Stegosaurids

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, predominantly in what is now North America, Europe and China, though one species (Kentrosaurus aethiopicus) is known to have lived in Africa. Their geographical origins are unclear; the earliest unequivocal stegosaurian, Huayangosaurus taibaii, lived in China.
Like the spikes and shields of ankylosaurs, the bony plates and spines of stegosaurians evolved from the low-keeled osteoderms characteristic of basal thyreophorans.One such described genus, Scelidosaurus, is proposed to be morphologically close to the last common ancestor of the clade uniting stegosaurians and ankylosaurians, the Eurypoda.Galton (2019) interpreted plates of an armored dinosaur from the Lower Jurassic (Sinemurian-Pliensbachian) Lower Kota Formation of India as fossils of a member of Ankylosauria; the author argued that this finding indicates a probable early Early Jurassic origin for both Ankylosauria and its sister group Stegosauria.Footprints attributed to the ichnotaxon Deltapodus brodricki from the Middle Jurassic (Aalenian) of England represent the oldest probable record of stegosaurians reported so far.The perhaps most basal known stegosaurian, the four-metre-long Huayangosaurus, is still close to Scelidosaurus in build, with a higher and shorter skull, a short neck, a low torso, long slender forelimbs, short hindlimbs, large condyles on the thighbone, a narrow pelvis, long ischial and pubic shafts, and a relatively long tail. Its small tail club might be a eurypodan synapomorphy. Huayangosaurus lived during the Bathonian stage of the Middle Jurassic, about 166 million years ago.

1. Stegosaurus 2. Huayangosaurus 3. Gigantspinosaurus 4. Wuerhosaurus 5. Tuojiangosaurus
A few million years later, during the Callovian-Oxfordian, from China much larger species are known, with long, "graviportal", i.e., a type adapted for moving only in a slow manner on land due to a high body weight, hindlimbs: Chungkingosaurus, Chialingosaurus, Tuojiangosaurus and Gigantspinosaurus. Most of these are considered members of the derived Stegosauridae. Lexovisaurus and Loricatosaurus, stegosaurid finds from England and France of approximately equivalent age to the Chinese specimens, are likely the same taxon. During the Late Jurassic, stegosaurids seem to have experienced their greatest radiation. In Europe, Dacentrurus and the closely related Miragaia were present. While older finds had been limited to the northern continents, in this phase Gondwana was colonised also as shown by Kentrosaurus living in Africa. No unequivocal stegosaurian fossils have been reported from South-America, India, Madagascar, Australia, or Antarctica, though. A Late Jurassic Chinese stegosaurian is Jiangjunosaurus. The most derived Jurassic stegosaurians are known from North-America: Stegosaurus (perhaps several species thereof) and the somewhat older Hesperosaurus. Stegosaurus was quite large (some specimens indicate a length of at least seven metres), had high plates, no shoulder spine, and a short, deep rump.

From the Early Cretaceous, far fewer finds are known and it seems that the group had declined in diversity. Some fragmentary fossils have been described, such as Craterosaurus from England and Paranthodon from South Africa. The only more substantial discoveries are those of Wuerhosaurus, the exact age of which is highly uncertain.In the autumn of 2016, the teeth and tail spikes of stegosaurs were found in a Russian ravine that was once part of an Early Cretaceous river. The remains of these stegosaurs and other dinosaurs in the area were all miniature, implying that either large amounts of baby dinosaurs spent the first stage in their lives here, or that the area was home to dwarf dinosaurs. Several Russian scientists also theorize that the remains of these stegosaurs, as well as the remains of allosaurids in the same area, could imply that the area could have once been a "refugium", where these Jurassic dinosaurs managed to survive into the Cretaceous period.
It has often been suggested that the decline in stegosaur diversity was part of a Jurassic-Cretaceous transition, where angiosperms become the dominant plants, causing a faunal turnover where new groups of herbivores evolved.Although in general the case for such a causal relation is poorly supported by the data, stegosaurians are an exception in that their decline coincides with that of the Cycadophyta.

Though Late Cretaceous stegosaurian fossils have been reported, these have mostly turned out to be misidentified. A well-known example is Dravidosaurus, known from Coniacian fossils found in India. Though originally thought to be stegosaurian, in 1991 these badly-eroded fossils were suggested to instead have been based on plesiosaurian pelvis and hindlimb material and none of the fossils are demonstrably stegosaurian.The reinterpretation of Dravidosaurus as a plesiosaur wasn"t accepted by Galton and Upchurch (2004), who stated that the skull and plates of Dravidosaurus are certainly not plesiosaurian, and noted the need to redescribe the fossil material of Dravidosaurus.Purported stegosaurian dermal plate was reported from the latest Cretaceous (Maastrichtian) Kallamedu Formation (southern India); however, Galton & Ayyasami (2017) interpreted the specimen as a bone of a sauropod dinosaur. Nevertheless, the authors considered the survival of stegosaurians into the Maastrichtian to be possible, noting the presence of the stegosaurian ichnotaxon Deltapodus in the Maastrichtian Lameta Formation (western India)


The largest known stegosaurids was Stegosaurus at 9 metres (30 ft) in length and 6 tonnes in weight

Stegosaurus ungulatus, meaning "hoofed roof lizard", was named by Marsh in 1879 from remains recovered at Como Bluff, Wyoming (Quarry 12, near Robber"s Roost). It might be synonymous with S. stenops. At 9 m (30 ft), it was the longest species within the genus Stegosaurus.
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 (30 ft) in length the various species of Stegosaurus were dwarfed by their 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 ft) long and 2 m (6.6 ft) high, and is estimated to have weighed 2.3 metric tons (2.6 t) while alive. It is on display in the University of Wyoming Geological Museum.
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 of Stegosaurus had a flat upward extension that would have completely hidden the teeth when viewed from the side, and which probably supported a turtle-like beak in life. Other researchers have interpreted these ridges as modified versions of similar structures in other ornithischians which might have supported fleshy cheeks, rather than beaks. Stegosaurian teeth were small, triangular, and flat; wear facets show that they did grind their food. The jaws of Stegosaurus had flat downward and upward extensions that would have completely hidden the teeth when viewed from the side, and these probably supported a beak in life. The presence of a beak extended along much of the jaws may have precluded the presence of cheeks in these species. 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.
A fragmentary Stegosaurus specimen discovered in Portugal and dating from the upper Kimmeridgian-lower Tithonian stage has been tentatively assigned to this species. Stegosaurus ungulatus can be distinguished from S. stenops by the presence of smaller, more triangular plates and by several pairs of small, flat, pointed plates just before the spikes on the tail. These spine-like plates appear to have been paired, due to the presence of at least one pair that are identical but mirrored. S. ungulatus also appears to have had longer legs (femora) and hip bones than other species. The type specimen of S. ungulatus was discovered with eight spikes, though they were scattered away from their original positions. These have often been interpreted as indicating that the animal had four pairs of tail spikes. No specimens have been found with complete or articulated sets of tail spikes, but no additional specimens have been found that preserve eight spikes together. It is possible the extra pair of spikes came from a different individual, and though no other extra bones were found with the specimen, these may be found if more digging were done at the original site. Specimens from other quarries (such as a tail from Quarry 13, now forming part of the composite skeleton AMNH 650 at the American Museum of Natural History), referred to S. ungulatus on the basis of their notched tail vertebrae, are preserved with only four tail spikes. The type specimen of S. ungulatus (YPM 1853) was incorporated into the first ever mounted skeleton of a stegosaur at the Peabody Museum of Natural History in 1910 by Richard Swann Lull. It was initially mounted with paired plates set wide, above the base of the ribs, but was remounted in 1924 with two staggered rows of plates along the midline of the back. Additional specimens recovered from the same quarry by the United States National Museum of Natural History, including tail vertebrae and an additional large plate (USNM 7414), belong to the same individual as YPM 1853.
Stegosaurus stenops, meaning "narrow-faced roof  lizard", was named by Marsh in 1887 with the holotype having been collected by Marshal Felch at Garden Park, north of Caсon City, Colorado, in 1886. This is the best-known species of Stegosaurus, mainly because its remains include at least one complete articulated skeleton. It had proportionately large, broad plates and rounded tail plates. Articulated specimens show that the plates were arranged alternating in a staggered double row. S. stenops is known from at least 50 partial skeletons of adults and juveniles, one complete skull, and four partial skulls. It was shorter than other species, at 7 m (23 ft). Found in the Morrison Formation, Colorado, Wyoming, and Utah.
  

Dacentrurus armatus (Very sharp tail).

1.Dacentrurus 2.Lexovisaurus 3.Huayangosaurus 4.Wuerhosaurus 5.Gigantspinosaurus
Dacentrurus was a large stegosaurid. Some specimens have been estimated to reach lengths between 7–8 m (23–26 ft) and to weigh up to 5 t (5.5 short tons). Many books claim that Dacentrurus was a small stegosaur, when in fact finds such as a 1.5 metres (4.9 ft) wide pelvis indicate that Dacentrurus was among the largest of them. For a stegosaur the gut was especially broad and a massive rump is also indicated by exceptionally wide dorsal vertebrae centra.The hindlimb was rather short but the forelimb relatively long, largely because of a long lower arm.
Dacentrurus was originally named Omosaurus armatus in 1875 by the famous British palaeontologist Richard Owen, however the genus name of Omosaurus was already used to name another animal. This led to the 1902 renaming by Frederic Lucas, although the species name was still retained in creating the type species of the new genus, as is standard procedure for such a renaming.
Unlike more famous genera that have plates all the way down the back to a spiked ‘thagomizer’ on the end of the tail, Dacentrurus had eight pairs of triangular plates that ran from the neck to the posterior end of the sacrum (hip), which were then followed by four pairs of large spikes that ran down to the thagomizer (four more pairs of spikes that pointed to the sides). This arrangement is very similar to the African stegosaur Kentrosaurus, though analysis suggests that the closest relative of Dacentrurus was Miragaia. Another study has also revealed that Dacentrurus is one of if not the closest relative of the North American Hesperosaurus.
Dacentrurus lived in the Kimmeridgian area of the late Jurassic period, the heyday for the stegosaurs where they seem to have been at their most successful. Aside from being discovered in England, further remains have been in France and Spain with a particularly large number coming from Portugal. Study of late Jurassic ecosystems in North America has brought the strong suggestion that stegosaurs regularly came into conflict with theropod dinosaurs like Allosaurus. This predator/prey interaction may have also happened in late Jurassic Europe, although most of the large theropods such as Dubreuillosaurus and Poekilopleuron are so far only known from earlier in the Jurassic.

 Wuerhosaurus

Wuerhosaurus was fairly big for a stegosaur and is known from both China and Mongolia. Wuerhosaurus homheni was probably a broad-bodied animal. Gregory S. Paul in 2010 estimated the length at 7 metres (23 ft) and the weight at four tonnes. Only a few scattered bones have been found, making a full restoration difficult. Its dorsal plates were at first thought to have been much rounder or flatter than other stegosaurids but Maidment established this was an illusion caused by breakage: their actual form is unknown. W. homheni had a pelvis of which the front of the ilia strongly flared outwards indicating a very broad belly. The neural spines on the tail base were exceptionally tall. What is really significant about this Stegosaur is that it lived during the early Cretaceous. Most other stegosaur genera are known from the earlier in late Jurassic, which means that Wuerhosaurus is one of the latest surviving stegosaurs that we know about. The first reconstructions of Wuerhosaurus show the plates on the back to be quite low, but it has now been proven that this is a result of the plates being damaged, perhaps as a result of animal rolling over on its back after death. The true form of these plates is still not currently known.