| Stegosaurs | |
|---|---|
 | |
| Six stegosaurs (top left to bottom right):Stegosaurus ungulatus,Kentrosaurus,Huayangosaurus,Gigantspinosaurus,Miragaia (or possiblyDacentrurus),Tuojiangosaurus | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Thyreophora |
| Clade: | †Eurypoda |
| Clade: | †Stegosauria Marsh, 1877 |
| Subgroups[4][5] | |
Stegosauria is a group ofherbivorousornithischiandinosaurs that lived during theJurassic and earlyCretaceousperiods. Stegosaurian fossils have been found mostly in theNorthern Hemisphere (North America,Europe andAsia),Africa andSouth America. Their geographical origins are unclear; the earliest unequivocal stegosaurian,Bashanosaurus primitivus, was found in theBathonianShaximiao Formation of China.
Stegosaurians were armored dinosaurs (thyreophorans). Originally, they did not differ much from more primitive members of that group, being small, low-slung, running animals protected by armoredscutes. An early evolutionary innovation was the development of spikes as defensive weapons. Later species, belonging to a subgroup called theStegosauridae, became larger, and developed long hindlimbs that no longer allowed them to run. This increased the importance of active defence by thethagomizer, which could ward off even large predators because the tail was in a higher position, pointing horizontally to the rear from the broad pelvis. Stegosaurids had complex arrays of spikes and plates running along their backs, hips and tails.
Stegosauria includes two families, the primitiveHuayangosauridae and the more derivedStegosauridae. The stegosaurids like all other stegosaurians were quadrupedal herbivores that exhibited the characteristic stegosaurian dorsal dermal plates. These large, thin, erect plates are thought to be aligned parasagittally from the neck to near the end of the tail. The end of the tail has pairs of spikes, sometimes referred to as athagomizer.[6][7] It may be that this is the only scientific term derived from a joke (in this case aThe Far Side comic). Although defense, thermo-regulation and display have been theorized to be the possible functions of these dorsal plates, a study of the ontogenetic histology of the plates and spikes suggests that the plates serve different functions at different stages of the stegosaurids' life histories. The terminal spikes in the tail are thought to have been used in old adults, at least, as a weapon for defence.[8] However, the function of stegosaurid plates and spikes, at different life stages, still remains a matter of great debate.
The first stegosaurian finds in the early 19th century were fragmentary. Better fossil material, of the genusDacentrurus, was discovered in 1874 in England. Soon after, in 1877, the first nearly-complete skeleton was discovered in the United States. ProfessorOthniel Charles Marsh that year classified such specimens in the new genusStegosaurus, from which the group acquired its name, and which is still by far the most famous stegosaurian. During the latter half of the twentieth century, many important Chinese finds were made, representing about half of the presently known diversity of stegosaurians.
The first known discovery of a possible stegosaurian was probably made in the early nineteenth century in England. It consisted of a lower jaw fragment and was in 1848 namedRegnosaurus. In 1845, in the area of the present state ofSouth Africa, remains were discovered that much later would be namedParanthodon. In 1874, other remains from England were namedCraterosaurus. All threetaxa were based on fragmentary material and were not recognised as possible stegosaurians until the twentieth century. They gave no reason to suspect the existence of a new distinctive group of dinosaurs.
In 1874, extensive remains of what was clearly a large herbivore equipped with spikes were uncovered in England; the first partial stegosaurian skeleton known.[9] They were namedOmosaurus byRichard Owen in 1875. Later, this name was shown to be preoccupied by thephytosaurOmosaurus and the stegosaurian was renamedDacentrurus. Other English nineteenth century and early twentieth century finds would be assigned toOmosaurus; later they would, together with French fossils, be partly renamedLexovisaurus andLoricatosaurus.
In 1877,Arthur Lakes, a fossil hunter working for ProfessorOthniel Charles Marsh, inWyoming excavated a fossil that Marsh the same year namedStegosaurus. At first, Marsh still entertained some incorrect notions about its morphology. He assumed that the plates formed a flat skin cover — hence the name, meaning "roof saurian" — and that the animal was bipedal with the spikes sticking out sideways from the rear of the skull. A succession of additional discoveries from theComo Bluff sites allowed a quick update of the presumed build. In 1882, Marsh was able to publish the first skeletal reconstruction of a stegosaur. Hereby, stegosaurians became much better known to the general public. The American finds at the time represented the bulk of known stegosaurian fossils, with about twenty skeletons collected.[9]
The next important discovery was made when a German expedition to theTendaguru, then part ofGerman East Africa, from 1909 to 1912 excavated over a thousand bones ofKentrosaurus. The finds increased the known variability of the group,Kentrosaurus being rather small and having long rows of spikes on the hip and tail.
From the 1950s onwards, thegeology of China was systematically surveyed in detail andinfrastructural works led to a vast increase of excavation in the PRC. This resulted in a new wave of Chinese stegosaurian discoveries, starting withChialingosaurus in 1957. Chinese finds of the 1970s and 1980s includedWuerhosaurus,Tuojiangosaurus,Chungkingosaurus,Huayangosaurus,Yingshanosaurus andGigantspinosaurus. This increased the age range of stegosaurian materials of goodfossil preservation, as they represented the first relatively complete skeletons from theMiddle Jurassic and theEarly Cretaceous. Especially important wasHuayangosaurus, which provided unique information about the early evolution of the group. In 2007,Jiangjunosaurus was reported, the first Chinese dinosaur named since 1994.
Towards the end of the twentieth century, the so-calledDinosaur Renaissance took place in which a vast increase in scientific attention was given to the Dinosauria. During the 1990s, European and North-American sites became productive again, with fossils such asMiragaia having been found in theLourinhã Formation inPortugal, and a number of relatively completeHesperosaurus skeletons having been excavated in Wyoming. Apart from the fossils per se, important new insights have been gained by applying the analytic method ofcladistics, which allowed for precise calculations of interrelationships and the construction of stegosaurianphylogenetic trees.
Stegosaurids are distinguished from other stegosaurians in that the former have lost the plesiomorphic pre-maxillary teeth and lateral scute rows along the trunk.[10] Furthermore, stegosaurids have long narrow skulls and longer hindlimbs compared to their forelimbs.[7] However, these two features are not diagnostic of Stegosauridae because they may also be present in non-stegosaurid stegosaurians.[1]
Stegosaurians had characteristic small, long, flat, narrow heads and a horn-covered beak orrhamphotheca,[1] which covered the front of the snout (twopremaxillaries) and lower jaw (a singlepredentary) bones. Similar structures are seen inturtles andbirds. Apart fromHuayangosaurus, stegosaurians subsequently lost all premaxillary teeth within the upper beak.Huayangosaurus still had seven per side. The upper and lower jaws are equipped with rows of small teeth. Later species have a vertical bone plate covering the outer side of the lower jaw teeth. The structure of the upper jaw, with a low ridge above, and running parallel to, the tooth row, indicates the presence of a fleshy cheek. In stegosaurians, the typicalarchosaurian skull opening, theantorbital fenestra in front of the eye socket, is small, sometimes reduced to a narrow horizontal slit. In general, stegosaurids have proportionally long, low and narrow snouts with a deep mandible, compared to that ofHuayangosaurus. Stegosaurids also lack premaxillary teeth.[10]
All stegosaurians arequadrupedal, with hoof-like toes on all four limbs. All stegosaurians afterHuayangosaurus have forelimbs much shorter than their hindlimbs. Their hindlimbs are long and straight, designed to carry the weight of the animal while stepping. Thecondyles of the lower thighbone are short from the front to the rear. This would have limited the supported rotation of the knee joint, making running impossible.Huayangosaurus had a thighbone like a running animal. The upper leg was always longer than the lower leg.
Huayangosaurus had relatively long and slender arms. The forelimbs of later forms are very robust, with a massivehumerus andulna. Thewrist bones were reinforced by a fusion into two blocks, an ulnar and a radial. The front feet of stegosaurians are commonly depicted in art and in museum displays with fingers splayed out and slanted downward. However, in this position, most bones in the hand would be disarticulated. In reality, the hand bones of stegosaurians were arranged into vertical columns, with the main fingers, orientated outwards, forming a tube-like structure. This is similar to the hands ofsauropod dinosaurs, and is also supported by evidence from stegosaurian footprints and fossils found in a lifelike pose.[11]
The long hindlimbs elevated the tail base, such that the tail pointed out behind the animal almost horizontally from that high position. While walking, the tail would not have sloped downwards as this would have impeded the function of the tail base retractor muscles, to pull the thighbones backwards. However, it has been suggested byRobert Thomas Bakker that stegosaurians could rear on their hind legs to reach higher layers of plants, the tail then being used as a "third leg". The mobility of the tail was increased by a reduction or absence of ossified tendons, that with many Ornithischia stiffen the hip region.Huayangosaurus still possessed them. In species that had short forelimbs, the relatively short torso towards the front curved strongly downwards. The dorsal vertebrae typically were very high, with very tallneural arches andtransverse processes pointing obliquely upwards to almost the level of the neural spine top. Stegosaurian back vertebrae can easily be identified by this unique configuration. The tall neural arches often house deepneural canals; enlarged canals in thesacral vertebrae have given rise to the incorrect notion of a "second brain". Despite the downwards curvature of the rump, the neck base was not very low and the head was held a considerable distance off the ground. The neck was flexible and moderately long.Huayangosaurus still had the probably original number of nine cervical vertebrae;Miragaia has an elongated neck with seventeen.[12]
The stegosaurianshoulder girdle was very robust. InHuayangosaurus, theacromion, a process on the lower front edge of theshoulderblade, was moderately developed; thecoracoid was about as wide as the lower end of thescapula, with which it formed theshoulder joint. Later forms tend to have a strongly expanded acromion, while the coracoid, largely attached to the acromion, no longer extends to the rear lower corner of the scapula.
The stegosaurian pelvis was originally moderately large, as shown byHuayangosaurus. Later species, however, convergent to the Ankylosauria developed very broadpelves, in which theiliac bones formed wide horizontal plates with flaring front blades to allow for an enormous belly-gut. The ilia were attached to the sacral vertebrae via a sacral yoke formed by fused sacral ribs.Huayangosaurus still had rather long and obliquely orientedischia andpubic bones. In more derived species, these became more horizontal and shorter to the rear, while the front prepubic process lengthened.
Like allThyreophora, stegosaurians were protected by bonyscutes that were not part of the skeleton proper but skin ossifications instead: the so-calledosteoderms.Huayangosaurus had several types. On its neck, back, and tail were two rows of paired small vertical plates and spikes. The very tail end bore a small club. Each flank had a row of smaller osteoderms, culminating in a long shoulder spine in front, curving to the rear.[citation needed] Later forms show very variable configurations, combining plates of various shape and size on the neck and front torso with spikes more to the rear of the animal. They seem to have lost the tail club and the flank rows are apparently absent also, with the exception of the shoulder spine, still shown byKentrosaurus and extremely developed, as its name indicates, inGigantspinosaurus. As far as is known, all forms possessed some sort of thagomizer, though these are rarely preserved articulated allowing to establish the exact arrangement. A fossil ofChungkingosaurus sp. has been reported with three pairs of spikes pointing outwards and a fourth pair pointing to the rear.[13] The most derived species, likeStegosaurus,Hesperosaurus andWuerhosaurus, have very large and flat back plates. Stegosaurid plates have a thick base and central portion, but are transversely thin elsewhere. The plates become remarkably large and thin inStegosaurus. They are found in varying sizes along the dorsum, with the central region of the back usually having the largest and tallest plates. The arrangement of these parasagittal dorsal plates has been intensely debated in the past. DiscovererOthniel Charles Marsh suggested a single median row of plates running post-cranially along the longitudinal axis[14] andLull argued in favour of bilaterally paired arrangement throughout the series.[15] Current scientific consensus lies in the arrangement proposed byGilmore - two parasagittal rows of staggered alternates, after the discovery of an almost complete skeleton preserved in this manner in rock.[16] Furthermore, no two plates share the same size and shape, making the possibility of bilaterally paired rows even less likely. Plates are usually found with distinct vascular grooves on their lateral surfaces, suggesting the presence of a circulatory network. Stegosaurids also have osteoderms on the throat in the form of small depressed ossicles and two pairs of elongated spike-like tail-spines.[1] WithStegosaurus fossils also ossicles have been found in the throat region, bony skin discs that protected the lower neck.[17]
Many basal stegosaurs likeGigantspinosaurus andHuayangosaurus have been discovered with parascapular spines, or spines emerging from the shoulder region. Among stegosaurids, onlyKentrosaurus has been found with parascapular spines, which project posteriorly out of the lower part of the shoulder plates. These spines are long, rounded and comma-shaped in lateral view and have an enlarged base.Loricatosaurus was also believed to have a parascapular spine, but Maidmentet al. (2008) observed that the discovered specimen, from which the spine is described, has a completely different morphology than the parascapular spine specimens of other stegosaurs. They suggest it may be a fragmentary tail spine instead.[18] Stegosaurids also lack lateral scute rows that run longitudinally on either side of the trunk inHuayangosaurus andankylosaurs, indicating yet another secondary loss of a plesiomorphic characters.[10] However, the absence of lateral scutes as well as pre-maxillary teeth mentioned above are not specifically diagnostic of stegosaurids, since these features are also present in some other stegosaurians, whose phylogenetic relationships are unclear.[10][18]
The discovery of an impression of the skin covering the dorsal plates has implications for all possible functions of stegosaurian plates. Christiansen and Tschopp (2010)[19] found that the skin was smooth with long, parallel, shallow grooves indicating a keratinous structure covering the plates. The addition ofbeta-keratin, a strong protein, would indeed allow the plates to bear more weight, suggesting they may have been used for active defense. A keratinous covering would also allow greater surface area for the plates to be uses as a mating display structures, which could be potentially coloured like the beaks of modern birds. At the same time this finding implies that the use of plates for thermo-regulation may be less likely because the keratinous covering would make heat transfer from the bone highly ineffective.[19]
In 1877, Othniel Marsh discovered and namedStegosaurus armatus, from which the name of the family 'Stegosauridae' was erected in 1880.[9] In comparison to basal stegosaurians, notable synapomorphies of Stegosauridae include a large antitrochanter (supracetabular process) in theilium, a long prepubic process and longfemur relative to the length of thehumerus.[20] Furthermore, stegosaurid sacral ribs are T-shaped in parasagittal cross-section[1] and the dorsalvertebrae have an elongated neural arch.[9] The first exact clade definition of Stegosauria was given byPeter Malcolm Galton in 1997: allthyreophoranOrnithischia more closely related toStegosaurus than toAnkylosaurus.[21] This definition was formalized in thePhyloCode by Daniel Madzia and colleagues in 2021 as "the largest clade containingStegosaurus stenops, but notAnkylosaurus magniventris".[4] Thus defined, the Stegosauria are by definition thesister group of theAnkylosauria within theEurypoda. 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 byPaul Sereno as all stegosaurians more closely related toStegosaurus than toHuayangosaurus.[22] This definition was also formalized in thePhyloCode by Daniel Madzia and colleagues in 2021 as "the largest clade containingStegosaurus stenops, but notHuayangosaurus taibaii".[4] They include per definition the well-knownStegosaurus. This group is widespread, with members across the Northern Hemisphere,Africa andSouth America.[23][24]
Huayangosauridae (derived fromHuayangosaurus, "Huayang reptile") is a family of stegosauriandinosaurs from theJurassic ofChina.[25] The group is defined as all taxa closer to the namesake genusHuayangosaurus thanStegosaurus, and was originally named as the family Huayangosaurinae byDong Zhiming and colleagues in the description ofHuayangosaurus.[25][4] Huayangosaurinae was originally differentiated by the remaining taxa withinStegosauridae by the presence ofteeth in thepremaxilla, anantorbital fenestra, and amandibular fenestra. Huayangosaurinae, known from theMiddle Jurassic of theShaximiao Formation, was proposed to be intermediate betweenScelidosaurinae andStegosaurinae, suggesting that the origins of stegosaurs lay in Asia.[25] Following phylogenetic analyses, Huayangosauridae was expanded to also include the taxonChungkingosaurus, known from specimens from younger Late Jurassic deposits of the Shaximiao Formation.[18] Huayangosauridae is either the sister taxon to all other stegosaurs,[18][9] or close to the origin of the clade, with taxa likeGigantspinosaurus orIsaberrysaura outside the Stegosauridae-Huayangosauridae split.[4][9] Huayangosauridae was formally defined in2021 by Daniel Madzia and colleagues, who used the previous definitions of all taxa closer toHuayangosaurus taibaii thanStegosaurus stenops.[4]
In 2017, Raven and Maidment published a comprehensive phylogenetic framework including most valid stegosaurian genera. Several subsequent publications expanded and corrected this matrix based on novel taxa and revised anatomical interpretations. In 2025, Sánchez-Fenollosa & Cobos compiled these variations and other observations into an updated and expanded dataset. The authors further coined the nameNeostegosauria for the clade comprising the Dacentrurinae (includingKentrosaurus) and the Stegosaurinae. These results are displayed in thecladogram below:[5]
| Eurypoda |
| ||||||||||||||||||||||||
To date, several genera from China bearing names have been proposed but not formally described, including "Changdusaurus".[26] Until formal descriptions are published, these genera are regarded asnomina nuda.
Like the spikes and shields ofankylosaurs, thebony plates and spines of stegosaurians evolved from the low-keeled osteoderms characteristic ofbasal thyreophorans.[27] One such described genus,Scelidosaurus, is proposed to be morphologically close to thelast common ancestor of the clade uniting stegosaurians and ankylosaurians, theEurypoda.[28]Galton (2019) interpreted plates of an armored dinosaur from theLower Jurassic (Sinemurian-Pliensbachian) LowerKota Formation ofIndia as fossils of a member ofAnkylosauria; the author argued that this finding indicates a probable earlyEarly Jurassic origin for both Ankylosauria and itssister group Stegosauria.[29] Footprints attributed to theichnotaxonDeltapodus brodricki from the Middle Jurassic (Aalenian) ofEngland represent the oldest probable record of stegosaurians reported so far.[2] Outside that, there are assigned fossils to stegosauria from theToarcian: the specimen "IVPP V.219", a chimaera with bones of the sauropodSanpasaurus is known from the Maanshan Member of theZiliujing Formation.[30] The earliest possible trackways of stegosaurians are discovered from theHettangian-aged deposits ofFrance, indicating a possibly earlier origin.[1] The perhaps most basal known stegosaurian, the four-metre-longHuayangosaurus, is still close toScelidosaurus in build, with a higher and shorter skull, a short neck, a low torso, long slender forelimbs, short hindlimbs, large condyles on the thighbone, a narrow pelvis, long ischial and pubic shafts, and a relatively long tail. Its small tail club might be aeurypodansynapomorphy.Huayangosaurus lived during theBathonian stage of theMiddle Jurassic, about 166 million years ago.
A few million years later, during theCallovian-Oxfordian, from China much larger species are known, with long, "graviportal" (adapted for moving only in a slow manner on land due to a high body weight) hindlimbs:Chungkingosaurus,Chialingosaurus,Tuojiangosaurus andGigantspinosaurus. Most of these are considered members of the derivedStegosauridae.Lexovisaurus andLoricatosaurus, stegosaurid finds from England and France of approximately equivalent age to the Chinese specimens, are likely the sametaxon. During theLate Jurassic, stegosaurids seem to have experienced their greatest radiation. In Europe,Dacentrurus and the closely relatedMiragaia were present. While older finds had been limited to the northern continents, in this phaseGondwana was colonised also as shown byKentrosaurus living inAfrica. No unequivocal stegosaurian fossils have been reported fromSouth-America, India,Madagascar, Australia, orAntarctica, though. A Late Jurassic Chinese stegosaurian isJiangjunosaurus. The most derived Jurassic stegosaurians are known fromNorth-America:Stegosaurus (perhaps several species thereof) and the somewhat olderHesperosaurus.Stegosaurus was quite large (some specimens indicate a length of at least seven metres), had high plates, no shoulder spine, and a short, deep rump.
From theEarly Cretaceous, far fewer finds are known and it seems that the group had declined in diversity. Some fragmentary fossils have been described, such asCraterosaurus from England andParanthodon fromSouth Africa. Up until recently, the only substantial discoveries were those ofWuerhosaurus from Northern China, the exact age of which is highly uncertain[31] More recent discoveries from Asia however would later begin to fill out the Early Cretaceous diversity of the group. Indeterminate stegosaurs are known from the Early Cretaceous ofSiberia, including theIlek Formation[32] andBatylykh Formation.[33] The youngest known definitive remains of stegosaurs are those ofMongolostegus from Mongolia, a stegosaurine from theHekou Group of China, andYanbeilong of theZuoyun Formation of China, all of which date to theAptian-Albian.[34][35][36]
It has often been suggested that the decline in stegosaur diversity was part of a Jurassic-Cretaceous transition, whereangiosperms become the dominant plants, causing afaunal turnover where new groups of herbivores evolved.[37] Although in general the case for such a causal relation is poorly supported by the data, stegosaurians are an exception in that their decline coincides with that of theCycadophyta.[38]
Though Late Cretaceous stegosaurian fossils have been reported, these have mostly turned out to be misidentified. A well-known example isDravidosaurus, known fromConiacian fossils found in India. Though originally thought to be stegosaurian, in 1991 these badly-eroded fossils were suggested to instead have been based onplesiosaurian pelvis and hindlimb material,[39] and none of the fossils are demonstrably stegosaurian.[40] The reinterpretation ofDravidosaurus as a plesiosaur wasn't accepted byGalton and Upchurch (2004), who stated that the skull and plates ofDravidosaurus are certainly not plesiosaurian, and noted the need to redescribe the fossil material ofDravidosaurus.[1] A purported stegosaurian dermal plate was reported from the latest Cretaceous (Maastrichtian)Kallamedu Formation (southern India); however, Galton & Ayyasami (2017) interpreted the specimen as a bone of a sauropod dinosaur. Nevertheless, the authors considered the survival of stegosaurians into the Maastrichtian to be possible, noting the presence of the stegosaurian ichnotaxonDeltapodus in the MaastrichtianLameta Formation (western India).[3]
In anontogenetic histological analysis ofStegosaurus plates and spikes, Hayashiet al. (2012)[8] examined their structure and function through juveniles to old adults. They found that throughout the ontogeny, the dorsal osteoderms are composed of dense ossified collagen fibres in both thecortical andcancellous sections of the bone, suggesting that plates and spikes are formed from the directmineralization of already existing fibrous networks in the skin. However, the many structural features, seen in the spikes and plates of old adults specimens, are acquired at different stages of development. Extensive vascular networks form in the plates during the change from juveniles to young adults and persist in old adults but spikes acquire a thick cortex with a large axial vascular channel only in old adults. Hayashiet al. argue that the formation of nourishing vascular networks in young adults supported the growth of large plates. This would have enhanced the size of the animal, which may have helped attract mates and deter rivals.[8] Furthermore, the presence of the vascular networks in the plates of the young adult indicate a secondary use of the plates as a thermoregulatory device for heat loss much like theelephant ear,toucan bill oralligator osteoderms. The thickening of the cortical section of the bone and the compaction of bone in the terminal tail-spikes in old adults suggest that they were used as defence weapons, but not until an ontogenetically late stage. The development of the large axial channel in old adults from small canals in young adults, facilitated the further enlargement of the spikes by increasing the amount of nourishment supplied. On the other hand, plates do not show a similar degree of bone compaction or cortical thickening indicating they would not be capable of taking much weight from above. This suggests they were not as important as spikes in active defense.[8]
The protective nature of dorsal plates has also been questioned in the past - Davitashvili (1961) noted that narrow dorsal location of the plates still left the sides vulnerable. Since the pattern of plates and spines vary between species, he suggested it could be important for intraspecific recognition and as a display for sexual selection.[1] This is corroborated by Spassov's (1982) observations that the plates are arranged for maximum visible effect when viewed laterally during non-aggressive agonistic behaviour, as opposed to from a head-on aggressive stance.[41]
Stegosaurian tracks were first recognized in 1996 from a hindprint-onlytrackway discovered at theCleveland-Lloyd quarry, which is located near Price, Utah.[42] Two years later, a newichnogenus calledStegopodus was erected for another set of stegosaurian tracks which were found nearArches National Park, also in Utah.[42] Unlike the first, this trackway preserved traces of the forefeet. Fossil remains indicate that stegosaurians have five digits on the forefeet and three weight-bearing digits on the hind feet.[42] From this, scientists were able to predict the appearance of stegosaurian tracks in 1990, six years in advance of the first actual discovery ofMorrison stegosaurian tracks.[42] More trackways have been found since the erection ofStegopodus. None, however, have preserved traces of the front feet and stegosaurian traces remain rare.[42]
Deltapodus is an ichnogenus attributed as stegosaurian prints, and are known across Europe,[43] North Africa,[44] and China.[45] OneDeltapodus footprint measures less than 6 cm in length and represents the smallest known stegosaurian track.[45][46] Some tracks preserve exquisite scaly skin pattern.[43]
Australia's 'Dinosaur Coast' inBroome, Western Australia includes tracks of several different thyreophoran track-makers. Of these, the ichnogenusGarbina (a Nyulnyulan word for 'shield') andLuluichnus (honours the late Paddy Roe, OAM who went by the name 'Lulu') have been considered registered by stegosaurs.[47]Garbina includes the largest stegosaur tracks measuring 80 cm in length. Trackway data showGarbina track-makers were capable of bipedal and quadrupedal progression, suggesting an adaptation to facultative bipedalism amongst some stegosaurs.
While has no body fossil evidence currently known for stegosaurs, handprints from underground coal mines nearOakey, Queensland, resemblingGarbina tracks suggests their occurrence in this country from at least the Middle to Upper Jurassic (Callovian–Tithonian).[48] A single plaster cast of one of these handprints is in the collections of theQueensland Museum.
There has been debate about whether the spikes were used simply for display, as posited by Gilmore in 1914,[16] or used as a weapon.Robert Bakker noted that it is likely that the stegosaur tail was much more flexible than those of otherornithischian dinosaurs because it lacked ossified tendons, thus lending credence to the idea of the tail as a weapon. He also observed thatStegosaurus could have maneuvered its rear easily by keeping its large hindlimbs stationary and pushing off with its very powerfully muscled but short forelimbs, allowing it to swivel deftly to deal with attack.[49] In 2010, analysis of a digitized model ofKentrosaurus aethiopicus showed that the tail could bring the thagomizer around to the sides of the dinosaur, possibly striking an attacker beside it.[50]
In 2001, a study of tail spikes by McWhinney et al.,[51] showed a high incidence of trauma-related damage. This too supports the theory that the spikes were used in combat. There is also evidence forStegosaurus defending itself, in the form of anAllosaurus tail vertebra with a partially healed puncture wound that fits aStegosaurus tail spike.[52]Stegosaurus stenops had four dermal spikes, each about 60–90 cm (2–3 ft) long. Discoveries of articulated stegosaur armor show that, at least in some species, these spikes protruded horizontally from the tail, not vertically as is often depicted. Initially, Marsh describedS. armatus as having eight spikes in its tail, unlikeS. stenops. However, recent research re-examined this and concluded this species also had four.[53][54]
A digital articulation and manipulation of digital scans of specimen material ofKentrosaurus inferred that stegosaurids may have used an erect limb posture, like that of most mammals, for habitual locomotion while using a sprawled crocodilian pose for defensive behavior. The sprawled pose would allow them to tolerate the large lateral forces used in swinging the spiked tail against predators as a clubbing device.[55]
There have been several findings of possiblesexual dimorphism in stegosaurids. Saitta (2015)[56] presents evidence of two morphs ofHesperosaurus dorsal plates, with one morph having a wide, oval plate with a surface area 45% larger than the narrow, tall morph. Considering that dorsal plates most likely functioned as display structures and that the wide oval shape allowed a broad continuous display, Saitta assigns the wider morph with larger surface area as male.
Kevin Padian, a paleontologist at the University of California, Berkeley, remarked that Saitta had misidentified features in his specimen's bone tissue sections and said "there's no evidence the animal has stopped growing". Paidan also expressed ethical concerns about the use of private specimens in the study.[57]
Kentrosaurus,Dacentrurus andStegosaurus are also suggested to have exhibited dimorphism in the form of three extra sacral ribs in the females.[1]
In order to explore the feeding habits of stegosaurids, Reichel (2010)[58] created a 3-D model ofStegosaurus teeth using the softwareZBrush. The model finds that the bite forces ofStegosaurus was significantly weaker than that ofLabradors,wolves and humans. The finding suggests that these dinosaurs would be capable of breaking smaller branches and leaves with their teeth, but would not be able to bite through a thick object (12 mm or more in diameter). Parrishet al.'s (2004)[59] description of Jurassic flora in the stegosaurid-richMorrison Formation supports this finding. The flora during this time-period was dominated by seasonal small, fast-growing herbaceous plants, which stegosaurids could consume easily if Reichel's reconstruction is accurate.[58]
Mallison (2010)[55] suggested thatKentrosaurus may have used a tripodal stance on their hindlimbs and tail to double the foraging height from the general low browsing height under one metre for stegosaurids. This challenged the view that stegosaurs are primarily low vegetation feeders because of their small heads, short necks and short forelimbs, since the tripodal stance would also give them access to young trees and high bushes.
Another piece of evidence suggesting that some stegosaurids may have consumed more than just low vegetation was the discovery of the long-necked stegosauridMiragaia longicollum. This dinosaur's neck has at least 17 cervical vertebrae achieved through the transformation of thoracic vertebrae into cervical vertebrae and possible lengthening of the centrum. This is more than mostsauropod dinosaurs, which also achieved the elongation of the neck through similar mechanisms and had access to fodder higher off the ground.[12]
Evidence fromYakutia suggests thatEarly Cretaceous stegosaurs living in high latitude environments were capable of palinal jaw motion and exhibited high rates of tooth replacement and short tooth formation time.[60]
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