Stegosaurus
Summary 3
Stegosaurus (/ˌstɛɡəˈsɔːrəs/[1]), from Greek stegos (στέγος) which means roof and sauros (σαῦρος) which means lizard (Greek: Στεγόσαυρος), is a genus of herbivorous thyreophoran dinosaur. Fossils of this genus date to the Late Jurassic period, where they are found in Kimmeridgian to early Tithonian aged strata, between 155 and 150 million years ago, in the western United States and Portugal. Of the species that have been classified in the upper Morrison Formation of the western US, only three are universally recognized; S. stenops, S. ungulatus and S. sulcatus. The remains of over 80 individual animals of this genus have been found. Stegosaurus would have lived alongside dinosaurs such as Apatosaurus, Diplodocus, Brachiosaurus, Allosaurus, and Ceratosaurus; the latter two may have been predators of it.
These were large, heavily built, herbivorous quadrupeds with rounded backs, short fore limbs, long hind limbs, and tails held high in the air. Due to their distinctive combination of broad, upright plates and tail tipped with spikes, Stegosaurus is one of the most recognizable kinds of dinosaurs. The function of this array of plates and spikes has been the subject of much speculation among scientists. Today, it is generally agreed that their spiked tails were most likely used for defense against predators, while their plates may have been used primarily for display, and secondarily for thermoregulatory functions. Stegosaurus had a relatively low brain-to-body mass ratio. It had a short neck and a small head, meaning it most likely ate low-lying bushes and shrubs. One species, Stegosaurus ungulatus, is the largest known of all the stegosaurians (bigger than related dinosaurs such as Kentrosaurus and Huayangosaurus).
Stegosaurus remains were first identified during the "Bone Wars" by Othniel Charles Marsh. The first known skeletons were fragmentary and the bones were scattered, and it would be many years before the true appearance of these animals, including their posture and plate arrangement, became well understood. Despite its popularity in books and film, mounted skeletons of Stegosaurus did not become a staple of major natural history museums until the mid-20th century, and many museums have had to assemble composite displays from several different specimens due to a lack of complete skeletons. Stegosaurus is one of the best-known dinosaurs, and has been featured in film, postal stamps, and many other types of media.
The quadrupedal Stegosaurus is one of the most easily identifiable dinosaur genera, due to the distinctive double row of kite-shaped plates rising vertically along the rounded back and the two pairs of long spikes extending horizontally near the end of the tail. Although large individuals could grow up to 9 m (29.5 ft) in length[2] and 5.3–7 metric tons (5.8–7.7 short tons) in weight,[3][4] the various species of Stegosaurus were dwarfed by contemporaries, the giant sauropods. Some form of armor appears to have been necessary, as Stegosaurus species coexisted with large predatory theropod dinosaurs, such as Allosaurus and Ceratosaurus.
Most of the information known about Stegosaurus comes from the remains of mature animals; more recently, though, juvenile remains of Stegosaurus have been found. One subadult specimen, discovered in 1994 in Wyoming, is 4.6 m (15.1 ft) long and 2 m (6.6 ft) high, and is estimated to have weighed 2.4 metric tons (2.6 short tons) while alive. It is on display in the University of Wyoming Geological Museum.[5]
The long and narrow skull was small in proportion to the body. It had a small antorbital fenestra, the hole between the nose and eye common to most archosaurs, including modern birds, though lost in extant crocodylians. The skull's low position suggests that Stegosaurus may have been a browser of low-growing vegetation. This interpretation is supported by the absence of front teeth and their likely replacement by a horny beak or rhamphotheca. The lower jaw had flat downward and upward extensions that would have completely hidden the teeth when viewed from the side, and these probably supported a turtle-like beak in life. The presence of a beak extended along much of the jaws may have precluded the presence of cheeks in these species.[6][7] Such an extensive beak was probably unique to Stegosaurus and some other advanced stegosaurids among ornithischians, which usually had beaks restricted to the jaw tips.[6][8] Other researchers have interpreted these ridges as modified versions of similar structures in other ornithischians which might have supported fleshy cheeks, rather than beaks.[9] Stegosaurian teeth were small, triangular, and flat; wear facets show that they did grind their food.[10]
Despite the animal's overall size, the braincase of Stegosaurus was small, being no larger than that of a dog. A well-preserved Stegosaurus braincase allowed Othniel Charles Marsh to obtain, in the 1880s, a cast of the brain cavity or endocast of the animal, which gave an indication of the brain size. The endocast showed the brain was indeed very small, the smallest proportionally of all dinosaur endocasts then known. The fact that an animal weighing over 4.5 metric tons (5 short tons) could have a brain of no more than 80 g (2.8 oz) contributed to the popular old idea that all dinosaurs were unintelligent, an idea now largely rejected.[11] Actual brain anatomy in Stegosaurus is poorly known, but the brain itself was small even for a dinosaur, fitting well with a slow, herbivorous lifestyle and limited behavioral complexity.[12]
In Stegosaurus stenops there are 27 bones in the vertebral column anterior to the sacrum, a varying number of vertebrae in the sacrum, with four in most subadults, and around 46 caudal (tail) vertebrae. The presacrals are divided into cervical (neck) and dorsal (back) vertebrae, with around 10 cervicals and 17 dorsals, the total number being one greater than in Hesperosaurus, two greater than Huayangosaurus, although Miragaia preserves 17 cervicals and an unknown number of dorsals. The first cervical vertebra is the axis bone, which is connected and often fused to the atlas bone. Farther posteriorly, the proportionately larger the cervicals become, although they do not change greatly in anything other than size. Past the first few dorsals, the centrum of the bones become more elongate front-to-back, and the transverse processes become more elevated dorsal. The sacrum of S. stenops includes four sacral vertebrae, but one of the dorsals is also incorporated into the structure. In some specimens of S. stenops, a caudal is also incorporated, as a caudosacral. In Hesperosaurus there are two dorsosacrals, and only four fused sacrals, but in Kentrosaurus there may be as many as seven vertebrae in the sacrum, with both dorsosacrals and caudosacrals. S. stenops preserves 46 caudal vertebrae, and up to 49, and along the series both the centrums and the neural spines become smaller, until the neural spines disappear at caudal 35. Around the middle of the tail, the neural spines become bifurcated, meaning they are divided near the top.[13]
With multiple well-preserved skeletons, S. stenops preserves all regions of the body, including the limbs. The scapula (shoulder blade) is sub-rectangular, with a robust blade. Though it is not always perfectly preserved, the acromion ridge is slightly larger than in Kentrosaurus. The blade is relatively straight, although it curves towards the back. There is a small bump on the back of the blade, that would have served as the base of the triceps muscle. Articulated with the scapula, the coracoid is sub-circular.[13] The hind feet each had three short toes, while each fore foot had five toes; only the inner two toes had a blunt hoof. The phalangeal formula is 2-2-2-2-1, meaning the innermost finger of the fore limb has two bones, the next has two, etc.[14] All four limbs were supported by pads behind the toes.[15] The fore limbs were much shorter than the stocky hind limbs, which resulted in an unusual posture. The tail appears to have been held well clear of the ground, while the head of Stegosaurus was positioned relatively low down, probably no higher than 1 m (3.3 ft) above the ground.[16]
The most recognizable features of Stegosaurus are its dermal plates, which consisted of between 17 and 22 separate plates and flat spines.[17] These were highly modified osteoderms (bony-cored scales), similar to those seen in crocodiles and many lizards today. They were not directly attached to the animal's skeleton, instead arising from the skin. The largest plates were found over the hips and could measure over 60 cm (24 in) wide and 60 cm (24 in) tall.[17]
In a 2010 review of Stegosaurus species, Peter Galton suggested that the arrangement of the plates on the back may have varied between species, and that the pattern of plates as viewed in profile may have been important for species recognition. Galton noted that the plates in S. stenops have been found articulated in two staggered rows, rather than paired. Fewer S. ungulatus plates have been found, and none articulated, making the arrangement in this species more difficult to determine. However, the type specimen of S. ungulatus preserves two flattened spine-like plates from the tail that are nearly identical in shape and size, but are mirror images of each other, suggesting that at least these were arranged in pairs.[18] Many of the plates are manifestly chiral[19][20] and no two plates of the same size and shape have been found for an individual; however plates have been correlated between individuals. Well preserved integumentary impressions of the plates of Hesperosaurus show a smooth surface with long and parallel, shallow grooves. This indicates that the plates were covered in keratinous sheaths.[21]
Fontes e Créditos
- (c) Andreone93, alguns direitos reservados (CC BY-SA), http://commons.wikimedia.org/wiki/File:Museo_Nacional_de_Ciencias_Naturales_Stegosaurus.jpg
- (c) calgaryzoo, todos os direitos reservados
- Adaptado por calgaryzoo de uma obra de (c) Wikipedia, alguns direitos reservados (CC BY-SA), https://en.wikipedia.org/wiki/Stegosaurus
