The stegosaurians, generally speaking, are medium to large herbivores with small heads (and necks longer than usually illustrated), arms a good deal shorter than the legs, a pair of spikes jutting either up (traditionally) or out (as suggested by new finds) from the end of the tail (known informally as the "thagomizer," from a Far Side joke), heavily-built shoulders often with a spike jutting out from them at an uncertain angle, and two rows of either spines or plates going from behind the head to the end of the tail. Most stegosaurians have spines from the mid-back to the tail, and small plates over the neck; Stegosaurus is unique in its large plates and not having either spines graded from plates or shoulder spikes. They appear to have retained five fingers per hand, but only three functional toes per foot. The limbs are unusually columnar for dinosaurs, and the arms are robust; the hindlimbs become longer relative to the forelimbs in derived stegosaurians. As mentioned on the previous page, the dorsals are unusually stretched vertically, particularly in the derived stegosaurians, as if they had been pulled up by the transverse processes, which jut up and away from the neural spine (an illustration is appropriate here, so I'll have to make one). Stegosaurids are unique among ornithischians for their lack of ossified tendons, a feature which could be related to the peculiar form of the dorsals (or maybe not, as this is not prominent in basal stegosaurians). Unlike most groups of dinosaurs, they dwindled and bowed out well before the final curtain on the Mesozoic, controversial and poorly-substantiated reports of LK material notwithstanding. As you might guess from the length of some of the entries, I'm rather fond of stegosaurians. 

      |    |--Chunkingosaurus
      |    `--Huayangosaurus
                          |     `--Miragaia
Stegosauria i.s.:

Taxon or Taxa: Time/Place: Comments:
"Changdusaurus laminaplacodus" (N.N.) Zhao, 1985 MJ of China Little is yet known about "Changdusaurus" (or "Changtusaurus") beyond its funky species name, and whatever material was assigned to it is now lost.
Chialingosaurus kuani (N.D.) Yang, 1959 Oxfordian (LJ) of China This is an obscure stegosaurid, but was China's first.  It is known from one partial subadult skeleton, including a skull described as high and narrow.
Craterosaurus pottonensis (?N.D.) Seeley, 1874 ?Aptian (EK) of England Craterosaurus is based on the partial neural arch of a dorsal vertebra, originally thought to be the partial braincase of a lizard.  It is suspected that this belongs to Regnosaurus, although this can't be shown right now with the material available. The age is uncertain, as the damaged fossil was probably reworked (eroded from one layer and redeposited, making it look younger than it appears) from another formation. 
Hypsirophus discursus (N.D.) Cope, 1878 (?Stegosaurus) ?early Tithonian (LJ) of Colorado Based on 2 dorsals and part of a caudal neural arch, this taxon is usually tossed off as a synonym of Stegosaurus, but may be valid.
Monkonosaurus lawulacus (N.D.) Zhao vide Dong, 1990 Kimmeridgian (LJ) of China Monkonosaurus is a stegosaurid based on a partial hip including sacrum, two vertebrae, and three plates, not all of which can be located. So far little has been written about it yet. It is the first dinosaur named from Tibet.
"Omosaurus" durobrivensis (N.D.)  Hulke, 1887 (Lexovisaurus Callovian (MJ) of England You might think Lexovisaurus had tenure, being named in 1957. Well, you'd be mistaken. It's a stegosaur, but not diagnostic.
?Regnosaurus northamptoni (?N.D.) Mantell, 1848 Barremian (EK) of England Based on a jaw, this animal from the early years of dinosaur paleontology has been at times referred to the ankylosaurians, the nodosaurids, and the sauropods, until currently coming to rest in Stegosauria. Regnosaurus and Huayangosaurus have similar lower jaws.
"Stegosaurus": "S. affinis" (N.N.) Marsh, 1881 Kimmeridgian (LJ) of Wyoming This is an inadequately-described and unfigured stegosaurid based on a pubis.
"S." longispinus (N.D.) Gilmore, 1914 Kimmeridgian (LJ) of Wyoming This animal, based on fragmentary postcranial remains including long tail spines, has usually been tossed off as a forgotten species of Stegosaurus, but occasionally gets some press as a possible Morrison species of Kentrosaurus (this seems to have been a late 1990s-early 2000s Internet thing).
"S." sulcatus (N.D.) Marsh, 1887 Kimmeridgian (LJ) of Wyoming Just another dubious Stegosaurus species; nothing to see here, folks.
"Wuerhosaurus" ordosensis (N.D.) Dong, 1993 ?Barremian (EK) of China "W." ordosensis is smaller than the type, with only 11 dorsal vertebrae (the number for the type unknown). Its ilium also seems to be short and deep fore and aft.
"Yingshanosaurus jichuanensis" (N.N.) Zhou S., 1984 LJ of China This stegosaurid is apparently closely related to Tuojiangosaurus. Unfortunately, its "type" material is missing.


Taxon or Taxa: Time/Place: Comments:
Chungkingosaurus jiangbeiensis Dong, Zhou, and Zhang, 1983 Oxfordian (MJ) of China Chungkingosaurus is one of the better known Chinese stegosaurians, its remains including four partial skeletons. It has thick plates among its armor, between plates and spines in form.  Interestingly, a tail with spikes preserved in position seems to show at least a total of five, suggesting more than the traditional two pairs at the tip.
It is also one of the smallest stegosaurians, with an overall length possible 3 to 4 meters.
Huayangosaurus taibaii Dong, Tang, and Zhou, 1982 Bathonian-Callovian (MJ) of China Huayangosaurus is one of the best-known stegosaurians, with remains belonging to over a half dozen individuals including a fine skull and fragmentary postcrania. The skull has little postorbital "horns," which may be a gender-related character.  
Compared to more derived stegosaurians, its arms are fairly long relative to its hindlimbs, its skull is large, and it has premaxillary teeth.  It is also unique for having armor scutes in a row down its sides, ossified tendons (only known stegosaurian to have them), and a small, knob-like bone at the tip of the spined tail. Armor, besides the scutes, appears to be a mix of spines and tall, narrow plates, with large shoulder spines.
Some of the postcranial remains referred to Huayangosaurus actually belong to an unnamed, more derived stegosaurid from the same time and place.

Stegosauridae:  There are several possible levels within Stegosauridae, but for the sake of clarity, and since there aren't particularly major differences, I won't subject you to Stegosaurinae and Stegosaurini.

Taxon or Taxa: Time/Place: Comments:
Gigantspinosaurus sichuanensis Ouyang, 1992 (also "Gigantospinosaurus") Oxfordian (MJ) of China This is a stegosaurid based on a mostly complete skeleton lacking the upper jaws, feet, and tail. Although often thought to be undescribed, publications starting in 2006 have been treating it as a valid stegosaurid, possibly a basal form, currently under study, with massive shoulder spines (twice the length as the scapula) that point backward. I'd thought that it might be the same as "Yingshanosaurus", with somewhat similar scapular spines, but Tracy Ford informed me that they are different.
Paranthodon africanus Nopcsa, 1929 (originally Palaeoscincus africanus Broom, 1912) mid Tithonian-early Valanginian (LJ-EK) of South Africa This animal spent a lot of time bouncing around classifications in its early taxonomic history, and in fact for a time was considered non-dinosaurian. It is based on a partial skull, including most of the front end and some teeth.
Tuojiangosaurus multispinus Dong, Li, Zhou, and Zhang, 1977 Oxfordian (LJ) of China Tuojiangosaurus, known from two partial skeletons including cranial material, is the largest known Jurassic Asian stegosaurid. It is more pointy, with many tall, narrow plates, than most other stegosaurids, and has shoulder spines. Interestingly, the nostrils seem to face dorsally, unlike the lateral nostrils of most other stegosaurids. Reports of shoulder spines seem to be based on "Yingshanosaurus" or Gigantspinosaurus.
Kentrosaurus aethiopicus Hennig, 1915 late Kimmeridgian-Tithonian (LJ) of Tanzania Kentrosaurus, a signature Tendaguru dinosaur, is an excellent example of a "spiky" stegosaurid. It has few plates, and these are clustered in the neck to the middle back. Beyond this are several pairs of spines. It has prominent ?shoulder spines as well (originally assigned as pelvic spines, and possibly they belong there). Although the partial remains of over thirty individuals, including two mounted skeletons (composite) and more than thirty femora, were known at one time, WWII bombing raids destroyed much material.
It was one of the smaller known stegosaurids, at a possible length of 5 meters or so.  It may have been sexually dimorphic.
At one time, Kentrosaurus was though to be a possible homonym of Centrosaurus, the centrosaurine ceratopsid, spawning the replacement names Kentrurosaurus Hennig, 1916 and Dorphyorosaurus Nopcsa, 1916. However, this was not needed, as the pronunciations are different.
Loricatosaurus priscus Maidment, Norman, Barrett, and Upchurch, 2008 (originally Stegosaurus priscus Nopcsa, 1911) Callovian (MJ) of England and France Loricatosaurus, while named in 2008, is just your old friend Lexovisaurus under a new name. There were three specimens of high regard under the old genus, and the least diagnostic got to be the holotype because it was attached to the oldest name. Now, that specimen gets Lexovisaurus all to its lonesome. Previously, the material had been assigned to Omosaurus (now Dacentrurus).  
It's been traditional to show Lexovi/Loricat as having large spikes on the hips (before the 1990s) or on the shoulders (more recently); however, the spike that this was based on is more likely a poorly preserved tail spike.
One story that has been circulating the Internet for years is that some "plates" assigned to Lexovisaurus turned out to be from an aquatic organism: gill rakers from the giant fish Leedsichthys. While this would be a fantastic coincidence with Dravidosaurus' plesiosaurian adventures, it also has the ring of a paleo urban legend. However, it does appear to be true.
Dacentrurus armatus Lucas, 1902 (originally Omosaurus armatus Owen, 1875) Kimmeridgian-Tithonian (LJ) of England, France, and Portugal (most of this is D. sp., not the type species per se) Dacentrurus, based on the first described stegosaurid (that original name, Omosaurus, was preoccupied), is one of the more basal stegosaurids. Some researchers suggest it deserves to be put in a separate family. It is also one of the largest, with some remains suggesting individuals of 10 meters in length, although many nontechnical works have described it as small. As with many stegosaurids, the armor includes paired plates in the neck region grading to spines. It has an unusually great temporal span, covering essentially the entire Late Jurassic, suggesting that there may be more than one species represented in the known remains. Much of the younger material is from Portugal, including five partial skeletons. Despite the large number of remains, it is an obscure animal. Like other basal stegosaurids, the forelimbs are still long relative to the hindlimb.
Miragaia longicollum Mateus, Maidment, and Christiansen, 2009 late Kimmeridgian-early Tithonian (LJ) of Portugal Known from much of the front end of a skeleton, including the first cranial material for a European stegosaurid, Miragaia is most notable for its long neck, made up of at least 17 verts; the extras were probably mostly donated from the back. This total is more than most sauropods, but since the verts aren't also similarly elongate, it's not quite the same thing. Also present were paired triangular plates and a typically stegosaurian skull. Miragaia and Dacentrurus were quite similar, befitting their similar locations and ages.
Hesperosaurus mjosi Carpenter, Miles, and Cloward, 2001 (?Stegosaurus) early Kimmeridgian (LJ) of Wyoming This is a stegosaurid from the early Morrison, based on a nearly complete skeleton, lacking the limbs. A partial skull is among the remains; it seems to be wider than that of Stegosaurus, and the back of the skull slopes down behind the eyes. Also unlike Stegosaurus, the cervical plates are longer than tall, and there is a high number of cervical vertebrae. It seems to be closest to Dacentrurus
Stegosaurus: Marsh, 1877 S. armatus Marsh, 1877 Kimmeridgian (LJ) of Utah, Colorado, and Wyoming Stegosaurus is one of the few dinosaurs the average person on the street could probably either name or recognize, having come a long way from Marsh's original conception of it as a sort of giant aquatic turtle.  Stegosaurus is also one of the most famous components of the Morrison dinosaur fauna. Remains from over 80 individuals have been assigned to this genus. 
It is famous for both its large plates and spikes, and for its mythical "second brain."  This myth developed because this animal seemed to be none too rich in "grey matter," and there was found to be an enlarged space for the spinal cord in the hips, which was called a second brain by some people. This space was filled not by a second brain, but was most probably used for nerves for the hindquarters and possibly some neuron upkeep (?glycogen body for making myelin sheaths). 
The spikes are usually shown as pointing up, but new material indicates that they pointed more laterally. Although the number of these spines has been debated, recent research has shown that, to paraphrase another discussion of offensive weaponry, four is the number of the spikes, and the number of the spikes is four; eight is right out. There are subtle differences between the anterior and posterior pair of spines; the spines of the anterior pair are larger and their bases are less angled than those of the posterior pair. There was likely a short sheath over them, adding perhaps a centimeter or less of length. The tail supporting them could do about 13 degrees total movement to either side (mostly at the tip; the tail plates helped to stiffen the tail in the absence of ossified tendons, keeping it mostly straight through its first two-thirds), and maybe reach a speed of 29 km/hour (18 mph) or so, fast enough to stick them in something with painful results. Given that most stegosaurians lacked large caudal plates and thus their stiffening action (spines instead), they would have had much more flexible tails than their illustrious namesake.  
The plates, 17 for S. stenops, were most likely arranged in an alternating pattern close to the midline of the beast, although paired and single-line rows have at times been fashionable (and make an excellent marker for guessing either the age of a restoration, or the artist or researcher responsible for making it or influencing it). The alternating pattern is based on articulated skeletons found with plates overlapping on their margins.  The plates were heavily crossed with blood vessel markings, which suggests the reasonable idea that one of the functions of the plates was as heat-exchange units, although recent research suggests that the markings are actually artifacts of growth, being more similar to antlers or a ceratopsian's frill. Given the large apparent blood supply, they may have also provided some coloring; imagine the plates flushed with blood (red-tinged armor has also been suggested for ankylosaurians as well).  Although some artists illustrate the plates with extensive horny sheaths, researchers have found no evidence of such coatings.  
The shapes are quite variable within a single individual, with the transitions possibly varying by species: the first five along the neck are small, but relatively tall fore and aft; the next few are more diamond-shaped as the plates come up the curve of the spine, with wide, asymmetrical bases allowing for neck motion; from the middle of the back to the base of the tail, the plates increase in size, with a shape like an elongated triangle stacked on a rectangle, and have a smaller, more symmetric base; the last few plates retain this shape as they decrease in size. Stegosaurus has been shown with a large gap between plates and spines, but this appears to have not been the case. Some specimens have lower plates than others; some have very elongated points on the plates. These differences could be a matter of individual or sexual variation, or could be traits of different species. (Oddly, I've also noticed that the size of the shoulder girdle also seems to vary, with some specimens having their scapulae much more expanded, with larger coracoids, than other specimens)  It was believed that juveniles lacked plates, but new remains show their presence.
Besides the plates and spikes, many small nodules have been found associated with Stegosaurus remains, often around the neck. It appears that the underside of the neck was covered in these bony nodules, and more of them were scattered over the body. Scalation appears to have been made up of six-sided nonoverlapping tubercles, a common dinosaurian fashion. 
    Stegosaurus, for all its fame, may have some taxonomic trouble on the horizon: its type specimen has never been fully prepared or described, so what precisely Stegosaurus constitutes has never been settled. It may be that some species, like S. stenops, belong in other genera, such as Diracodon Marsh, 1881. Most images of Stegosaurus are based on either S. stenops or S. ungulatus, which may be the same as the type species.  
It has been suggested, based on the build of the vertebral column, that Stegosaurus reared up on its hindlimbs to gather food, and this may well have been an important method of foraging. On the other hand, studies of the plates on the tail indicate very limited mobility there, which would have been a great hindrance to a tripodal posture. Occasionally it has been suggested that it also walked around or even ran bipedally, but that seems a bit much.
Often thought of as a huge dinosaur, Stegosaurus may have gotten up to 9 meters in length, but was usually closer to 6 or 7 meters. Meanwhile, Iguanodon bernissartensis, not usually called to mind by the layman when considering long or large dinosaurs, easily clears 9 meters on its way to the double-digits and is in about the same mass class. The point of this?  Mainly, that Stegosaurus wasn't a particularly gigantic dinosaur (and that I. bernissartensis is a lot bigger than you might think!).
Maidment et al. (2008), in their recent review, condensed Morrison stegosaurids to S. armatus and mjosi (being of course the type of Hesperosaurus). While I would not be surprised if Hesperosaurus mjosi turned out to be a species of Stegosaurus, or if the other species should be condensed into armatus, I think it's a bit premature given our lack of understanding of variation in Stegosaurus, and I'd like to see a dedicated report just on that topic before I go for it. That said, I don't agree with their inclusion of Wuerhosaurus in the genus; going beyond the issue of our lack of understanding of variation in Stegosaurus, Wuerhosaurus is based on limited material (not all of which can be found), from a distant continent, in rocks at least ten million years younger.
 S. ungulatus Marsh, 1879 (?S. armatus)
S. stenops Marsh, 1887
Wuerhosaurus homheni Dong, 1973 ?Valanginian-Albian (EK) of China One of the last stegosaurids, Wuerhosaurus seems to be like Stegosaurus. Like Stegosaurus, it has plates instead of spines along its back; Wuerhosaurus's plates are commonly described as long and low, unlike the tall plates of Stegosaurus, but they are broken, so the exact shape is unknown. Remains from a couple of individuals have been mentioned, but have not necessarily been seen recently.

Stegosauridae i.s.:

Taxon or Taxa: Time/Place: Comments:
Jiangjunosaurus junggarensis Jia C., Forster, Xu X., and Clark, 2007 Oxfordian (LJ) of China This stegosaur, the first from the Shishugou Formation (which, if recent SVPs have taught me anything, will become more important in the near future), is known from an articulated front end from a subadult, consisting of a lower jaw, some skull pieces, eleven cervicals, and a couple of in-situ neck plates. It has both derived and basal features.
"Omosaurus" leedsi (N.D.) Seeley vide Huene, 1901 Callovian (MJ) of England "Omosaurus" leedsi is a dubious species based on a stegosaurid plate.


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