Stegosauria is a group of herbivorous ornithischian dinosaurs that lived during the Jurassic and early Cretaceous periods. Stegosaurian fossils have been found mostly in the Northern Hemisphere (North America, Europe and Asia), Africa and South America. Although their geographical origins are unclear, the earliest unequivocal stegosaurians are known from the Middle Jurassic (Bajocian-Bathonian stages), including Adratiklit, Bashanosaurus, Isaberrysaura and Thyreosaurus. Stegosaurians belong to a clade of armored dinosaurs known as Thyreophora. Originally, they did not differ much from more basal (early-diverging) members of that group, being small, low-slung, running animals protected by armored scutes. An early evolutionary innovation was the development of spikes as defensive weapons. Later species were larger and developed long hindlimbs that no longer allowed them to run. This increased the importance of active defence by the thagomizer, which could ward off even large predators because the tail was in a higher position, pointing horizontally to the rear from the broad pelvis. Stegosaurs had complex arrays of spikes and plates running along their backs, hips and tails. Stegosauria includes two families, the Huayangosauridae and the more diverse Stegosauridae. All species were quadrupedal herbivores with characteristic dorsal dermal plates. These large, thin, erect plates are thought to be aligned parasagittally from the neck to near the end of the tail. The end of the tail has pairs of spikes, sometimes referred to as a thagomizer. It may be that this is the only scientific term derived from a joke (in this case a The Far Side comic). Although defense, thermo-regulation, and display have been theorized to be the possible functions of these dorsal plates, a study of the ontogenetic histology of the plates and spikes suggests that the plates serve different functions at different stages of the stegosaurids' life histories. The terminal spikes in the tail are thought to have been used in old adults, at least, as a weapon for defence. However, the function of stegosaurid plates and spikes, at different life stages, still remains a matter of great debate. The first stegosaurian finds in the early 19th century were fragmentary. Better fossil material, of the genus Dacentrurus, was discovered in 1874 in England. Soon after, in 1877, the first nearly complete skeleton was discovered in the United States. Professor Othniel Charles Marsh that year classified such specimens in the new genus Stegosaurus, from which the group acquired its name, and which is still by far the most famous stegosaurian. During the latter half of the twentieth century, many important Chinese finds were made, representing about half of the presently known diversity of stegosaurians.
History of research The first known discovery of a possible stegosaurian was probably made in the early nineteenth century in England. It consisted of a lower jaw fragment and was in 1848 named Regnosaurus. In 1845, in the area of the present state of South Africa, remains were discovered that much later would be named Paranthodon. In 1874, other remains from England were named Craterosaurus. All three taxa were based on fragmentary material and were not recognised as possible stegosaurians until the twentieth century. They gave no reason to suspect the existence of a new, distinctive group of dinosaurs.
In 1874, extensive remains of what was clearly a large herbivore equipped with spikes were uncovered in England; the first partial stegosaurian skeleton known. They were named Omosaurus by Richard Owen in 1875. Later, this name was shown to be preoccupied by the phytosaur Omosaurus and the stegosaurian was renamed Dacentrurus. Other English nineteenth-century and early twentieth-century finds would be assigned to Omosaurus; later they would, together with French fossils, be partly renamed Lexovisaurus and Loricatosaurus. In 1877, Arthur Lakes, a fossil hunter working for Professor Othniel Charles Marsh in Wyoming, excavated a fossil that Marsh named Stegosaurus the same year. At first, Marsh still entertained some incorrect notions about its morphology. He assumed that the plates formed a flat skin cover—hence the name, meaning "roof saurian"—and that the animal was bipedal with the spikes sticking out sideways from the rear of the skull. A succession of additional discoveries from the Como Bluff sites allowed a quick update of the presumed build. In 1882, Marsh was able to publish the first skeletal reconstruction of a stegosaur. Hereby, stegosaurians became much better known to the general public. The American finds at the time represented the bulk of known stegosaurian fossils, with about twenty skeletons collected. The next important discovery was made when a German expedition to the Tendaguru, then part of German East Africa, from 1909 to 1912 excavated over a thousand bones of Kentrosaurus. The finds increased the known variability of the group, Kentrosaurus being rather small and having long rows of spikes on the hip and tail.
… excerpt ends here. Continue reading the full article.
