Stegosauria

Stegosaurs Temporal range: Middle Jurassic - Early Cretaceous, 169–100.5 Ma PreꞒ Ꞓ O S D C P T J K Pg N Possible Early Jurassic, Aalenian and Late Maastrichtian records in the form of fossil tracks and referred fossils.[1][2][3]
Six stegosaurs (top left to bottom right): Stegosaurus ungulatus, Kentrosaurus, Huayangosaurus, Gigantspinosaurus, Miragaia (or possibly Dacentrurus), Tuojiangosaurus
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	†Ornithischia
Clade:	†Thyreophora
Clade:	†Eurypoda
Clade:	†Stegosauria Marsh, 1877
Subgroups[4][5]
†Baiyinosaurus †Bashanosaurus †Craterosaurus †Gigantspinosaurus †Isaberrysaura †Lexovisaurus †Monkonosaurus †Yanbeilong †Huayangosauridae †Chungkingosaurus †Huayangosaurus †Regnosaurus? †Stegosauridae †Kentrosaurus †Loricatosaurus †Mongolostegus †Paranthodon †Tuojiangosaurus †Dacentrurinae †Alcovasaurus? †Adratiklit †Dacentrurus †Miragaia †Thyreosaurus †Stegosaurinae †Chialingosaurus? †Hesperosaurus †Hypsirhophus? †Jiangjunosaurus? †Stegosaurus †Wuerhosaurus †Yingshanosaurus?

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 possibly South America. Their geographical origins are unclear; the earliest unequivocal stegosaurian, Bashanosaurus primitivus, was found in the Bathonian Shaximiao Formation of China.

Stegosaurians were armored dinosaurs (thyreophorans). Originally, they did not differ much from more primitive 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, belonging to a subgroup called the Stegosauridae, became 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. Stegosaurids had complex arrays of spikes and plates running along their backs, hips and tails.

Stegosauria includes two families, the primitive Huayangosauridae and the more derived Stegosauridae. The stegosaurids like all other stegosaurians were quadrupedal herbivores that exhibited the characteristic stegosaurian 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.^[6][7] 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.^[8] 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.