Stegosauria

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]} 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.^[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.

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.^[9] 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 the same year named Stegosaurus . 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.^[9]

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.

From the 1950s onwards, the geology of China was systematically surveyed in detail and infrastructural works led to a vast increase of digging activities in that country. This resulted in a new wave of Chinese stegosaurian discoveries, starting with Chialingosaurus in 1957. Chinese finds of the 1970s and 1980s included Wuerhosaurus , Tuojiangosaurus , Chungkingosaurus , Huayangosaurus , Yingshanosaurus and Gigantspinosaurus . This increased the age range of good fossil stegosaurian material, as they represented the first relatively complete skeletons from the Middle Jurassic and the Early Cretaceous. Especially important was Huayangosaurus, which provided unique information about the early evolution of the group.

Towards the end of the twentieth century, the so-called Dinosaur Renaissance took place in which a vast increase in scientific attention was given to the Dinosauria. In 2007, Jiangjunosaurus was reported, the first Chinese dinosaur named since 1994. Nevertheless, European and North-American sites have become productive again during the 1990s, Miragaia having been found in the Lourinhã Formation in Portugal and a number of relatively complete Hesperosaurus skeletons having been excavated in Wyoming. Apart from the fossils per se, important new insights have been gained by applying the method of cladistics, allowing for the first time to exactly calculate stegosaurian evolutionary relationships.

Stegosaurids are distinguished from other stegosaurians in that the former have lost the plesiomorphic pre-maxillary teeth and lateral scute rows along the trunk.^[10] Furthermore, stegosaurids have long narrow skulls and longer hindlimbs compared to their forelimbs.^[7] However, these two features are not diagnostic of Stegosauridae because they may also be present in non-stegosaurid stegosaurians.^[1]

Stegosaurians had characteristic small, long, flat, narrow heads and a horn-covered beak or rhamphotheca,^[1] which covered the front of the snout (two premaxillaries) and lower jaw (a single predentary) bones. Similar structures are seen in turtles and birds. Apart from Huayangosaurus , stegosaurians subsequently lost all premaxillary teeth within the upper beak. Huayangosaurus still had seven per side. The upper and lower jaws are equipped with rows of small teeth. Later species have a vertical bone plate covering the outer side of the lower jaw teeth. The structure of the upper jaw, with a low ridge above, and running parallel to, the tooth row, indicates the presence of a fleshy cheek. In stegosaurians, the typical archosaurian skull opening, the antorbital fenestra in front of the eye socket, is small, sometimes reduced to a narrow horizontal slit. In general, stegosaurids have proportionally long, low and narrow snouts with a deep mandible, compared to that of Huayangosaurus . Stegosaurids also lack premaxillary teeth.^[10]

All stegosaurians are quadrupedal, with hoof-like toes on all four limbs. All stegosaurians after Huayangosaurus have forelimbs much shorter than their hindlimbs. Their hindlimbs are long and straight, designed to carry the weight of the animal while stepping. The condyles of the lower thighbone are short from the front to the rear. This would have limited the supported rotation of the knee joint, making running impossible. Huayangosaurus had a thighbone like a running animal. The upper leg was always longer than the lower leg.

Huayangosaurus had relatively long and slender arms. The forelimbs of later forms are very robust, with a massive humerus and ulna. The wrist bones were reinforced by a fusion into two blocks, an ulnar and a radial. The front feet of stegosaurians are commonly depicted in art and in museum displays with fingers splayed out and slanted downward. However, in this position, most bones in the hand would be disarticulated. In reality, the hand bones of stegosaurians were arranged into vertical columns, with the main fingers, orientated outwards, forming a tube-like structure. This is similar to the hands of sauropod dinosaurs, and is also supported by evidence from stegosaurian footprints and fossils found in a lifelike pose.^[11]

The long hindlimbs elevated the tail base, such that the tail pointed out behind the animal almost horizontally from that high position. While walking, the tail would not have sloped downwards as this would have impeded the function of the tail base retractor muscles, to pull the thighbones backwards. However, it has been suggested by Robert Thomas Bakker that stegosaurians could rear on their hind legs to reach higher layers of plants, the tail then being used as a "third leg". The mobility of the tail was increased by a reduction or absence of ossified tendons, that with many Ornithischia stiffen the hip region. Huayangosaurus still possessed them. In species that had short forelimbs, the relatively short torso towards the front curved strongly downwards. The dorsal vertebrae typically were very high, with very tall neural arches and transverse processes pointing obliquely upwards to almost the level of the neural spine top. Stegosaurian back vertebrae can easily be identified by this unique configuration. The tall neural arches often house deep neural canals; enlarged canals in the sacral vertebrae have given rise to the incorrect notion of a "second brain". Despite the downwards curvature of the rump, the neck base was not very low and the head was held a considerable distance off the ground. The neck was flexible and moderately long. Huayangosaurus still had the probably original number of nine cervical vertebrae; Miragaia has an elongated neck with seventeen.^[12]

The stegosaurian shoulder girdle was very robust. In Huayangosaurus, the acromion, a process on the lower front edge of the shoulderblade, was moderately developed; the coracoid was about as wide as the lower end of the scapula, with which it formed the shoulder joint. Later forms tend to have a strongly expanded acromion, while the coracoid, largely attached to the acromion, no longer extends to the rear lower corner of the scapula.

The stegosaurian pelvis was originally moderately large, as shown by Huayangosaurus. Later species, however, convergent to the Ankylosauria developed very broad pelves, in which the iliac bones formed wide horizontal plates with flaring front blades to allow for an enormous belly-gut. The ilia were attached to the sacral vertebrae via a sacral yoke formed by fused sacral ribs. Huayangosaurus still had rather long and obliquely oriented ischia and pubic bones. In more derived species, these became more horizontal and shorter to the rear, while the front prepubic process lengthened.

Like all Thyreophora, stegosaurians were protected by bony scutes that were not part of the skeleton proper but skin ossifications instead: the so-called osteoderms. Huayangosaurus had several types. On its neck, back, and tail were two rows of paired small vertical plates and spikes. The very tail end bore a small club. Each flank had a row of smaller osteoderms, culminating in a long shoulder spine in front, curving to the rear.^{[citation needed ]} Later forms show very variable configurations, combining plates of various shape and size on the neck and front torso with spikes more to the rear of the animal. They seem to have lost the tail club and the flank rows are apparently absent also, with the exception of the shoulder spine, still shown by Kentrosaurus and extremely developed, as its name indicates, in Gigantspinosaurus . As far as is known, all forms possessed some sort of thagomizer, though these are rarely preserved articulated allowing to establish the exact arrangement. A fossil of Chungkingosaurus sp. has been reported with three pairs of spikes pointing outwards and a fourth pair pointing to the rear.^[13] The most derived species, like Stegosaurus , Hesperosaurus and Wuerhosaurus , have very large and flat back plates. Stegosaurid plates have a thick base and central portion, but are transversely thin elsewhere. The plates become remarkably large and thin in Stegosaurus. They are found in varying sizes along the dorsum, with the central region of the back usually having the largest and tallest plates. The arrangement of these parasagittal dorsal plates has been intensely debated in the past. Discoverer Othniel Charles Marsh suggested a single median row of plates running post-cranially along the longitudinal axis^[14] and Lull argued in favour of bilaterally paired arrangement throughout the series.^[15] Current scientific consensus lies in the arrangement proposed by Gilmore - two parasagittal rows of staggered alternates, after the discovery of an almost complete skeleton preserved in this manner in rock.^[16] Furthermore, no two plates share the same size and shape, making the possibility of bilaterally paired rows even less likely. Plates are usually found with distinct vascular grooves on their lateral surfaces, suggesting the presence of a circulatory network. Stegosaurids also have osteoderms on the throat in the form of small depressed ossicles and two pairs of elongated spike-like tail-spines.^[1] With Stegosaurus fossils also ossicles have been found in the throat region, bony skin discs that protected the lower neck.^[17]

Many basal stegosaurs like Gigantspinosaurus and Huayangosaurus have been discovered with parascapular spines, or spines emerging from the shoulder region. Among stegosaurids, only Kentrosaurus has been found with parascapular spines, which project posteriorly out of the lower part of the shoulder plates. These spines are long, rounded and comma-shaped in lateral view and have an enlarged base. Loricatosaurus was also believed to have a parascapular spine, but Maidment et al. (2008) observed that the discovered specimen, from which the spine is described, has a completely different morphology than the parascapular spine specimens of other stegosaurs. They suggest it may be a fragmentary tail spine instead.^[18] Stegosaurids also lack lateral scute rows that run longitudinally on either side of the trunk in Huayangosaurus and ankylosaurs, indicating yet another secondary loss of a plesiomorphic characters.^[10] However, the absence of lateral scutes as well as pre-maxillary teeth mentioned above are not specifically diagnostic of stegosaurids, since these features are also present in some other stegosaurians, whose phylogenetic relationships are unclear.^{[10] [18]}

The discovery of an impression of the skin covering the dorsal plates has implications for all possible functions of stegosaurian plates. Christiansen and Tschopp (2010)^[19] found that the skin was smooth with long, parallel, shallow grooves indicating a keratinous structure covering the plates. The addition of beta-keratin, a strong protein, would indeed allow the plates to bear more weight, suggesting they may have been used for active defense. A keratinous covering would also allow greater surface area for the plates to be uses as a mating display structures, which could be potentially coloured like the beaks of modern birds. At the same time this finding implies that the use of plates for thermo-regulation may be less likely because the keratinous covering would make heat transfer from the bone highly ineffective.^[19]

In 1877, Othniel Marsh discovered and named Stegosaurus armatus, from which the name of the family 'Stegosauridae' was erected in 1880.^[9] In comparison to basal stegosaurians, notable synapomorphies of Stegosauridae include a large antitrochanter (supracetabular process) in the ilium, a long prepubic process and long femur relative to the length of the humerus.^[20] Furthermore, stegosaurid sacral ribs are T-shaped in parasagittal cross-section^[1] and the dorsal vertebrae have an elongated neural arch.^[9] The first exact clade definition of Stegosauria was given by Peter Malcolm Galton in 1997: all thyreophoran Ornithischia more closely related to Stegosaurus than to Ankylosaurus .^[21] This definition was formalized in the PhyloCode by Daniel Madzia and colleagues in 2021 as "the largest clade containing Stegosaurus stenops , but not Ankylosaurus magniventris ".^[4] Thus defined, the Stegosauria are by definition the sister group of the Ankylosauria within the Eurypoda. The vast majority of stegosaurian dinosaurs thus far recovered belong to the Stegosauridae, which lived in the later part of the Jurassic and early Cretaceous, and which were defined by Paul Sereno as all stegosaurians more closely related to Stegosaurus than to Huayangosaurus.^[22] This definition was also formalized in the PhyloCode by Daniel Madzia and colleagues in 2021 as "the largest clade containing Stegosaurus stenops , but not Huayangosaurus taibaii ".^[4] They include per definition the well-known Stegosaurus . This group is widespread, with members across the Northern Hemisphere, Africa and possibly South America.^{[23] [24]}

Huayangosauridae (derived from Huayangosaurus , "Huayang reptile") is a family of stegosaurian dinosaurs from the Jurassic of China.^[25] The group is defined as all taxa closer to the namesake genus Huayangosaurus than Stegosaurus , and was originally named as the family Huayangosaurinae by Dong Zhiming and colleagues in the description of Huayangosaurus.^{[25] [4]} Huayangosaurinae was originally differentiated by the remaining taxa within Stegosauridae by the presence of teeth in the premaxilla , an antorbital fenestra , and a mandibular fenestra . Huayangosaurinae, known from the Middle Jurassic of the Shaximiao Formation, was proposed to be intermediate between Scelidosaurinae and Stegosaurinae, suggesting that the origins of stegosaurs lay in Asia.^[25] Following phylogenetic analyses, Huayangosauridae was expanded to also include the taxon Chungkingosaurus , known from specimens from younger Late Jurassic deposits of the Shaximiao Formation.^[18] Huayangosauridae is either the sister taxon to all other stegosaurs,^{[18] [9]} or close to the origin of the clade, with taxa like Gigantspinosaurus or Isaberrysaura outside the Stegosauridae-Huayangosauridae split.^{[4] [9]} Huayangosauridae was formally defined in 2021 by Daniel Madzia and colleagues, who used the previous definitions of all taxa closer to Huayangosaurus taibaii than Stegosaurus stenops .^[4]

In 2017, Raven and Maidment published a new phylogenetic analysis, including almost every known stegosaurian genus:^{[26] [4]}