| Stegosaurians Temporal range: Middle Jurassic - Early Cretaceous, 176–100 Ma |
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|---|---|
| Fossil skeleton of a Stegosaurus, National Museum of Natural History |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | Dinosauria |
| Order: | †Ornithischia |
| Node: | †Eurypoda |
| Suborder: | †Stegosauria Marsh, 1877 |
| Superfamily: | †Stegosauroidea Marsh, 1880 |
| Type species | |
| †Stegosaurus stenops Marsh, 1877 |
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| Subgroups | |
Known colloquially as stegosaurs, the Stegosauria are a group of herbivorous dinosaurs of the Jurassic and Early Cretaceous Periods, being found mostly in the Northern Hemisphere, predominantly in what is now North America and China. Their geographical origins are unclear; the earliest stegosaurs have been found in China, although fragmentary material hails from southern England.
The genus Stegosaurus, from which the group acquires its name, is by far the most famous stegosaurian.
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All stegosaurs have rows of special bones, called osteoderms, which develop into plates and spines along the back and tail (forming the so-called "thagomizer"). Many also have intermediate spines, called 'splates'.
They had characteristic long, narrow heads and a horn-covered beak or rhamphotheca, which covered the front of the upper jaw (premaxillary) and lower jaw (predentary) bones. Similar structures are seen in turtles and birds. Apart from Huayangosaurus, stegosaurs subsequently lost nearby premaxillary teeth.[1]
All are quadrupedal, with hoof-like toes on all four limbs. All stegosaurians after Huayangosaurus have forelimbs much shorter than their hindlimbs. Given that their speed would have been limited by their shortest limb and their size is likely to have precluded them from being bipedal, this suggests that they were not able to run quickly.
The front feet of stegosaurs 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 stegosaurs were arranged into vertical columns, with the main fingers forming a tube-like structure. This is similar to the hands of sauropod dinosaurs, and is also supported by evidence from stegosaur footprints and fossils found in a life-like pose.[2]
Stegosaur tracks were first recognized in 1996 from a hindprint-only trackway discovered at the Clevland-Lloyd quarry, which is located near Price, Utah.[3] Two years later, a new ichnogenus called Stegopodus was erected for another set of stegosaur tracks which were found near Arches National Park, also in Utah.[3] Unlike the first, this trackway preserved traces of the forefeet. Fossil remains indicate that stegosaurs have five digits on the forefeet and three weight-bearing digits on the hind feet.[3] From this scientists were able to predict the appearance of stegosaur tracks in 1990, six years in advance of the first actual discovery of Morrison stegosaur tracks.[3] More trackways have been found since the erection of Stegopodus. None, however, have preserved traces of the front feet and stegosaur traces remain rare.[3]
The Stegosauria was originally named as an order within Reptilia by O.C. Marsh in 1877,[4] although today it is generally treated as an infraorder or suborder (or simply a clade) within Thyreophora, the armored dinosaurs. It includes the families Huayangosauridae and Stegosauridae.
The Huayangosauridae were an early family of stegosaurs which lived during the early to middle Jurassic Period. In general, they were smaller than later stegosaurs and had shorter and higher skulls. Currently, the only confirmed genus included is the type genus Huayangosaurus of China. The poorly-known remains of Regnosaurus from England, however, indicate it too could be a member. Its lower jaw is very similar to the former.
The vast majority of Stegosaurian dinosaurs thus far recovered being to the Stegosauridae, which lived in the later part of the Jurassic and early Cretaceous. It includes the well-known Stegosaurus. The family is widespread, with members across the Northern Hemisphere and Africa.
Following is a list of stegosaurian genera by classification and location:
Suborder Thyreophora
Infraorder Stegosauria
Kenneth Carpenter of the Denver Museum of Nature and Science published a preliminary phyletic tree[7] of stegosaurs, in the 2001 description of Hesperosaurus. Here, the basal stegosaur Huayangosaurus is used as the outgroup. The Stegosauridae are then defined as all stegosaurs closer to Stegosaurus than to Huayangosaurus. The position of Chungkingosaurus is uncertain due to lack of data.
Stegosauria ├──Huayangosaurus └──Stegosauridae └──┬─?Chungkingosaurus └──┬──Chialingosaurus └──┬──┬──Wuerhosaurus │ └──┬──Dacentrurus │ └──Hesperosaurus └──┬──Tuojiangosaurus └──┬──┬──Kentrosaurus │ └──Lexovisaurus └──┬──Stegosaurus stenops └──S. ungulatus (=?S. armatus)
To date, several genera from China bearing names have not been formally described, including "Changdusaurus" and "Yingshanosaurus". Until formal descriptions are published, these genera are regarded as nomina nuda.