Stegosaurus

''Stegosaurus'' (; ) is a genus of herbivorous, four-legged, armored dinosaur from the Late Jurassic, characterized by the distinctive kite-shaped upright plates along their backs and spikes on their tails. Fossils of the genus have been found in the western United States and in Portugal, where they are found in Kimmeridgian- to early Tithonian-aged strata, dating to between 155 and 145 million years ago. Of the species that have been classified in the upper Morrison Formation of the western US, only three are universally recognized: ''S. stenops'', ''S. ungulatus'' and ''S. sulcatus''. The remains of over 80 individual animals of this genus have been found. ''Stegosaurus'' would have lived alongside dinosaurs such as ''Apatosaurus'', ''Diplodocus'', ''Brachiosaurus'', ''Ceratosaurus'', and ''Allosaurus''; the latter two may have preyed on it.

They were large, heavily built, herbivorous quadrupeds with rounded backs, short fore limbs, long hind limbs, and tails held high in the air. Due to their distinctive combination of broad, upright plates and tail tipped with spikes, ''Stegosaurus'' is one of the most recognizable kinds of dinosaurs. The function of this array of plates and spikes has been the subject of much speculation among scientists. Today, it is generally agreed that their spiked tails were most likely used for defense against predators, while their plates may have been used primarily for display, and secondarily for thermoregulatory functions. ''Stegosaurus'' had a relatively low brain-to-body mass ratio. It had a short neck and a small head, meaning it most likely ate low-lying bushes and shrubs. One species, ''Stegosaurus ungulatus'', is one of the largest known of all the stegosaurians, reaching in length and in body mass, and some specimens indicate an even larger body size.

''Stegosaurus'' remains were first identified during the "Bone Wars" by Othniel Charles Marsh at Dinosaur Ridge National Landmark. The first known skeletons were fragmentary and the bones were scattered, and it would be many years before the true appearance of these animals, including their posture and plate arrangement, became well understood. Despite its popularity in books and film, mounted skeletons of ''Stegosaurus'' did not become a staple of major natural history museums until the mid-20th century, and many museums have had to assemble composite displays from several different specimens due to a lack of complete skeletons. ''Stegosaurus'' is one of the better-known dinosaurs, and has been featured in film, postal stamps, and many other types of media.

History and naming

Bone Wars and ''Stegosaurus armatus''

''Stegosaurus'', one of the many dinosaurs described in the Bone Wars, was first collected by Arthur Lakes and consisted of several caudal vertebrae, a dermal plate, and several additional postcranial elements that were collected north of Morrison, Colorado at Lakes’ YPM Quarry 5. These first, fragmented bones (YPM 1850) became the holotype of ''Stegosaurus armatus'' when Yale paleontologist Othniel Charles Marsh described them in 1877. Marsh initially believed the remains were from an aquatic turtle-like animal, and the basis for its scientific name, 'roof(ed) lizard' was due to his early belief that the plates lay flat over the animal's back, overlapping like the shingles (tiles) on a roof. Though several more complete specimens have been attributed to ''Stegosaurus armatus'', preparation of the bones and analysis has discovered that this type specimen is actually dubious, which is not an ideal situation for the type species of a well-known genus like ''Stegosaurus''. Because of this, the International Code of Zoological Nomenclature decided to replace the type species with the more well known species ''Stegosaurus stenops''. Marsh also incorrectly referred several fossils to ''S. armatus'', including the dentary and teeth of the sauropod ''Diplodocus'' and putting sauropod limb bones and an ''Allosaurus'' tibia under YPM 1850.

On the other side of the Bone Wars, Edward Drinker Cope named '' Hypsirhophus discurus'' as another stegosaurian based on fragmentary fossils from Cope's Quarry 3 near the "Cope's Nipple" site in Garden Park, Colorado in 1878. Many later researchers have considered ''Hypsirhophus'' to be a synonym of ''Stegosaurus'', though Peter Galton (2010) suggested that it is distinct based on differences in the vertebrae. F. F. Hubbell, a collector for Cope, also found a partial ''Stegosaurus'' skeleton while digging at Como Bluff in 1877 or ‘78 that are now part of the ''Stegosaurus'' mount (AMNH 5752) at the American Museum of Natural History.

Arthur Lakes made another discovery later in 1879 at Como Bluff in Albany County, Wyoming, the site also dating to the Upper Jurassic of the Morrison Formation, when he found several large ''Stegosaurus'' fossils in August of that year. The majority of the fossils came from Quarry 13, including the type specimen of ''Stegosaurus ungulatus (''YPM 1853), which was collected by Lakes and William Harlow Reed the same year and named by Marsh. The specimen was one of many found at the quarry, the specimen consisting of a partial skull, several vertebrae, an ischium, partial limbs, several plates, and four thagomizers, though eight thagomizers were referred based on a specimen preserved alongside the type. The type specimen also preserved the pes, which was the namesake of the species, meaning "hoofed roofed lizard". In 1881, he named a third species ''Stegosaurus'' "affinis", based only on a hip bone, though the fossil has since been lost and the species declared a nomen nudum. Later in 1887, Marsh described two more species of ''Stegosaurus'' from Como Bluff, ''Stegosaurus duplex,'' based on a partial vertebral column, partial pelvis, and partial left hindlimb (YPM 1858) from Reed's Quarry 11, though the species is now seen as synonymous with ''Stegosaurus ungulatus''. The other, ''Stegosaurus sulcatus'', was named based on a left forelimb, scapula, left femur, several vertebrae, and several plates and dermal armor elements (USNM V 4937) collected in 1883. ''Stegosaurus sulcatus'' most notably preserves a large spike that has been speculated to have been a shoulder spike that is used to diagnose the species.
left, Type specimen of ''S. stenops'' on display at the National Museum of Natural History.
The greatest ''Stegosaurus'' discovery came in 1885 with the discovery of a nearly complete, articulated skeleton of a subadult that included previously undiscovered elements like a complete skull, throat ossicles, and articulated plates. Marshall P. Felch collected the skeleton throughout 1885 and 1886 from Morrison Formation strata at his quarry in Garden Park, a town near Cañon City, Colorado. The skeleton was expertly unearthed by Felch, who first divided the skeleton into labeled blocks and prepared them separately. The skeleton was shipped to Marsh in 1887, who named it ''Stegosaurus stenops'' ( "narrow-faced roof lizard") that year. Though it had not yet been completely prepared, the nearly complete and articulated type specimen of ''Stegosaurus stenops'' allowed Marsh to complete the first attempt at a reconstructed ''Stegosaurus'' skeleton. This first reconstruction, of ''S. ungulatus'' with missing parts filled in from ''S. stenops'', was published by Marsh in 1891. (In 1893, Richard Lydekker mistakenly re-published Marsh's drawing under the label ''Hypsirhophus''.)

Early skeletal mounts and plate interpretation

The skeleton of ''S. stenops'' has since been deposited at the National Museum of Natural History in Washington D. C., where it has been on display since 1915. Another mount was made for the NMNH in the form of a mounted composite skeleton consisting of several specimens referred to ''S. stenops'' that were collected at Quarry 13 at Como Bluff in 1887, the most complete being USNM 6531. 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.Revan, A. (2011)
Reconstructing an Icon: Historical Significance of the Peabody’s Mounted Skeleton of ''Stegosaurus'' and the Changes Necessary to Make It Correct Anatomically.
Doctoral dissertation, faculty of the Department of Geology and Geophysics, Yale University. 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.

The next species of ''Stegosaurus'' to be named was ''S. marshi'' by Frederick Lucas in 1901. Lucas reclassified this species in the new genus ''Hoplitosaurus'' later that year. Lucas also re-examined the issue of the life appearance of ''Stegosaurus'', coming to the conclusion that the plates were arranged in pairs in two rows along the back, arranged above the bases of the ribs. Lucas commissioned Charles R. Knight to produce a life restoration of ''S. ungulatus'' based on his new interpretation. However, the following year, Lucas wrote that he now believed the plates were probably attached in staggered rows. In 1910, Richard Swann Lull wrote that the alternating pattern seen in ''S. stenops'' was probably due to shifting of the skeleton after death. He led the construction of the first ever ''Stegosaurus'' skeletal mount at the Peabody Museum of Natural History, which was depicted with paired plates. In 1914, Charles Gilmore argued against Lull's interpretation, noting that several specimens of ''S. stenops'', including the now-completely prepared holotype, preserved the plates in alternating rows near the peak of the back, and that there was no evidence of the plates having shifted relative to the body during fossilization. Gilmore and Lucas' interpretation became the generally accepted standard, and Lull's mount at the Peabody Museum was changed to reflect this in 1924.

Plate arrangement

One of the major subjects of books and articles about ''Stegosaurus'' is the plate arrangement. The argument has been a major one in the history of dinosaur reconstruction. Four possible plate arrangements have been proposed over the years:
* The plates lie flat along the back, as a shingle-like armor. This was Marsh's initial interpretation, which led to the name 'roof lizard'. As further and complete plates were found, their form showed they stood on edge, rather than lying flat.
* By 1891, Marsh published a more familiar view of ''Stegosaurus'', with a single row of plates. This was dropped fairly early on (apparently because it was poorly understood how the plates were embedded in the skin and they were thought to overlap too much in this arrangement). It was revived, in somewhat modified form, in the 1980s, by Stephen Czerkas, based on the arrangement of iguana dorsal spines.
* The plates were paired in a double row along the back, such as in Knight's 1901 reconstruction and the 1933 film ''King Kong''.
* Two rows of alternating plates. By the early 1960s, this had become (and remains) the prevalent idea, mainly because some ''S. stenops'' fossils in which the plates are still partially articulated show this arrangement. This arrangement is chiral and so demands that a specimen be distinguished from its distinct, hypothetical mirror-image form.

Second Dinosaur Rush

After the end of the Bone Wars, many major institutions in the eastern United States were inspired by the depictions and finds by Marsh and Cope to assemble their own dinosaur fossil collections. The competition was foremost started by the American Museum of Natural History, the Carnegie Museum of Natural History, and the Field Museum of Natural History which all sent expeditions to the west to make their own dinosaur collections and mount skeletons in their fossil halls.Brinkman, P. D. (2010). The second Jurassic dinosaur rush. University of Chicago Press. The American Museum of Natural History was the first to launch an expedition in 1897, finding several assorted, but incomplete, ''Stegosaurus'' specimens at Bone Cabin Quarry in Como Bluff. These remains haven't been described and were mounted in 1932, the mount being a composite primarily of specimens AMNH 650 & 470 from Bone Cabin Quarry. The AMNH mount is cast and on display at the Field Museum, which didn't collect any ''Stegosaurus'' skeletons during the Second Dinosaur Rush. The Carnegie Museum in Pittsburgh on the other hand collected many ''Stegosaurus'' specimens, first at Freezout Hills in Carbon County, Wyoming in 1902–03. The fossils included only a couple postcranial remains, though in the 1900s-1920s Carnegie crews at Dinosaur National Monument discovered dozens of ''Stegosaurus'' specimens in one of the greatest single sites for the taxon.McIntosh, J. S. (1981). Annotated catalogue of the dinosaurs (Reptilia, Archosauria) in the collections of Carnegie Museum of Natural History. CM 11341, the most complete skeleton found at the quarry, was used for the basis of a composite ''Stegosaurus'' mount in 1940 along with several other specimens to finish the mount. A cranium (CM 12000) was also found by Carnegie crews, one of the few known. Both the AMNH and CM material has been referred to ''Stegosaurus ungulatus''.

Resurgent Discoveries

As part of the Dinosaur Renaissance and the resurgent interest in dinosaurs by museums and the public, fossils of ''Stegosaurus'' were once again being collected, though few have been fully described. An important discovery came in 1937 again at Garden Park by a high school teacher named Frank Kessler in while leading a nature hike. Kessler contacted the Denver Museum of Nature and Science, who sent paleontologist Robert Landberg. Landberg excavated the skeleton with the DMNS crews, recovering a 70% complete ''Stegosaurus'' skeleton along with turtles, crocodiles, and isolated dinosaur fossils at the quarry that would be nicknamed "The Kessler Site". Phillip Reinheimer, a steel worker, mounted the ''Stegosaurus'' skeleton at the DMNS in 1938. The skeleton remained mounted until 1989 when the museum curator of the DMNS began a revision of the museum's fossil hall and dispatched an expedition to find additional ''Stegosaurus'' remains. The expedition was successful in finding a nearly complete ''Stegosaurus'' near the Kessler site by Bryan Small, whose name would become the namesake of the new site. The "Small Quarry" ''Stegosaurus articulation and completeness clarified the position of plates and spikes on the back of ''Stegosaurus'' and the position and size of the throat ossicles found earlier first by Felch with the ''Stegosaurus stenops'' holotype, though like the ''S. stenops'' type, the fossils were flattened in a "roadkill" condition. The ''Stegosaurus'' skeletons have been mounted alongside an ''Allosaurus'' skeleton collected in Moffat County, Colorado originally in 1979.

1987 saw the discovery of a 40% complete ''Stegosaurus'' skeleton in Rabbit Valley in Mesa County, Colorado by Harold Bollan near the Dinosaur Journey Museum. The skeleton was nicknamed the "Bollan ''Stegosaurus"'' and is in the collections of the Dinosaur Journey Museum. At Jensen-Jensen Quarry, an articulated torso including several dorsal plates from a small individual were collected and briefly described in 2014, though the specimen was collected years before and is still in preparation at Brigham Young University. 2007 saw the description of a ''Stegosaurus'' specimen from the Upper Jurassic Lourinha Formation of Portugal, the specimen was placed as cf. ''Stegosaurus ungulatus'' by the describers. The specimen is one of the few associated ''Stegosaurus'' skeletons known, though it only contains a tooth, 13 vertebrae, partial limbs, a cervical plate, and several assorted postcranial elements.

Sophie the ''Stegosaurus'' is the best preserved ''Stegosaurus'' specimen, being 85% intact and containing 360 bones. Sophie was first discovered by Bob Simon in 2003 at the Red Canyon Quarry near Shell, Wyoming and was excavated by crews from the Swiss Sauriermuseum in 2004.Siber, H. J., & Möckli, U. (2009)
The stegosaurs of the Sauriermuseum Aathal.
/ref> The skeleton was excavated on private land, so it was interned by US federal authorities who then gave Sophie to the Natural History Museum, London where it was put on display in December of 2014 and later described in 2015. It is a young adult of undetermined sex, 5.8 m (19 ft) long and 2.9 m (9.5 ft) tall. The Sauriermuseum found several partial Stegosaurid skeletons throughout their excavations at Howe Quarry, Wyoming in the 1990s, though only Sophie has been described in detail. One skeleton collected at the site known as "Victoria" is very well preserved including many of the vertebrae preserved in semi-articulation and next to an ''Allosaurus'' skeleton found nicknamed "Big Al II".

Description

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. ''S. stenops'' reached in length and in body mass, while ''S. ungulatus'' reached in length and in body mass. Some large individuals may have reached in length and in body mass.

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 long and high, and is estimated to have weighed 1.5-2.2 metric tons (1.6-2.4 short tons) while alive. It is on display in the University of Wyoming Geological Museum.

Skull

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 had flat downward and upward extensions that would have completely hidden the teeth when viewed from the side, and these probably supported a turtle-like 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.Barrett, P.M. (2001). Tooth wear and possible jaw action of ''Scelidosaurus harrisonii'' Owen and a review of feeding mechanisms in other thyreophoran dinosaurs. Pp. 25-52 in Carpenter, K. (ed.): ''The Armored Dinosaurs''. Bloomington: Indiana University Press. 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.

Despite the animal's overall size, the braincase of ''Stegosaurus'' was small, being no larger than that of a dog. A well-preserved ''Stegosaurus'' braincase allowed Othniel Charles Marsh to obtain, in the 1880s, a cast of the brain cavity or endocast of the animal, which gave an indication of the brain size. The endocast showed the brain was indeed very small, the smallest proportionally of all dinosaur endocasts then known. The fact that an animal weighing over 4.5  metric tons (5 short tons) could have a brain of no more than contributed to the popular old idea that all dinosaurs were unintelligent, an idea now largely rejected. Actual brain anatomy in ''Stegosaurus'' is poorly known, but the brain itself was small even for a dinosaur.

Skeleton

In ''Stegosaurus stenops'' there are 27 bones in the vertebral column anterior to the sacrum, a varying number of vertebrae in the sacrum, with four in most subadults, and around 46 caudal (tail) vertebrae. The presacrals are divided into cervical (neck) and dorsal (back) vertebrae, with around 10 cervicals and 17 dorsals, the total number being one greater than in ''Hesperosaurus'', two greater than ''Huayangosaurus'', although '' Miragaia'' preserves 17 cervicals and an unknown number of dorsals. The first cervical vertebra is the axis bone, which is connected and often fused to the atlas bone. Farther posteriorly, the proportionately larger the cervicals become, although they do not change greatly in anything other than size. Past the first few dorsals, the centrum of the bones become more elongate front-to-back, and the transverse processes become more elevated dorsal. The sacrum of ''S. stenops'' includes four sacral vertebrae, but one of the dorsals is also incorporated into the structure. In some specimens of ''S. stenops'', a caudal is also incorporated, as a caudosacral. In ''Hesperosaurus'' there are two dorsosacrals, and only four fused sacrals, but in ''Kentrosaurus'' there may be as many as seven vertebrae in the sacrum, with both dorsosacrals and caudosacrals. ''S. stenops'' preserves 46 caudal vertebrae, and up to 49, and along the series both the centrums and the neural spines become smaller, until the neural spines disappear at caudal 35. Around the middle of the tail, the neural spines become bifurcated, meaning they are divided near the top.

With multiple well-preserved skeletons, ''S. stenops'' preserves all regions of the body, including the limbs. The scapula (shoulder blade) is sub-rectangular, with a robust blade. Though it is not always perfectly preserved, the acromion ridge is slightly larger than in ''Kentrosaurus''. The blade is relatively straight, although it curves towards the back. There is a small bump on the back of the blade, that would have served as the base of the ''triceps'' muscle. Articulated with the scapula, the coracoid is sub-circular. The hind feet each had three short toes, while each fore foot had five toes; only the inner two toes had a blunt hoof. The phalangeal formula is 2-2-2-2-1, meaning the innermost finger of the fore limb has two bones, the next has two, etc. All four limbs were supported by pads behind the toes. The fore limbs were much shorter than the stocky hind limbs, which resulted in an unusual posture. The tail appears to have been held well clear of the ground, while the head of ''Stegosaurus'' was positioned relatively low down, probably no higher than above the ground.

Plates

The most recognizable features of ''Stegosaurus'' are its dermal plates, which consisted of between 17 and 22 separate plates and flat spines. These were highly modified osteoderms (bony-cored scales), similar to those seen in crocodiles and many lizards today. They were not directly attached to the animal's skeleton, instead arising from the skin. The largest plates were found over the hips and could measure over wide and tall.

In a 2010 review of ''Stegosaurus'' species, Peter Galton suggested that the arrangement of the plates on the back may have varied between species, and that the pattern of plates as viewed in profile may have been important for species recognition. Galton noted that the plates in ''S. stenops'' have been found articulated in two staggered rows, rather than paired. Fewer ''S. ungulatus'' plates have been found, and none articulated, making the arrangement in this species more difficult to determine. However, the type specimen of ''S. ungulatus'' preserves two flattened spine-like plates from the tail that are nearly identical in shape and size, but are mirror images of each other, suggesting that at least these were arranged in pairs. Many of the plates are manifestly chiral and no two plates of the same size and shape have been found for an individual; however plates have been correlated between individuals. Well preserved integumentary impressions of the plates of ''Hesperosaurus'' show a smooth surface with long and parallel, shallow grooves. This indicates that the plates were covered in keratinous sheaths.

Classification and species

Like the spikes and shields of ankylosaurs, the bony plates and spines of stegosaurians evolved from the low-keeled osteoderms characteristic of basal thyreophorans.Norman, David (2001). "''Scelidosaurus'', the earliest complete dinosaur" in ''The Armored Dinosaurs'', pp 3-24. Bloomington: Indiana University Press. . 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.

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''. This group is widespread, with members across the Northern Hemisphere, Africa and possibly South America.

''Stegosaurus'' frequently is discovered in its own clade in Stegosauridae called Stegosauridae, usually including the taxa ''Wuerhosaurus'' and ''Loricatosaurus'', though ''Hesperosaurus'' is sometimes found in the group. in 2017, Raven and Maidment published a new phylogenetic analysis, including almost every known stegosaurian genus:

Species

Many of the species initially described have since been considered to be invalid or synonymous with earlier named species, leaving two well-known and one poorly known species. Confirmed ''Stegosaurus'' remains have been found in the Morrison Formation's stratigraphic zones 2–6, with additional remains possibly referrable to ''Stegosaurus'' recovered from stratigraphic zone 1.
* ''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 , it was the longest species within the genus ''Stegosaurus''. 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 longer hind limbs, proportionately smaller, more pointed plates with wide bases and narrow tips, and by several small, flat, spine-like 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 Marshall Felch at Garden Park, north of Cañon 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 . Found in the Morrison Formation, Colorado, Wyoming, and Utah.
* ''Stegosaurus sulcatus'', meaning "furrowed roof lizard", was described by Marsh in 1887 based on a partial skeleton. It has traditionally been considered a synonym of ''S. armatus'', though more recent studies suggest it is not. ''S. sulcatus'' is distinguished mainly by its unusually large, furrowed spikes with very large bases. A spike associated with the type specimen, originally thought to be a tail spike, may in fact come from the shoulder or hip, since its base is much larger than the corresponding tail vertebrae. A review published by Maidment and colleagues in 2008 regarded it as an indeterminate species possibly not even belonging to ''Stegosaurus'' at all, but to a different genus. Peter Galton suggested it should be considered a valid species due to its unique spikes.

Susannah Maidment and colleagues in 2008 proposed extensive alterations to the taxonomy of ''Stegosaurus''. They advocated synonymizing ''S. stenops'' and ''S. ungulatus'' with ''S. armatus'', and sinking ''Hesperosaurus'' and ''Wuerhosaurus'' into ''Stegosaurus'', with their type species becoming ''Stegosaurus mjosi'' and ''Stegosaurus homheni'', respectively. They regarded ''S. longispinus'' as dubious. Thus, their conception of ''Stegosaurus'' would include three valid species (''S. armatus'', ''S. homheni'', and ''S. mjosi'') and would range from the Late Jurassic of North America and Europe to the Early Cretaceous of Asia. However, this classification scheme was not followed by other researchers, and a 2017 cladistic analysis co-authored by Maidment with Thomas Raven rejects the synonymy of ''Hesperosaurus'' with ''Stegosaurus''. In 2015, Maidment ''et al.'' revised their suggestion due to the recognition by Galton of ''S. armatus'' as a ''nomen dubium'' and its replacement by ''S. stenops'' as type species.

Doubtful species and junior synonyms

* ''Stegosaurus armatus'', meaning "armored roof lizard", was the first species to be found and the original type species named by O.C. Marsh in 1877. It is known from a partial skeleton, and more than 30 fragmentary specimens have been referred to it. However, the type specimen was very fragmentary, consisting only of a partial tail, hips, and leg, parts of some back vertebrae, and a single fragmentary plate (the presence of which was used to give the animal its name). No other plates or spikes were found, and the entire front half of the animal appears not to have been preserved. Because the type specimen is very fragmentary, it is extremely difficult to compare it with other species based on better specimens, and it is now generally considered to be a ''nomen dubium''. Because of this, it was replaced by ''S. stenops'' as the type species of ''Stegosaurus'' in a ruling of the ICZN in 2013.
* ''Stegosaurus'' "affinis", named by Marsh in 1881, is only known from a pubis which has since been lost. Because Marsh did not provide an adequate description of the bone with which to distinguish a new species, this name is considered a '' nomen nudum''.
* ''Diracodon laticeps'' was described by Marsh in 1881, from some jawbone fragments. Bakker