Stegosauria
Stegosaurs | |
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Mounted skeleton of Stegosaurus stenops, Natural History Museum, London
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Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauria |
Clade: | †Ornithischia |
Clade: | †Thyreophora |
Clade: | † Eurypoda
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Clade: | †Stegosauria Marsh, 1877 |
Subgroups[6][7] | |
Stegosauria is a group of
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 armored scutes. An early evolutionary innovation was the development of spikes as defensive weapons. Later species, belonging to a subgroup called the Stegosauridae, became larger, and developed long hindlimbs that no longer allowed them to run. This increased the importance of active defence by the thagomizer, 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.
The first stegosaurian finds in the early 19th century were fragmentary. Better fossil material, of the genus Dacentrurus, was discovered in 1874 in England. Soon after, in 1877, the first nearly-complete skeleton was discovered in the United States. Professor Othniel Charles Marsh that year classified such specimens in the new genus Stegosaurus, 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.
Description
Skull
Stegosaurians had characteristic small, long, flat, narrow heads and a horn-covered beak or
Postcranial skeleton
All stegosaurians are
Huayangosaurus had relatively long and slender arms. The forelimbs of later forms are very robust, with a massive humerus and ulna. The wrist 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 of sauropod dinosaurs, and is also supported by evidence from stegosaurian footprints and fossils found in a lifelike pose.[10]
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 by
The stegosaurian
The stegosaurian pelvis was originally moderately large, as shown by Huayangosaurus. Later species, however, convergent to the Ankylosauria developed very broad
Osteoderms
Like all Thyreophora, stegosaurians were protected by bony scutes that were not part of the skeleton proper but skin ossifications instead: the so-called osteoderms. 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.[12] 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 by Kentrosaurus and extremely developed, as its name indicates, in Gigantspinosaurus. 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 of Chungkingosaurus sp. has been reported with three pairs of spikes pointing outwards and a fourth pair pointing to the rear.[13] The most derived species, like Stegosaurus, Hesperosaurus and Wuerhosaurus, have very large and flat back plates. To discern them from the smaller plates, which are intermediate to spines in having a thickened central section, these latter are sometimes called 'splates'. Stegosaurus plates are so large that it has been suggested that they were not arranged in paired but alternated rows or even formed a single overlapping midline row. With Stegosaurus fossils also ossicles have been found in the throat region, bony skin discs that protected the lower neck. Apart from protection, suggested functions of the osteoderms include display, species recognition and thermoregulation.[14]
Discovery
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 named Regnosaurus. In 1845, in the area of the present state of South Africa, remains were discovered that much later would be named Paranthodon. In 1874, other remains from England were named Craterosaurus. All three taxa 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.[15] They were named Omosaurus by Richard Owen in 1875. Later, this name was shown to be preoccupied by the phytosaur Omosaurus and the stegosaurian was renamed Dacentrurus. Other English nineteenth century and early twentieth century finds would be assigned to Omosaurus; later they would, together with French fossils, be partly renamed Lexovisaurus and Loricatosaurus.
In 1877, Arthur Lakes, a fossil hunter working for Professor Othniel Charles Marsh, in Wyoming excavated a fossil that Marsh the same year named Stegosaurus. 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 the Como 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.[15]
The next important discovery was made when a German expedition to the
From the 1950s onwards, the geology of China was systematically surveyed in detail and infrastructural works led to a vast increase of digging activities in that country. This resulted in a new wave of Chinese stegosaurian discoveries, starting with Chialingosaurus in 1957. Chinese finds of the 1970s and 1980s included Wuerhosaurus, Tuojiangosaurus, Chungkingosaurus, Huayangosaurus, Yingshanosaurus and Gigantspinosaurus. This increased the age range of good fossil stegosaurian material, as they represented the first relatively complete skeletons from the Middle Jurassic and the Early Cretaceous. Especially important was Huayangosaurus, which provided unique information about the early evolution of the group.
Towards the end of the twentieth century, the so-called
Classification
The Stegosauria was originally named as an order within
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.[17] They include per definition the well-known Stegosaurus. This group is widespread, with members across the Northern Hemisphere, Africa and possibly South America.[18]
The first exact clade definition of Stegosauria was given by
Phylogeny
Kenneth Carpenter of the Denver Museum of Nature and Science published a preliminary phyletic tree[20] of stegosaurians, in the 2001 description of Hesperosaurus. An updated phylogeny was published by Mateus et al. (2009), which is shown below.[11]
Stegosauria | |
Alternately, in 2017, Raven and Maidment published a new phylogenetic analysis, including almost every known stegosaurian genus:[21][6]
Thyreophora |
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Undescribed species
To date, several genera from China bearing names have been proposed but not formally described, including "
Evolutionary history
Like the spikes and shields of
A few million years later, during the
From the Early Cretaceous, far fewer finds are known and it seems that the group had declined in diversity. Some fragmentary fossils have been described, such as Craterosaurus from England and Paranthodon from South Africa. Up until recently, the only substantial discoveries were those of Wuerhosaurus from Northern China, the exact age of which is highly uncertain[27] 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 of Siberia, including the Ilek Formation[28] and Batylykh Formation.[29] The youngest known definitive remains of stegosaurs are those of Mongolostegus from Mongolia, possibly Stegosaurus from the Hekou Group of China, and Yanbeilong of the Zuoyun Formation of China, all of which date to the Aptian-Albian.[30][31][5]
It has often been suggested that the decline in stegosaur diversity was part of a Jurassic-Cretaceous transition, where
Though Late Cretaceous stegosaurian fossils have been reported, these have mostly turned out to be misidentified. A well-known example is
Paleobiology
Trace fossils
Stegosaurian tracks were first recognized in 1996 from a hindprint-only
Deltapodus is an ichnogenus attributed as stegosaurian prints, and are known across Europe,[38] North Africa,[39] and China.[40] One Deltapodus footprint measures less than 6 cm in length and represents the smallest known stegosaurian track.[40][41] Some tracks preserve exquisite scaly skin pattern.[42]
Australia's 'Dinosaur Coast' in Broome, Western Australia includes tracks of several different thyreophoran track-makers. Of these, the ichnogenus Garbina (a Nyulnyulan word for 'shield') and Luluichnus (honours the late Paddy Roe, OAM who went by the name 'Lulu') have been considered registered by stegosaurs.[43] Garbina includes the largest stegosaur tracks measuring 80 cm in length. Trackway data show Garbina track-makers were capable of bipedal and quadrupedal progression.
While has no body fossil evidence currently known for stegosaurs, handprints from underground coal mines near Oakey, Queensland, resembling Garbina tracks suggests their occurrence in this country from at least the Middle to Upper Jurassic (Callovian–Tithonian).[44] A single plaster cast of one of these handprints is in the collections of the Queensland Museum.
Tail spikes
There has been debate about whether the spikes were used simply for display, as posited by Gilmore in 1914,[45] or used as a weapon. Robert Bakker noted that it is likely that the stegosaur tail was much more flexible than those of other ornithischian dinosaurs because it lacked ossified tendons, thus lending credence to the idea of the tail as a weapon. He also observed that Stegosaurus 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.[46] In 2010, analysis of a digitized model of Kentrosaurus aethiopicus showed that the tail could bring the thagomizer around to the sides of the dinosaur, possibly striking an attacker beside it.[47]
In 2001, a study of tail spikes by McWhinney et al.,[48] showed a high incidence of trauma-related damage. This too supports the theory that the spikes were used in combat. There is also evidence for Stegosaurus defending itself, in the form of an Allosaurus tail vertebra with a partially healed puncture wound that fits a Stegosaurus tail spike.[49] 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 described S. armatus as having eight spikes in its tail, unlike S. stenops. However, recent research re-examined this and concluded this species also had four.[50][51]
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