulcatus by James H. Madsen and Samuel P. Welles in a 2000 monograph, were substantially larger than the holotype.[8][9] The larger of these, C. dentisuclatus, was informally estimated by Madsen to have been around 8.8 metres (29 ft) long.[10] American science writer Gregory S. Paul, in 1988, estimated the C. dentisulcatus specimen at 980 kilograms (2,160 lb).[11] A considerably lower figure, 275 kilograms (606 lb) for C. magnicornis and 452 kilograms (996 lb) for C. dentisulcatus, was proposed by John Foster in 2007.[12]

 
Distinguishing skull features of Ceratosaurus: The co-ossificated left and right nasal bones form a prominent nasal horn (top), and the teeth of the upper jaw are exceptionally long (bottom). Fossils are on display at the Dinosaur Journey Museum of Fruita, Colorado and have been found near the museum.

The skull was quite large in proportion to the rest of its body, measuring 62.5 cm in length in the holotype.[6][7] Its most distinctive feature was a prominent horn, which was situated on the midline of the skull behind the nostrils. Only the bony horn core is known from fossils – in the living animal, this core would have supported a keratinous sheath. In the holotype specimen, the horn core is 13 centimetres (5.1 in) long and 2 centimetres (0.79 in) wide at its base but quickly narrows down to only 1.2 centimetres (0.47 in) further up; it is 7 centimetres (2.8 in) in height. The horn core formed from co-ossified protuberances of the left and right paired nasal bones.[4] In juveniles, the halves of the horn core were not yet co-ossified.[13] In addition to the large nasal horn, Ceratosaurus possessed smaller hornlike ridges in front of each eye, similar to those of Allosaurus; these ridges were formed by the paired lacrimal bones.[12] All three horns were larger in adults than in juveniles.[13]

The upper jaws were lined with between 12 and 15 blade-like teeth on each side. The paired premaxillary bone, which formed the tip of the snout, contained merely three teeth on each half, less than in most other theropods.[5] Each half of the lower jaw was equipped with 11 to 15 teeth that were slightly straighter and less sturdy than those of the upper jaw.[9] The tooth crowns of the upper jaw were exceptionally long, measuring up to 9.3 cm in length in the largest specimen, which is equal to the minimum height of the lower jaw. In the smaller holotype specimen, the length of the upper tooth crowns (7 cm) even surpasses the minimum height of the lower jaw (6.3 cm) – in other theropods, this feature is only known from the possibly closely related Genyodectes.[14] In contrast, several members of the related Abelisauridae feature very low tooth crowns.[5]

 
Artist's impression of C. nasicornis

The exact number of vertebrae is unknown due to several gaps in the holotype's spine. The sacrum consisted of 6 fused sacral vertebrae. At least 20 presacral vertebrae formed the spine of the neck and back and ca. 50 caudal vertebrae the tail. The tail comprised about half of the body's total length;[6] 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 tigh backwards when contracted. The tail of Ceratosaurus was characterized by comparatively high neural spines (upwards directed bony processes of the caudal vertebrae) and elongated chevrons (bones located below the tail vertebrae), giving the tail a deep profile in lateral view.[5]

The type specimen was found with an articulated left forelimb including an incomplete manus. 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 type specimen or other specimens, opening the possibility that they were lost in the genus. In a 2016 paper, Matthew Carrano and Jonah Choiniere analyzed the cast, concluding that the presence of cartilaginous carpals is indicated by a gap present between the forearm bones and the metacarpals as well as by the surface texture within this gap, which is different from that of the surrounding sediment.[15] The manus retained four digits, with digit IV being reduced in size, and is 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 type specimen, respectively; the total number of phalanges and unguals thus is unknown. The anatomy of metacarpal I indicates that phalanges had been present originally on this digit as well. The 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 specimen. Marsh, in is original 1884 description, assumed that this digit was lost in Ceratosaurus entirely, while Gilmore, in his 1920 monography, noted an attachment area on the second metatarsal demonstrating the presence of this digit.[4]:112

Uniquely among theropods, Ceratosaurus possessed a row of small, elongated and irregularly formed osteoderms (skin bones) running down the middle of its neck, back and most of its tail. Apart from the body midline, the skin contained additional osteoderms, as indicated by a 6 × 7 cm large plate found together with the holotype specimen; the position of this plate on the body is unknown.[4]

History of discoveryEdit

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

The first specimen, the holotype specimen USNM 4737, was discovered by the farmer Marshall Parker Felch in 1883 to 1884.[16] An articulated skeleton (with bones still in their original anatomical position), it was nearly complete, including the skull. The specimen was found encased in hard sandstone; skull and spine had been heavily distorted during fossilization.[4] The site of discovery, located in the Garden Park area north of Cañon City, Colorado and known as the Felch Quarry 1, is regarded one of the richest fossil sites of the Morrison Formation. Numerous dinosaur fossils had been rescued from this quarry even before the discovery of Ceratosaurus, most notably the holotype specimen of Allosaurus. Shortly after its discovery, paleontologist Othniel Charles Marsh published a description of the skeleton, naming the new genus and species Ceratosaurus nasicornis.[6] Given the completeness of the specimen, the newly described genus was the at the time best-known theropod discovered in America. The name Ceratosaurus ("Horn lizard"; from Greek κερας/κερατος, keras/keratos meaning "horn" and σαυρος/sauros meaning "lizard") alludes to the animal's prominent nose horn. In 1920, paleontologist Charles Gilmore published an extensive re-description of the skeleton.[4]

 
C. 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

In a 1892 paper, Marsh published the first skeletal reconstruction of Ceratosaurus. As noted by Gilmore in 1920, the trunk was depicted much too long in this reconstruction, incorporating at least six supernumerary dorsal vertebrae. 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 specimen was mounted by Gilmore in 1910 and 1911 and since was on exhibit at the National Museum of Natural History in Washington, D.C.. Most early reconstructions show Ceratosaurus in an upright posture, with the tail dragged over the ground.[4] Gilmore's mount, in contrast, was ahead of its time:[17] 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.[4] 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.[18][19] 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.[19]

 
Partial juvenile specimen, North American Museum of Ancient Life

After the discovery of the holotype skeleton, a significant find was not made until the early 1960's, when paleontologist James Madsen and his team unearthed a fragmentary, disarticulated skeleton including the skull (specimen number UMNH VP 5278) in the Cleveland-Lloyd Dinosaur Quarry in Utah. This find represents the largest known Ceratosaurus specimen.[20] A second, articulated specimen including the skull (specimen number MWC 1) was discovered by Thor Erikson, the son of paleontologist Lance Erikson, in 1976 near Fruita, Colorado.[10] A fairly complete specimen, it lacks lower jaws, forearms and gastralia. It was a large, but not fully grown individual, as indicated by open sutures between the skull bones. Both 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.[20] The validity of both species, however, was questioned in subsequent publications.[21][22][23] A further specimen (specimen number BYUVP 12893) was discovered in 1992 in the Agate Basin Quarry in Utah, but still awaits description. The specimen, one of the largest Ceratosaurus specimens known, includes the front half of a skull, seven fragmentary pelvic dorsal vertebrae, and fragmentary pelvic bones. In 1999, paleontologist Brooks Britt reported the discovery of the first Ceratosaurus skeleton pertaining to a juvenile individual. Discovered in Bone Cabin Quarry in Wyoming, it is 34% smaller than the holotype specimen 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,[24] including some of the major fossil sites of the formation. Dinosaur National Monument, Utah, yielded an isolated right premaxilla bone (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.[20]

Finds outside of North AmericaEdit

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.[25] Although commonly considered the most important African dinosaur locality,[25] large theropod dinosaurs are only known through few and very fragmentary remains.[26] 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 holotype individual.[26] 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.[20] Other authors question the referral of any of the Tendaguru finds to Ceratosaurus, noting that none of these specimens displays features diagnostic for that genus.[27][5][23][28] Ceratosaurus ingens is now believed to be a dubious carcharodontosaurid, and Labrosaurus(?) stechowi probably represents a taxon closely related to Ceratosaurus.[28] In 1990, Rowe and Gauthier mention a second Ceratosaurus species from Tendaguru, Ceratosaurus ingens, which purportedly was erected by Janensch in 1920 and is based on 25 isolated, very large teeth up to 15 cm in length.[27][26] Janensch, however, did in fact not refer this species to Ceratosaurus but to Megalosaurus; this name therefore might be a simple copying error.[20][26]

In 2000 and 2006, Paleontologists around Octávio Mateus described a find from 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.[29][30] The specimen was recovered from the cliffs of Valmitão beach, between the municipalities Lourinhã und Torres Vedras. The bones were found embedded in yellow to brown, fine-grained sandstones, which represent floodplain deposits and belong to the lower levels of the Porto Novo Member of the Lourinhã Formation, 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.[31] The specimen was ascribed to the species Ceratosaurus dentisulcatus by Mateus and colleagues in 2006.[30] A 2008 review by Carrano and Sampson (2008) confirmed the referral to Ceratosaurus, but concluded that the referral to a specific species is not possible at present.[23] Malafaia and colleagues (2015), who question the validity of C. dentisulcatus, assigned the specimen to Ceratosaurus aff. Ceratosaurus nasicornis.[31]

The specimen provides evidence of a Portuguese theropod fauna similar in composition to the North American Morrison Formation: Besides Ceratosaurus, the researchers also noted the presence of Allosaurus and Torvosaurus in the Portuguese rocks. 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, this similarity between the Portuguese and North American theropod fauna indicates the presence of a temporary land bridge, allowing for faunal interchange.[29][30] 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.[31]

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.[20] Soto and Perea, in 2008, described teeth from the Tacuarembó Formation in Uruguay that show vertical striation on the inner side of the teeth, a feature thought to be diagnostic for Ceratosaurus. These authors, however, stress that a referral to Ceratosaurus is infeasible due to the scant remains, and note that the referral of the European and African Material to Ceratosaurus has to be viewed with caution.[32]

ClassificationEdit

Definition and speciesEdit

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

According to Rauhut (2000), Ceratosaurus can be distinguished from related genera based on the following features: a narrow rounded horn core centrally placed on the fused nasals, a median oval groove on nasals behind horn core, a premaxilla with three teeth, premaxillary teeth with reduced extent of mesial serrations, chevrons that are extremely long, a pubis with a large, rounded notch underneath the obturator foramen, small epaxial osteoderms.[33]

Only the type species Ceratosaurus nasicornis is universally accepted as valid. Two additional species, Ceratosaurus magnicornis and Ceratosaurus dentisulcatus, were named by Madsen and Welles (2000) based on specimens significantly larger than the Ceratosaurus nasicornis holotype. The validity of both species is disputed: Britt and colleagues, in 2000, claim that the Ceratosaurus nasicornis holotype is in fact a juvenile individual, with the two larger species representing the adult state of a single species.[21] Rauhut (2003) and Carrano and Sampson (2008) consider the anatomical differences cited by Madsen and Welles to support these additional species to represent ontogenetic (age related) or individual variation.[22][23]

RelationshipsEdit

 
Cast of the hand of C. nasicornis (AMNH 27631).

In his original description of the Ceratosaurus type specimen and subsequent publications, Marsh noted a number of characteristics which were unknown in all other theropods known at the time.[23]: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.[34] In order to set the genus apart from Allosaurus, Megalosaurus and "coelurosaurs", Marsh made Ceratosaurus the eponymous and only member of a new infraorder, the Ceratosauria. For a whole century after its erection in 1884, only few and very fragmentary members of Ceratosauria were identified, and the clade was not widely used, with Ceratosaurus often regarded a primitive "Carnosaur". Significant content was only recognized with the establishment of cladistic analysis in the 1980s. Jacques 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 has been recognized since.[23]:185

The Ceratosauria splitted of early from the evolutionary line leading to modern birds, and thus is considered basal within theropods.[35] Ceratosauria itself contains a group of derived 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 basal Ceratosauria, forming the sister taxon of Abelisauroidea.[23][36] Oliver Rauhut, in 2004, however 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 inside the family Ceratosauridae,[14] which was followed by several later accounts.[37][38][39] The Ceratosauridae had already been erected by Marsh in 1884, but contained Ceratosaurus as the only species and therefore was considered redundant and not used.[14]

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.[40] Today, Proceratosaurus is considered a basal member of the Tyrannosauroidea, a much more derived clade of theropod dinosaurs;[41] the nasal horn therefore would have evolved independently in both genera.[23] 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.[41]

 
Mounted cast of a juvenile skeleton, Dinosaur Discovery Museum

The following is a cladogram based on the phylogenetic analysis conducted by Diego Pol and Oliver W. M. Rauhut in 2012,[37] showing the relationships of Ceratosaurus:

Ceratosauria 

Berberosaurus



Deltadromeus





Spinostropheus




Limusaurus



Elaphrosaurus




 Neoceratosauria 
 Ceratosauridae 

Ceratosaurus



Genyodectes



 Abelisauroidea 

Noasauridae



Abelisauridae






PaleobiologyEdit

Ecology and feedingEdit

 
Restoration of a feeding C. nasicornis

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.[42][43] Likewise, Como Bluff and nearby localities in Wyoming contained remains of Ceratosaurus, Allosaurus and at least one large megalosaurid.[44]

Several studies attempt to explain how these contemporary 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 is 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. The long-snouted skull of Ceratosaurus and the other Allosaurus morph, however, is better compared with that of a dog: 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. On the other hand, Henderson found great similarities between Ceratosaurus and the long-snouted Allosaurus morph, and concluded that both forms engaged in direct competition with each other. 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 Allosaurus morph. Thus, when acting as scavengers, both species could have preferred different parts of carcasses. The elongated teeth of Ceratosaurus also could 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 (6–8 m) would have tended to decrease competition, as the number of possible prey items increases with size.[42]

 
Skeletons of Allosaurus and C. nasicornis in fighting postures

In a 2004 study, Robert Bakker and Gray Bir suggested that Ceratosaurus was primarily specialized in aquatic prey such as lungfishes, 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 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 neural spines (upwardly directed bony 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.[44]

Allosaurus was itself a potential food item to other carnivores, as illustrated by an Allosaurus pubic foot marked by the teeth of another theropod, probably Ceratosaurus or Torvosaurus. The location of the bone in the body (along the bottom margin of the torso and partially shielded by the legs), and the fact that it was among the most massive in the skeleton, indicates that the Allosaurus was being scavenged.[45]

Function of the nasal horn, osteoderms, and forelimbsEdit

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

Marsh (1884) considered the nasal horn of Ceratosaurus to be a "most powerful weapon" for both offensive and defensive purposes, and Gilmore (1920) concurred with this analysis.[6]:331[4]:82 However, this interpretation is now generally considered unlikely.[10] Norman (1985) 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.[46] Paul (1988) suggested a similar function, and illustrated two Ceratosaurus engaged in a non-lethal butting contest.[11] Rowe and Gauthier (1990) went further, suggesting that the nasal horn of Ceratosaurus was "probably used for display purposes alone" and played no role in physical confrontations.[27] If used for display, it is likely that the horn would have been brightly colored.[12] A display function was also proposed for the row of osteoderms running down the body midline.[27]

The strongly shortened metacarpals and phalanges of Ceratosaurus rise 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,[47] 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 slightly inturned when flexed.[15]

Fusion of metatarsals and paleopathologyEdit

The holotype specimen of Ceratosaurus nasicornis, USMN 4735, was found with its left metatarsals II to IV fused together.[48] Marsh, in 1884, dedicated a short article to this feature, which was at the time unknown in dinosaurs, noting the close resemblance to the condition seen in modern birds.[34] The presence of this feature in Ceratosaurus became controversial in 1890, when Georg Baur speculated that the fusion in the type specimen was the result of a healed fracture. However, examples of fused metatarsii in dinosaurs that are not of pathological origin were described since, including taxa more basal than Ceratosaurus.[48] Osborn, in 1920, explained that no abnormal bone growth is evident, and that the co-ossification is unusual but likely not pathological.[4]:112 Ratkevich, in 1976, argued that this co-ossification had limited the running ability of the animal, but this claim was rejected by Paul (1988), who noted that the same feature occurs in many fast-moving animals of today, including ground birds and ungulates.[11] An analysis by Darren H. Tanke and Bruce Rothschild published in 1999 suggests that the fusion was indeed pathological, confirming the earlier claim of Baur.[48]

Other reports of pathologies in Ceratosaurus fossils include a stress fracture in a referred foot bone[49] as well as a broken tooth of an unidentified species of Ceratosaurus that shows signs of further wear received after the break.[48]

PaleoenvironmentEdit

 
Skeletons of C. nasicornis and Dryosaurus, Carnegie Museum

The Morrison Formation is a sequence of shallow marine and alluvial sediments which, according to radiometric dating, ranges between 156.3 million years old (Ma) at its base,[50] and 146.8 million years old at the top,[51] which places it in the late Oxfordian, Kimmeridgian, and early Tithonian stages of the Late Jurassic period. This formation is interpreted as a semiarid environment with distinct wet and dry seasons. The Morrison Basin where dinosaurs lived, 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.[52] This formation is similar in age to the Lourinha Formation in Portugal and the Tendaguru Formation in Tanzania.[53] In 1877, this formation became the center of the Bone Wars, a fossil-collecting rivalry between early paleontologists Othniel Charles Marsh and Edward Drinker Cope.[54]

The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs.[55] 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.[56] Diplodocus is commonly found at the same sites as Apatosaurus, Allosaurus, Camarasaurus, and Stegosaurus.[57] Allosaurus, which accounted for 70 to 75% of theropod specimens and was at the top trophic level of the Morrison food web.[43] Many of the dinosaurs of the Morrison Formation are the same genera as those seen in Portuguese rocks of the Lourinha Formation (mainly Allosaurus, Ceratosaurus, Torvosaurus, and Stegosaurus), or have a close counterpart (Brachiosaurus and Lusotitan, Camptosaurus and Draconyx).[53] 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.[58]

In popular cultureEdit

With its distinctive nasal horn, Ceratosaurus is amongst the more popular dinosaurs, and is regularly featured in popular books.[10] It also appeared in a number of movies, most notably the 1914 silent movie Brute Force by D. W. Griffith, where a life-sized model of Ceratosaurus is seen threatening cave men.[59]

See alsoEdit

ReferencesEdit

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References citedEdit