Stegosaurus

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. 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. 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. 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. This historically significant specimen was re-mounted ahead of the opening of the new Peabody Museum building in 1925. A third mounted skeleton of Stegosaurus, referred to S. stenops, was put on display at the American Museum of Natural History in 1932. Mounted under the direction of Charles J. Long, the American Museum mount was a composite consisting of partial remains filled in with replicas based on other specimens. In his article about the new mount for the museum's journal, Barnum Brown described (and disputed) the popular misconception that the Stegosaurus had a "second brain" in its hips. Another composite mount, using specimens referred to S. ungulatus collected from Dinosaur National Monument between 1920 and 1922, was put on display at the Carnegie Museum of Natural History in 1940. Plate arrangement with paired dorsal plates and eight tail spikes 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, 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. 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. The AMNH mount is cast and on display at the Field Museum, which didn't collect any Stegosaurus skeletons during the Second Dinosaur Rush. 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. 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". In 1987, a 40% complete Stegosaurus skeleton was discovered 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. The skeleton had been excavated on private land and was available for purchase. The Natural History Museum, London worked with private donors, most notably Jeremy Herrmann, to find the funding and then arranged to purchase the specimen, which was given the new official museum collection specimen designation NHMUK PV R36730 and re-nicknamed Sophie after Jeremy Herrmann's daughter. The mounted skeleton went on display in December 2014 and was scientifically described in 2015. The specimen had been discovered in 2022 on private land in Colorado and so could be sold to a private owner. The current owner has made "Apex" available for scientific research, but private ownership of important fossil specimens is controversial, with many researchers insisting that fossils be permanently curated at a formal institution for universal scientific access. == 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. 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. 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 of Sophie, depicting the modern view of S. stenops The most recognizable features of Stegosaurus are its dermal plates, which consisted of between 17 and 22 separate plates and flat spines. 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==

, Bozeman, Montana Like the spikes and shields of ankylosaurs, the bony plates and spines of stegosaurians evolved from the low-keeled osteoderms characteristic of basal thyreophorans. 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 a clade within the Stegosauridae called Stegosaurinae, usually including the taxa Wuerhosaurus and Hesperosaurus. The cladogram below displays the results of the "preferred tree" phylogenetic analysis of Raven et al. (2023), showing the position of the Stegosaurinae within Stegosauria and Eurypoda. }}|label1=Eurypoda}} In 2017, Raven and Maidment published a phylogenetic analysis including almost every known stegosaurian genus. Their dataset was expanded upon in the following years with additional taxa. In their 2024 description of stegosaur fossil material from China's Hekou Group, Li et al. used a modified version of the dataset of Raven and Maidment to analyze the phylogenetic relations of the Stegosauria: |3= |3= }} }} }}|label1=Stegosauria}} Species Many of the species initially described have since been considered to be invalid or synonymous with earlier named species, • Stegosaurus stenops, meaning "narrow-faced roof lizard", was named by Marsh in 1887, 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. Peter Galton suggested it should be considered a valid species due to its unique spikes. 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. Bakker resurrected D. laticeps in 1986 as a senior synonym of S. stenops, although others note that the material is not diagnostic and is only referable to Stegosaurus sp., making it a nomen dubium. • Stegosaurus priscus, described by Nopcsa in 1911, was reassigned to Lexovisaurus, a hadrosaur or even a crocodylian, but is now considered a possible ankylosaur. It was referred to Stegosaurus in 2008 by Maidment et al., • Stegosaurus mjosi was described as Hesperosaurus mjosi by Carpenter et al. in 2001 based on a partial skull and incomplete postcranial skeleton from the Morrison Formation of Johnson County, Wyoming. The species was referred to Stegosaurus mostly by Maidment et al. starting in 2008, ==Paleobiology==

Posture and movement Soon after its discovery, Marsh considered Stegosaurus to have been bipedal, due to its short forelimbs. He had changed his mind, however, by 1891, after considering the heavy build of the animal. Although Stegosaurus is undoubtedly now considered to have been quadrupedal, some discussion has occurred over whether it could have reared up on its hind legs, using its tail to form a tripod with its hind limbs, to browse for higher foliage. and opposed by Carpenter. Stegosaurus had short fore limbs in relation to its hind limbs. Furthermore, within the hind limbs, the lower section (comprising the tibia and fibula) was short compared with the femur. This suggests it could not walk very fast, as the stride of the back legs at speed would have overtaken the front legs, giving a maximum speed of . As the plates would have been obstacles during copulation, it is possible the female stegosaur laid on her side as the male entered her from above and behind. Another suggestion is that the female would stand on all fours but squat down the fore limbs and raise the tail up and out of the male's way as he supports his fore limbs on her hips. However, their reproductive organs still could not touch as there is no evidence of muscle attachments for a mobile penis nor a baculum in male dinosaurs. Plate function '', Denver Museum of Nature and Science The function of Stegosaurus' plates has been much debated. Marsh suggested that they functioned as some form of armor, Nevertheless, others have continued to support a defensive function. Bakker suggested in 1986 that the plates were covered in horn comparing the surface of the fossilized plates to the bony cores of horns in other animals known or thought to bear horns. Christiansen and Tschopp (2010), having studied a well-preserved specimen of Hesperosaurus with skin impressions, concluded that the plates were covered in a keratin sheath which would have strengthened the plate as a whole and provided it with sharp cutting edges. in a similar way to the sails of the pelycosaurs Dimetrodon and Edaphosaurus (and modern elephant and rabbit ears). The plates had blood vessels running through grooves and air flowing around the plates would have cooled the blood. Buffrénil, et al. (1986) found "extreme vascularization of the outer layer of bone", The thermoregulation hypothesis has been seriously questioned, since other stegosaurs such as Kentrosaurus, had more low surface area spikes than plates, implying that cooling was not important enough to require specialized structural formations such as plates. However, it has also been suggested that the plates could have helped the animal increase heat absorption from the sun. Since a cooling trend occurred towards the end of the Jurassic, a large ectothermic reptile might have used the increased surface area afforded by the plates to absorb radiation from the sun. Christiansen and Tschopp (2010) state that the presence of a smooth, insulating keratin covering would have hampered thermoregulation, but such a function cannot be entirely ruled out as extant cattle and ducks use horns and beaks to dump excess heat despite the keratin covering. The vascular system of the plates have been theorized to have played a role in threat displaying as Stegosaurus could have pumped blood into them, causing them to "blush" and give a colorful, red warning. However, the stegosaur plates were covered in horn rather than skin. Christiansen and Tschopp (2010) proposed that the display function would have been reinforced by the horny sheath which would have increased the visible surface and such horn structures are often brightly colored. Thagomizer (tail spikes) There has been debate about whether the tail spikes were only used for display, as posited by Gilmore in 1914, Additional support for this idea was a punctured tail vertebra of an Allosaurus into which a tail spike fits perfectly. The damage shows that the spike entered at an angle from below and displaced a piece of the process upward, remodeled bone on the underside of the process shows that an infection developed. S. stenops had four dermal spikes, each about long. Discoveries of articulated stegosaur armor show, at least in some species, these spikes protruded horizontally from the tail, not vertically as is often depicted. One 2009 study of Stegosaurus specimens of various sizes found that the plates and spikes had slower histological growth than the skeleton at least until the dinosaur reached its mature size. in Utah A 2013 study concluded, based on the rapid deposition of highly vascularised fibrolamellar bone, that Kentrosaurus had a quicker growth rate than Stegosaurus, contradicting the general rule that larger dinosaurs grew faster than smaller ones. A 2022 study by Wiemann and colleagues of various dinosaur genera including Stegosaurus suggests that it had an ectothermic (cold blooded) or gigantothermic metabolism, on par with that of modern reptiles. This was uncovered using the spectroscopy of lipoxidation signals, which are byproducts of oxidative phosphorylation and correlate with metabolic rates. They suggested that such metabolisms may have been common for ornithischian dinosaurs in general, with the group evolving towards ectothermy from an ancestor with an endothermic (warm blooded) metabolism. Diet Stegosaurus and related genera were herbivores. However, their teeth and jaws are very different from those of other herbivorous ornithischian dinosaurs, suggesting a different feeding strategy that is not yet well understood. The other ornithischians possessed teeth capable of grinding plant material and a jaw structure capable of movements in planes other than simply orthal (i.e. not only the fused up-down motion to which stegosaur jaws were likely limited). Unlike the sturdy jaws and grinding teeth common to its fellow ornithischians, Stegosaurus (and all stegosaurians) had small, peg-shaped teeth that have been observed with horizontal wear facets associated with tooth-food contact and their unusual jaws were probably capable of only orthal (up-down) movements. and no evidence in the fossil record of stegosaurians indicates use of gastroliths—the stone(s) some dinosaurs (and some present-day bird species) ingested—to aid the grinding process, so how exactly Stegosaurus obtained and processed the amount of plant material required to sustain its size remains "poorly understood". One hypothesized feeding behavior strategy considers them to be low-level browsers, eating low-growing foliage of various nonflowering plants. This scenario has Stegosaurus foraging at most 1 m above the ground. Conversely, if Stegosaurus could have raised itself on two legs, as suggested by Bakker, then it could have browsed on vegetation quite high up, with adults being able to forage up to above the ground. However, a 2016 study indicates that Stegosaurus bite strength was stronger than previously believed. Comparisons were made between it (represented by a specimen known as "Sophie" from the United Kingdom's Natural History Museum) and two other herbivorous dinosaurs; Erlikosaurus and Plateosaurus to determine if all three had similar bite forces and similar niches. Based on the results of the study, it was revealed that the subadult Stegosaurus specimen had a bite similar in strength to that of modern herbivorous mammals, in particular, cattle and sheep. Based on this data, it is likely Stegosaurus also ate woodier, tougher plants such as cycads, perhaps even acting as a means of spreading cycad seeds. "Second brain" At one time, stegosaurs were described as having a "second brain" in their hips. Soon after describing Stegosaurus, Marsh noted a large canal in the hip region of the spinal cord, which could have accommodated a structure up to 20 times larger than the famously small brain. This has led to the influential idea that dinosaurs like Stegosaurus had a "second brain" in the tail, which may have been responsible for controlling reflexes in the rear portion of the body. This "brain" was proposed to have given a Stegosaurus a temporary boost when it was under threat from predators. It also may function as a balance organ, or reservoir of compounds to support the nervous system. ==Paleoecology==

The Morrison Formation is interpreted as a semiarid environment with distinct wet and dry seasons, and flat floodplains. Vegetation varied from river-lining forests of conifers, tree ferns, and ferns (gallery forests), to fern savannas with occasional trees such as the Araucaria-like conifer Brachyphyllum. The flora of the period has been revealed by fossils of green algae, fungi, mosses, horsetails, ferns, cycads, ginkoes, and several families of conifers. Animal fossils discovered include bivalves, snails, ray-finned fishes, frogs, salamanders, turtles like Glyptops, sphenodonts, lizards, terrestrial and aquatic crocodylomorphs like Hoplosuchus, several species of pterosaurs such as Harpactognathus and Mesadactylus, numerous dinosaur species, and early mammals such as docodonts (like Docodon), multituberculates, symmetrodonts, and triconodonts. Dinosaurs that lived alongside Stegosaurus included theropods Allosaurus, Saurophaganax, Torvosaurus, Ceratosaurus, Marshosaurus, Stokesosaurus, Ornitholestes, Coelurus and Tanycolagreus. Sauropods dominated the region, and included Brontosaurus, Brachiosaurus, Apatosaurus, Diplodocus, Camarasaurus, and Barosaurus. Other ornithischians included Camptosaurus, Gargoyleosaurus, Dryosaurus, and Nanosaurus. Stegosaurus is commonly found at the same sites as Allosaurus, Apatosaurus, Camarasaurus, and Diplodocus. Stegosaurus may have preferred drier settings than these other dinosaurs. ==Cultural significance==

One of the most recognizable of all dinosaurs, which appeared in the November 1884 issue of Scientific American and elsewhere, and which depicted the dinosaur amid a speculative Morrison age Jurassic landscape. Jobin restored the Stegosaurus as bipedal and long-necked, with the plates arranged along the tail and the back covered in spikes. This covering of spikes might have been based on a misinterpretation of the teeth, which Marsh had noted were oddly shaped, cylindrical, and found scattered, such that he thought they might turn out to be small dermal spines. Marsh published his more accurate skeletal reconstruction of Stegosaurus in 1891, and within a decade Stegosaurus had become among the most-illustrated types of dinosaur. Stegosaurus made its major public debut as a paper mache model commissioned by the U.S. National Museum of Natural History for the 1904 Louisiana Purchase Exposition. The model was based on Knight's latest miniature with the double row of staggered plates, File:Dinosaurs Sci Am 1884.png|Early restoration of Stegosaurus by A. Jobin, 1884 File:Stegosaurus stenops model Smithsonian.png|Life-sized restoration of Stegosaurus stenops in the U.S. National Museum, c. 1911 ==See also==