Saturday, November 19, 2016

Artist's impression of C. nasicornis

Ceratosaurus 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). 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 (i.e. similar to modern birds). A row of small osteoderms was present down the middle of the back.

Ceratosaurus, at first glance, looked like a fairly typical theropod, however its skull was quite large in proportion to the rest of its body, and large nasal and brow horns and possessed a prominent nose horn formed from protuberances of the nasal bones. 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 enlargement of the lacrimal bones. Uniquely among theropods, Ceratosaurus possessed dermal armor, in the form of small osteoderms running down the middle of its back. Its tail comprised about half of the body’s total length and was thin and flexible with high vertebral spines.

The type specimen was an individual about 18 feet (5.5 m) long; it is not clear whether this animal was fully grown. David B. Norman (1985) estimated that the maximum length of Ceratosaurus was 20 ft (6.1 m), an assessment supported by a particularly large Ceratosaurus specimen from the Cleveland-Lloyd Dinosaur Quarry (UMNH 5728), discovered in the mid-1960s, which may have been 22 ft (6.7 m) long assuming similar proportions to the holotype.

Ceratosaurus is known from the Cleveland-Lloyd Dinosaur Quarry in central Utah and the Dry Mesa Quarry in Colorado. The type species, described by O. C. Marsh in 1884 and redescribed by Gilmore in 1920, is Ceratosaurus nasicornis. The first skeleton was excavated by rancher Marshall Parker Felch in 1883.

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 gives its name to the Ceratosauria, a clade of theropod dinosaurs that diverged early from the evolutionary lineage leading to modern birds. Within the 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, was later found to be unrelated. Ceratosaurus shared its habitat with other large theropod genera including Torvosaurus and Allosaurus, and it has been suggested that these theropods occupied different ecological niches to reduce competition. Ceratosaurus may have preyed upon plant-eating dinosaurs, although some paleontologists suggested that it hunted aquatic prey such as fish. The nasal horn was probably not used as a weapon as was originally suggested by Marsh, but more likely was used solely for display.

Ceratosaurus followed the body plan typical for large theropod dinosaurs. A biped, it moved on powerful hind legs, while its arms were reduced in size. Specimen USNM 4735, the first discovered skeleton and holotype of Ceratosaurus nasicornis, was an individual 5.3 m (17 ft) or 5.69 m (18.7 ft) in length according to separate sources. Whether this animal was fully grown is not clear. Othniel Charles Marsh, in 1884, suggested that this specimen weighed about half as much as the contemporary Allosaurus. In more recent accounts, this was revised to 418 kilograms (922 lb), 524 kg (1,155 lb), or 670 kg (1,480 lb). Three additional skeletons discovered in the latter half of the 20th century were substantially larger. The first of these, UMNH VP 5278, was informally estimated by James Madsen to have been around 8.8 m (29 ft) long, but was later estimated at 7 m (23 ft) in length. Its weight was calculated at 980 kg (2,160 lb), 452 kg (996 lb), and 700 kg (1,540 lb) in separate works. The second skeleton, MWC 1, was somewhat smaller than UMNH VP 5278 and might have been 275 kg (606 lb) in weight. The third, yet undescribed, specimen BYUVP 12893 was claimed to be the largest yet discovered, although estimates have not been published. Another specimen (ML 352), discovered in Portugal in 2000, was estimated at 6 m (20 ft) in length and 600 kg (1,320 lb) in weight.



In his original description of the Ceratosaurus nasicornis holotype and subsequent publications, Marsh noted a number of characteristics that were unknown in all other theropods known at the time. Two of these features, the fused pelvis and fused metatarsus, were known from modern-day birds, and according to Marsh, clearly demonstrate the close relationship between the latter and dinosaurs. To set the genus apart from Allosaurus, Megalosaurus, and coelurosaurs, Marsh made Ceratosaurus the only member of both a new family, the Ceratosauridae, and a new infraorder, the Ceratosauria. 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 C. nasicornis was a distinct species, but were insufficient to justify a distinct genus. Consequently, he assigned C. nasicornis to the genus Megalosaurus, creating the new combination Megalosaurus nasicornis.

Although Ceratosaurus was retained as a distinct genus in all subsequent analyses, its relationships remained controversial during the following century. Both the Ceratosauridae and Ceratosauria were not widely accepted, with only few and poorly known additional members identified. Over the years, separate authors classified Ceratosaurus within the Deinodontidae, the Megalosauridae, the Coelurosauria, the Carnosauria, and the Deinodontoidea. In his 1920 revision, Gilmore argued that the genus was the most basal theropod known from after the Triassic, so not closely related to any other contemporary theropod known at that time; it thus warrants its own family, the Ceratosauridae. It was not until the establishment of cladistic analysis in the 1980s, however, that Marsh's original claim of the Ceratosauria as a distinct group gained ground. In 1985, the newly discovered South American genera Abelisaurus and Carnotaurus were found to be closely related to Ceratosaurus. Gauthier, in 1986, recognized the Coelophysoidea to be closely related to Ceratosaurus, although this clade falls outside of Ceratosauria in most recent analyses. Many additional members of the Ceratosauria have been recognized since.

Restoration of a feeding C. nasicornis

The Ceratosauria split off early from the evolutionary line leading to modern birds, thus is considered basal within theropods. Ceratosauria itself contains a group of derived (nonbasal) members of the families Noasauridae and Abelisauridae, which are 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 ceratosaurians is under debate. Some analyses considered Ceratosaurus as the most derived of the basal members, forming the sister taxon of the Abelisauroidea. Oliver Rauhut, in 2004, proposed Genyodectes as the sister taxon of Ceratosaurus, as both genera are characterized by exceptionally long teeth in the upper jaw. Rauhut grouped Ceratosaurus and Genyodectes within the family Ceratosauridae, which was followed by several later accounts.

Source: https://en.wikipedia.org