Stegosaurus

Marsh's 1896 illustration of S. ungulatus. Note the single row of twelve dorsal plates and eight tail spikes

Stegosaurus (meaning "roof-lizard") was a type of plant-eating dinosaur which lived in what is now western North America.

Stegosaurus lived in the Late Jurassic period around 155 to 145 million years ago. It is among the most easily recognized dinosaurs, due to the distinctive double row of kite-shaped plates on its back and the long spikes on its tail. The armor was necessary as it lived with such meat-eating theropods such as Allosaurus and Ceratosaurus. The use of the plates are still in dispute today. Some say it`s for mating porposes, others to ward off predators.

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. Although large animals at up to 9 metres (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.

Stegosaurus.

The hind feet each had three short toes, while each forefoot had five toes; only the inner two toes had a blunt hoof. The phalangeal formula is 2-2-2-2-1, meaning that the innermost finger of the forelimb has two bones, the next has two, etc. All four limbs were supported by pads behind the toes. The forelimbs were much shorter than the stocky hindlimbs, 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 the pelvis than 1 meter (3.3 ft) above the ground.

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 replacement by a horny beak or rhamphotheca. Stegosaurian teeth were small, triangular and flat; wear facets show that they did grind their food. The inset placement in the jaws suggests that Stegosaurus had cheeks to keep food in their mouths while they chewed.

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 that the brain was indeed very small, maybe the smallest among the dinosaurs. The fact that an animal weighing over 4.5 metric tons (5 short tons) could have a brain of no more than 80 grams (2.8 oz) 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 however small even for a dinosaur, fitting well with a slow herbivorous lifestyle and limited behavioural complexity.

Most of the information known about Stegosaurus comes from the remains of mature animals; however more recently juvenile remains of Stegosaurus have been found. One sub-adult specimen, discovered in 1994 in Wyoming, is 4.6 meters (15 ft) long and 2 meters (7 ft) high, and is estimated to have weighed 2.3 metric tons (2.6 short tons) while alive. It is on display in the University of Wyoming Geological Museum. Even smaller skeletons, 210 centimeters (6.9 ft) long and 80 centimeters (2.6 ft) tall at the back, are on display at the Denver Museum of Nature & Science.

Classification

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 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:

Thyreophora

Lesothosaurus diagnosticus Laquintasaura venezuelae Scutellosaurus lawleri Emausaurus ernsti Scelidosaurus harrisonii Alcovasaurus longispinus Eurypoda Ankylosauria Sauropelta edwardsi Gastonia burgei Euoplocephalus tutus Stegosauria Huayangosauridae Huayangosaurus taibaii Chungkingosaurus jiangbeiensis Tuojiangosaurus multispinus Paranthodon africanus Stegosauridae Jiangjunosaurus junggarensis Gigantspinosaurus sichuanensis Kentrosaurus aethiopicus Dacentrurus armatus Loricatosaurus priscus Hesperosaurus mjosi Miragaia longicollum Stegosaurus stenops Wuerhosaurus homheni

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 Stegosaurusrecovered 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 7 m (23.0 ft), 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.

https://upload.wikimedia.org/wikipedia/commons/thumb/a/a8/Stegosaurus_National_Museum_of_Natural_History.jpg/238px-Stegosaurus_National_Museum_of_Natural_History.jpg

https://upload.wikimedia.org/wikipedia/commons/thumb/0/04/Stenops.jpg/238px-Stenops.jpg

Type specimen of S. stenops on display at the National Museum of Natural History.

• 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 6.5 m (21 ft). 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.

[[null|link=https://en.m.wikipedia.org/wiki/File:%22Stegosaurus%22 sulcatus.png|alt=|thumb|250x250px|S. sulcatus reconstructed as a member of the Dacentrurinae]] 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 dubiumand 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 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 duplex, meaning "two plexus roof lizard" (in allusion to the greatly enlarged neural canal of the sacrum which Marsh characterized as a "posterior brain case"), was named by Marsh in 1887 (including the holotype specimen). The disarticulated bones were actually collected in 1879 by Edward Ashley at Como Bluff. Marsh initially distinguished it from S. ungulatus based on the fact that each sacral (hip) vertebra bore its own rib, which he claimed was unlike the anatomy of S. ungulatus; however, the sacrum of S. ungulatus had not actually been discovered. Marsh also suggested that S. duplex may have lacked armor, since no plates or spikes were found with the specimen, though a single spike may actually have been present nearby, and re-examination of the site maps has shown that the entire specimen was found highly disarticulated and scattered.It is generally considered a synonym of S. ungulatus today, and parts of the specimen were actually incorporated into the Peabody Museum S. ungulatus skeletal mount in 1910.

Reassigned species

• Stegosaurus marshi, which was described by Lucas in 1901, was renamed Hoplitosaurus in 1902.
• Stegosaurus priscus, described by Nopcsa in 1911, was reassigned to Lexovisaurus,and is now the type species of Loricatosaurus.
• Stegosaurus longispinus was named by Charles W. Gilmore in 1914 based on a fragmentary postcranial skeleton that has largely been lost. It is now the type species of the genus Alcovasaurus, though it has been referred to Miragaia.
• Stegosaurus madagascariensis from Madagascar is known solely from teeth and was described by Piveteau in 1926. The teeth were variously attributed to a stegosaur, the theropod Majungasaurus,a hadrosaur or even a crocodylian, but is now considered a possible ankylosaur.
• Stegosaurus homheni is an alternative combination for the Chinese Cretaceous stegosaur Wuerhosaurus homheni, which was described based on a partial postcranial skeleton in 1973 by Dong Zhiming. It was referred to Stegosaurus in 2008 by Maidment et al, but some still consider the species to be in its own genus.
• 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,but Hesperosaurus has been the more popular combination since the discovery of more remains.

Discovery and species

Stegosaurus was originally named by Othniel Charles Marsh in 1877, from fossils found near Morrison, Color Hunt These first bones became the first species of Stegosaur named: Stegosaurus armatus.

Several different Stegosaurus species have been found.

• Stegosaurus armatus: This was the first type of Stegosaurus to be found. Over thirty different skeletons have been discovered by scientists. This type had four tail spikes and small plates. At 9 meters (30 ft), it was the longest species of Stegosaurus.
• Stegosaurus stenops: Named by Marsh in 1887, it was discovered near Cañon City, Color Hunt, in 1886. This is the best known species of Stegosaurus, mainly because its fossils make at least one complete skeleton. It had large, broad plates and four tail spikes. S. stenops is known from at least 50 partial skeletons of both adults and juveniles, one complete skull and four partial skulls. It was shorter than S. armatus, at 7 m (23 ft).
• S. ungulatus: Named by Marsh in 1879 from remains recovered at Como Bluff, Wyoming, it is only known from a few backbones and armor plates. It is probably the same as S. armatus.
• S. sulcatus: This is another partial skeleton. It is probably the same as S. armatus.
• S. duplexibleThis animal is probably the same as S. armatus. It was also named by Marsh in 1887,. Its fossils were found in 1879 by Edward Ashley at Como Bluff, Wyoming.
• ?S. see? BUTTOCKS: Probably the same as S. armatus.
• ?S. (Diracodon) biceps: Named by Marsh in 1881 from some jawbone fragments.

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.This has been proposed by Bakker and opposed by Carpenter. A study by Mallison (2010) found support for a rearing up posture in Kentrosaurus, though not for ability for the tail to act as a tripod.

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 15.3–17.9 km/h (9.5–11.1 mph).Tracks discovered by Matthew Mossbrucker (Morrison Natural History Museum, Colorado) suggest that Stegosaurus lived and traveled in multiple-age herds. One group of tracks is interpreted as showing four or five baby stegosaurs moving in the same direction, while another has a juvenile stegosaur track with an adult track overprinting it.

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 baculumin male dinosaurs.

Plate function

[[null|link=https://en.m.wikipedia.org/wiki/File:Allosaurus attacks Stegosaurus.jpg|alt=|thumb|Adult and juvenile S. stenops mounted as if under attack from an Allosaurus fragilis, 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, though Davitashvili (1961) disputed this, claiming that they were too fragile and ill-placed for defensive purposes, leaving the animal's sides unprotected. 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. Bakker stated that Stegosaurus could flip its osteoderms from one side to another to present a predator with an array of spikes and blades that would impede it from closing sufficiently to attack the Stegosaurus effectively. He contends that they had insufficient width for them to stand erect easily in such a manner as to be useful in display without continuous muscular effort.Mobility of the plates, however, has been disputed by other paleontologists.

Another possible function of the plates is they may have helped to control the body temperature of the animal, 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", which was seen as evidence that the plates "acted as thermoregulatory devices".Likewise, 2010 structural comparisons of Stegosaurus plates to Alligator osteoderms seem to support the conclusion that the potential for a thermoregulatory role in the plates of Stegosaurus definitely exists. link=https://en.m.wikipedia.org/wiki/File:Stegosaurus plates.jpg|alt=|left|thumb|Plates of Stegosaurus specimen "Sophie" 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. Histologicalsurveys of plate microstructure attributed the vascularization to the need to transport nutrients for rapid plate growth.

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. The plates' large size suggests that they may have served to increase the apparent height of the animal, either to intimidate enemies or to impress other members of the same species in some form of sexual display. A 2015 study of the shapes and sizes of Hesperosaurus plates suggested that they were sexually dimorphic, with wide plates belonging to males and taller plates belonging to females. 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. Some have suggested that plates in stegosaurs were used to allow individuals to identify members of their species. The use of exaggerated structures in dinosaurs as species identification has been questioned, as no such function exists in modern species.

Thagomizer (tail spikes)edit

Main article: Thagomizer link=https://en.m.wikipedia.org/wiki/File:Thagomizer_01.jpg|alt=|left|thumb|Thagomizer on mounted tail Debate has been going on about whether the tail spikes were used for display only, as posited by Gilmore in 1914 or used as a weapon. Robert Bakker noted the tail was likely to have been much more flexible than that of other dinosaurs, as it lacked ossified tendons, thus lending credence to the idea of the tail as a weapon. However, as Carpenter has noted, the plates overlap so many tail vertebrae, movement would be limited. Bakker also observed that Stegosaurus could have maneuvered its rear easily, by keeping its large hind limbs stationary and pushing off with its very powerfully muscled but short forelimbs, allowing it to swivel deftly to deal with attack. [[null|link=https://en.m.wikipedia.org/wiki/File:Thagomizer_(spike).jpg%7Calt=%7Cright%7Cthumb%7CAllosaurus tail vertebra punctured by a Stegosaurus thagomizer]] More recently, a study of the tail spikes by McWhinney et al., which showed a high incidence of trauma-related damage, lends more weight to the position that the spikes were indeed used in combat. This study showed that 9.8% of Stegosaurus specimens examined had injuries to their tail spikes.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 60–90 cm (2.0–3.0 ft) 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. Initially, Marsh described S. ungulatus as having eight spikes in its tail, unlike S. stenops. However, recent research re-examined this and concluded this species also had four.

Growth and metabolism

Juveniles of Stegosaurus have been preserved, probably showing the growth of the genus. The two juveniles are both relatively small, with the smaller individual being 1.5 m (4.9 ft) long, and the larger having a length of 2.6 m (8.5 ft). The specimens can be identified as not mature because they lack the fusion of the scapula and coracoid, and the lower hind limbs. Also, the pelvic region of the specimens are similar to Kentrosaurus juveniles. 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. [[null|link=https://en.m.wikipedia.org/wiki/File:Stegosaurus juvenile.jpg|alt=|right|thumb|Partial juvenile Stegosaurusskeleton on display with cast bones at Dinosaur National Monument 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 Stegosaurussuggests 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

link=https://en.m.wikipedia.org/wiki/File:Stegotooth.svg|alt=|left|thumb|Tooth crown illustration 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. Their teeth were "not tightly pressed together in a block for efficient grinding", 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".

The stegosaurians were widely distributed geographically in the late Jurassic.Palaeontologists believe it would have eaten plants such as mosses, ferns, horsetails, cycads, and conifers. 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 Stegosauruscould 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 6 m (20 ft) above the ground.

A detailed computer analysis of the biomechanics of Stegosaurus's feeding behavior was performed in 2010, using two different three-dimensional models of Stegosaurus teeth given realistic physics and properties. Bite force was also calculated using these models and the known skull proportions of the animal, as well as simulated tree branches of different size and hardness. The resultant bite forces calculated for Stegosauruswere 140.1 newtons (N), 183.7 N, and 275 N (for anterior, middle and posterior teeth, respectively), which means its bite force was less than half that of a Labrador retriever. Stegosaurus could have easily bitten through smaller green branches, but would have had difficulty with anything over 12 mm in diameter. Stegosaurus, therefore, probably browsed primarily among smaller twigs and foliage, and would have been unable to handle larger plant parts unless the animal was capable of biting much more efficiently than predicted in this study. However, a 2016 study indicates that Stegosaurus's 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"

Casts of the and brain cavity of S. ungulatus (left), and brain cavity marked with red (right).

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.

This space, however, is more likely to have served other purposes. The sacro-lumbar expansion is not unique to stegosaurs, nor even ornithischians. It is also present in birds. In their case, it contains what is called the glycogen body, a structure whose function is not definitely known, but which is postulated to facilitate the supply of glycogen to the animal's nervous system. It also may function as a balance organ, or reservoir of compounds to support the nervous system.

Gallery

External links

• [1]
• Stegosaurus "roofed butt", by T. Mike Keesey at The Dinosauricon.