Stegosaurus

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Stegosaurus
Fossil range: Late Jurassic
Model Stegosaurus, Bałtów Jurassic Park, Poland.
Model Stegosaurus, Bałtów Jurassic Park, Poland.
Conservation status
Extinct (fossil)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Superorder: Dinosauria
Order: Ornithischia
Suborder: Thyreophora
Infraorder: Stegosauria
Family: Stegosauridae
Genus: Stegosaurus
Marsh, 1877
Species

Stegosaurus (IPA: /ˌstɛgəˈsɔːɹəs/) is a genus of stegosaurid armoured dinosaur from the Late Jurassic period (Kimmeridgian to Early Tithonian) in what is now western North America. In 2006, a specimen of Stegosaurus was announced from Portugal, suggesting that they were present in Europe as well.[1] Due to its distinctive tail spikes and plates, Stegosaurus is one of the most recognisable dinosaurs, along with Tyrannosaurus, Triceratops, and Apatosaurus. The name Stegosaurus means "roof-lizard" and is derived from the Greek στέγος-, stegos- ("roof") and σαῦρος, -sauros ("lizard").[2] At least three species have been identified in the upper Morrison Formation and are known from the remains of about 80 individuals. They lived some 155 to 145 million years ago, in an environment and time dominated by the giant sauropods Diplodocus, Camarasaurus, and Apatosaurus.

A large, heavily-built and herbivorous quadruped, Stegosaurus had a distinctive and unusual posture, with a heavily-arched back, short forelimbs, head held low to the ground and a stiffened tail held high in the air. Its array of plates and spikes have been the subject of much speculation. The spikes were most likely used for defence, while the plates have also been proposed as a defensive mechanism, as well as having display and thermoregulatory (heat control) functions. Stegosaurus was the largest of all the stegosaurians (bigger than genera such as Kentrosaurus and Huayangosaurus) and, although roughly bus-sized, it nonetheless shared many anatomical features (including the tail spines and plates) with the other stegosaurian genera.

Contents

[edit] Description

The size of a Stegosaurus compared to a human.
The size of a Stegosaurus compared to a human.

Averaging around 9 metres (30 ft) long and 4 metres (14 ft) tall, the quadrupedal Stegosaurus is one of the most easily-identifiable dinosaurs, due to the distinctive double row of kite-shaped plates rising vertically along its arched back and the two pairs of long spikes extending horizontally near the end of its tail. Although a large-bodied animal, it was dwarfed by its contemporaries, the giant sauropods. Some form of armour appears to have been necessary, as it coexisted with large predatory theropod dinosaurs, such as the fearsome Allosaurus and Ceratosaurus.

The hind feet each had three short toes, while each forefoot had five toes. All four limbs were supported by pads behind the toes.[3] 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 no higher than 1 metre (3.3 ft) above the ground.[4]

Stegosaurus tooth.
Stegosaurus tooth.

The long and narrow skull was small in proportion to the body. Unlike most dinosaurs, it did not have an antorbital fenestra, the hole between the nose and eye common to most archosaurs, including modern birds, though lost in extant crocodylians.[3] Its 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 apparently did little grinding, as they lacked wear surfaces (facets). However, their placement in the jaw suggests that Stegosaurus had cheeks.[5]

Despite the animal's overall size, the braincase of Stegosaurus was small, being no larger than that of a dog. A well preserved Stegosaurus skull had allowed Othniel Charles Marsh to obtain in the 1880s a cast representing the brain of the animal and showed that it was indeed very small, maybe the smallest among the dinosaurs. The fact that an animal weighing over 4.5 tonnes (5 US short tons) could have a brain of no more than 80 grams (2.8 oz) has largely contributed to the popular old idea that dinosaurs were extremely stupid, an idea now largely rejected.

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 metres (15 ft) long and 2 metres (7 ft) high, and is estimated to have weighed 2.3 tonnes (2.6 short tons) while alive. It is on display in the University of Wyoming Geological Museum.[6]

[edit] Classification

Stegosaurus was the first-named genus of the family Stegosauridae. It is the type genus, that gives its name to the family. Stegosauridae is one of two families within the infraorder Stegosauria, with the other being Huayangosauridae. Stegosauria lies within the Thyreophora, or armoured dinosaurs, a suborder which also includes the more diverse ankylosaurs. The stegosaurs were a clade of animals similar in appearance, posture and shape, that mainly differed in their array of spikes and plates. Among the closest relatives to Stegosaurus are Wuerhosaurus from China and Kentrosaurus from east Africa.

[edit] Origins

The origin of Stegosaurus is uncertain, as few remains of basal stegosaurs and their ancestors are known. Recently, stegosaurids have been shown to be present in the lower Morrison Formation, existing several million years before the occurrence of Stegosaurus itself, with the discovery of the related Hesperosaurus from the early Kimmeridgian.[7] The earliest stegosaurid (the genus Lexovisaurus) is known from the Oxford Clay Formation of England and France, giving it an age of early to middle Callovian.

The earlier, and more basal genus Huayangosaurus from the Middle Jurassic of China (some 165 million years ago) predates Stegosaurus by 20 million years and is the only genus in the family Huayangosauridae. Earlier still is Scelidosaurus, from Early Jurassic England, which lived approximately 190 million years ago. Interestingly, it possessed features of both stegosaurs and ankylosaurs. Emausaurus from Germany was another small quadruped, while Scutellosaurus from Arizona in the USA was an even earlier genus and was facultatively bipedal. These small, lightly-armoured dinosaurs were closely related to the direct ancestor of both stegosaurs and ankylosaurs. A trackway of a possible early armoured dinosaur, from around 195 million years ago, has been found in France.[8]

[edit] Discovery and species

Marsh's 1896 illustration of Stegosaurus. Note the twelve dorsal plates and eight tail spikes; Stegosaurus actually had 17 plates and just four spikes.
Marsh's 1896 illustration of Stegosaurus. Note the twelve dorsal plates and eight tail spikes; Stegosaurus actually had 17 plates and just four spikes.

Stegosaurus, one of the many dinosaurs first collected and described in the Bone Wars, was originally named by Othniel Charles Marsh in 1877,[9] from remains recovered north of Morrison, Colorado. These first bones became the holotype of Stegosaurus armatus. The basis for its scientific name, 'roof(ed) lizard' has been thought to have been Marsh's initial belief that the plates lay flat over the animal's back, overlapping like the shingles (tiles) on a roof. A wealth of Stegosaurus material was recovered over the next few years and Marsh published several papers on the genus. Initially, several species were described. However, many of these have since been considered to be invalid or synonymous with existing species,[10] leaving two well-known and one poorly-known species.

[edit] Valid species

Stegosaurus armatus, meaning "armoured roof lizard", was the first species to be found and is known from two partial skeletons, two partial skulls and at least thirty fragmentary individuals.[9] This species had four horizontal tail spikes and relatively small plates. At 9 metres (30 ft), it was the longest species within the genus Stegosaurus.

Stegosaurus stenops, meaning "narrow-faced roof lizard", was named by Marsh in 1887,[3] with the holotype having been collected by Marshal 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 large, broad plates and four tail spikes. Stegosaurus 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 metres (23 ft).

Stegosaurus longispinus, meaning "long-spined roof lizard", was named by Charles W. Gilmore[11] and known from one partial skeleton, from the Morrison Formation in Wyoming. Stegosaurus longispinus was notable for its set of four unusually long tail spines. Some consider it a species of Kentrosaurus. Like S. stenops, it grew to 7 metres (23 ft) in length.

[edit] Nomina dubia (dubious names)

Stegosaurus ungulatus, meaning "hoofed roof lizard", was named by Marsh in 1879, from remains recovered at Como Bluff, Wyoming.[12] It is known from a few vertebrae and armour plates. It might be a juvenile form of S. armatus,[13] although the original material of S. armatus is yet to be fully described. The specimen discovered in Portugal and dating from the upper Kimmeridgian-lower Tithonian stage has been ascribed to this species.[1]

Stegosaurus sulcatus, meaning "furrowed roof lizard" was described by Marsh in 1887 based on a partial skeleton.[3] It is considered a synonym of S. armatus.[13] 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"), is probably the same as S. armatus.[13] Although named by Marsh in 1887 (including the holotype specimen), the disarticulated bones were actually collected in 1879 by Edward Ashley at Como Bluff, Wyoming.

Stegosaurus seeleyanus, originally named Hypsirophus, is probably the same as S. armatus. Stegosaurus (Diracodon) laticeps was described by Marsh in 1881, from some jawbone fragments.[14] Just as some consider S. stenops a species of Diracodon, others consider Diracodon itself to be a species of Stegosaurus. Bakker had resurrected D. laticeps in 1986,[15] although others note that the material is non-diagnostic and likely synonymous with S. stenops.[10]

Stegosaurus affinis, described by Marsh in 1881, is only known from a pubis and is considered nomen dubium.[13] It is possibly synonymous with S. armatus.[11]

[edit] Dubious and invalid species

"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 Majungatholus,[16] a hadrosaur or even a crocodylian. Other remains, originally attributed to Stegosaurus, are now considered to belong to different genera. This is the case for "Stegosaurus" marshi, which was described by Lucas in 1901. It was renamed Hoplitosaurus in 1902. "Stegosaurus" priscus, described by Nopcsa in 1911, is a synonym of Lexovisaurus.[13]

[edit] Palaeobiology

Stegosaurus was the largest stegosaur, reaching up to 12 metres (39 ft) in length and possibly weighing up to 5,000 kilograms (5.5 short tons). However, 7 to 9 metres was a more usual length. Soon after its discovery, Marsh considered Stegosaurus to have been bipedal, due to its short forelimbs.[17] He had changed his mind however, by 1891, after considering the heavy build of the animal.[3] Although Stegosaurus is undoubtedly now considered to have been quadrupedal, there has been some discussion over whether it could have reared up on its hind legs, using its tail to form a tripod with its hind limbs and browsing for higher foliage.[13] This has been proposed by Bakker[18][15] and opposed by Carpenter.[4]

Stegosaurus did have very short forelimbs, in relation to its hind legs. Furthermore, within the hindlimbs, the lower section (comprising the tibia and fibula) was short compared with the femur. This suggests that it couldn't walk very fast, as the stride of the back legs at speed would have overtaken the front legs, giving a maximum speed of 6-7 kilometres per hour (4-5 mi/hr).[5]

[edit] "Second brain"

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 brain. This has led to the famous 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.It has also been suggested that this "brain" might have given the stegosaurus a temporary boost when it was under threat from predators.[5] More recently, it has been argued that this space (also found in sauropods) may have been the location of a glycogen body, a structure in living birds whose function is not definitely known but which is postulated to facilitate the supply of glycogen to the animal's nervous system.[19]

[edit] Plates

Stegosaurus mounted skeleton and small model at the American Museum of Natural History, New York.
Stegosaurus mounted skeleton and small model at the American Museum of Natural History, New York.

The most recognizable features of Stegosaurus are its dermal plates, which consisted of 17 separate flat plates. These were highly modified osteoderms (bony-cored scales), similar to those seen in crocodiles and many lizards today. They were not directly attached to the animal's skeleton, instead arising from the skin. In the past, some palaeontologists, notably Robert Bakker, have speculated the plates may have been mobile to some degree, although others disagree.[20] Bakker suggested that the plates were the bony cores of pointed horn-covered plates that a Stegosaurus could flip from one side to another in order to present a predator with an array of spikes and blades that would impede it from closing sufficiently to attack the Stegosaurus effectively. The plates would naturally sag to the sides of the Stegosaurus, the length of the plates reflecting the width of the animal at that point along its spine. His reasoning for these plates to be covered in horn is that the surface fossilised plates have a resemblance to the bony cores of horns in other animals known or thought to bear horns, and his reasoning for the plates to be defensive in nature is that the plates had insufficient width for them to stand erect easily in such a manner as to be useful in display without continuous muscular effort.[21] The largest plates were found over the animal's hips and measured 60 centimetres (2 ft) wide and 60 centimetres tall. The arrangement of the plates has long been a subject of debate but most palaeontologists now agree that they formed a pair of alternating rows, one running down each side of the midline of the animal's back.

The function of the plates has been much debated. Initially thought of as some form of armour,[17] they appear to have been too fragile and ill-placed for defensive purposes, leaving the animal's sides unprotected.[22] More recently, researchers have proposed that they may have helped to control the body temperature of the animal,[20] in a similar way to the sails of the large carnivorous Spinosaurus or of the pelycosaur Dimetrodon (and the ears of modern elephants and jackrabbits). The plates had blood vessels running through grooves and air flowing around the plates would have cooled the blood.[23] This theory has been seriously questioned,[24] since the closest relative to the common plate-wielding species, Stegosaurus stenops, had low surface area spikes instead of plates, implying that cooling was not important enough to require specialised structural formations such as plates.

Their large size suggests that the plates may have served to increase the apparent height of the animal, in order either to intimidate enemies[11] or to impress other members of the same species, in some form of sexual display,[22] although both male and female specimens seemed to have had them. A study published in 2005 supports the idea of their use in identification. Researchers believe this may be the function of other unique anatomical features, found in various dinosaur species.[25] Stegosaurus stenops also had disk-shaped plates on its hips.

One of the major subjects of books and articles about Stegosaurus is the plate arrangement.[26] The argument has been a major one in the history of dinosaur reconstruction. Four possible plate arrangements have been mooted over the years:

  1. The plates lay flat along the back, like armour. This was Marsh's initial interpretation, which led to the name 'Roof Lizard'. As further and complete plates were found, their form showed that they stood on edge, rather than lying flat.
  2. By 1891, Marsh published a more familiar view of Stegosaurus,[3] with a single row of plates. This was dropped fairly early on (apparently because it was poorly understood how the plates were embedded in the skin and it was thought that they would overlap too much in this arrangement). It was revived, in somewhat modified form, in the 1980s, by an artist (Stephen Czerkas),[27] based on the arrangement of iguana dorsal spines.
  3. The plates paired in a double row along the back. This is probably the most common arrangement in pictures, especially earlier ones (until the 'Dinosaur Renaissance' in the '70s). (The Stegosaurus in the 1933 film, King Kong has this arrangement.) However, no two plates of identical size and shape have ever been found within the same animal.
  4. Two rows of alternating plates. By the early 1960s, this had become (and remains) the prevalent idea, mainly because the one Stegosaurus fossil with the plates still articulated indicates this arrangement. An objection to it is that this phenomenon is unknown among other reptiles and it is difficult to understand how such a disparity could evolve.

[edit] Thagomizer (tail spikes)

Main article: Thagomizer

There has been debate about whether the tail spikes were used for display only, as posited by Gilmore in 1914[11] 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. He also observed that Stegosaurus could have manoeuvred 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.[15] More recently, a study of tail spikes by McWhinney et al.,[28] which showed a high incidence of trauma-related damage, confirms the spikes were indeed used in combat.

Stegosaurus stenops had four dermal spikes, each about 60-90 centimetres (2-3 ft) long. Discoveries of articulated stegosaur armour 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.[10]

[edit] Diet

Stegosaurus and related genera were herbivores. However, they adopted a feeding strategy different from that of the other herbivorous ornithischian dinosaurs. 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 they could chew plants). This contrasts with Stegosaurus (and all stegosaurians), which had small teeth lacking the wear facets associated with tooth-tooth occulsion and a jaw capable of only orthal movements.[13]

The stegosaurians must have been successful, as they became speciose and geographically widely distributed, in the late Jurassic.[13] Palaeontologists believe it would have eaten plants such as mosses, ferns, horsetails, cycads and conifers or fruits[29] and swallowed gastroliths to aid food processing (due to the lack of chewing ability), in the same manner used by modern birds and crocodiles.[30] Low-level browsing on grasses, seen in modern mammalian herbivores, would not have been possible for Stegosaurus, as grasses did not evolve until late into the Cretaceous Period, long after Stegosaurus had become extinct.

One hypothesised feeding behaviour strategy considers them to be low-level browsers, eating low-growing fruit of various non-flowering plants, as well as foliage. This scenario has Stegosaurus foraging at most one metre above the ground.[31] On the other hand, if Stegosaurus could have raised itself on two legs, as suggested by Bakker, then it could have browsed on vegetation and fruits quite high up, with adults being able to forage up to 6 metres (20 ft) above the ground.[5]

[edit] Popular culture

Main article: Stegosaurus in popular culture

As one of the most recognizable dinosaurs, Stegosaurus has been widely depicted on film, in cartoons, comics, as children's toys, and has even been declared the State Dinosaur of Colorado in 1982.[32]

[edit] See also

[edit] Footnotes

  1. ^ a b Escaso, F, Ortega, F., Dantas, P., Malafaia, E., Pimentel, N.L, Pereda-Suberbiola, X., Sanz, J.L., Kullberg, J.C., Kullberg, M.C., and Barriga, F. (2007). "New Evidence of Shared Dinosaur Across Upper Jurassic Proto-North Atlantic: Stegosaurus From Portugal." Naturwissenschaften,
  2. ^ Liddell & Scott (1980). Greek-English Lexicon, Abridged Edition. Oxford University Press, Oxford, UK. ISBN 0-19-910207-4. 
  3. ^ a b c d e f Lambert D (1993). The Ultimate Dinosaur Book. Dorling Kindersley, New York, 110-129. ISBN 1-56458-304-X. 
  4. ^ a b Carpenter K (1998). "Armor of Stegosaurus stenops, and the taphonomic history of a new specimen from Garden Park Colorado", The Upper Jurassic Morrison Formation: An Interdisciplinary Study. Part 1. Modern Geol., 127-144. 
  5. ^ a b c d Fastovsky DE, Weishampel DB (2005). "Stegosauria:Hot Plates", in Fastovsky DE, Weishampel DB: The Evolution and Extinction of the Dinosaurs (2nd Edition). Cambridge University Press, 107–130. ISBN 0-521-81172-4. 
  6. ^ Stegosaurus. University of Wyoming Geological Museum. 2006. Retrieved October 6, 2006. University of Wyoming Geological Museum
  7. ^ Carpenter K, Miles CA, Cloward K (2001). "New Primitive Stegosaur from the Morrison Formation, Wyoming", in Carpenter, Kenneth(ed): The Armored Dinosaurs. Indiana University Press, 55–75. ISBN 0-253-33964-2. 
  8. ^ Le Loeuff J, Lockley M, Meyer C, Petit J-P(1999) Discovery of a thyreophoran trackway in the Hettangian of central France. C. R. Acad. Sci. Paris 2 328, 215-219
  9. ^ a b Marsh OC (1877). "A new order of extinct Reptilia (Stegosauria) from the Jurassic of the Rocky Mountains". American Journal of Science 3 (14): 513–514. 
  10. ^ a b c Carpenter K & Galton PM (2001). "Othniel Charles Marsh and the Eight-Spiked Stegosaurus", in Carpenter, Kenneth(ed): The Armored Dinosaurs. Indiana University Press, 76–102. ISBN 0-253-33964-2. 
  11. ^ a b c d Gilmore CW (1914). "Osteology of the armored Dinosauria in the United States National Museum, with special reference to the genus Stegosaurus". Series: Smithsonian Institution. United States National Museum. Bulletin 89 (89). 
  12. ^ Marsh OC (1879). "Notice of new Jurassic reptiles". American Journal of Science 3 (18): 501-505. 
  13. ^ a b c d e f g h Galton PM, Upchurch P (2004). "Stegosauria", in Weishampel DB, Osmólska H, Dodson P: The Dinosauria (2nd Edition). University of California Press, 361. ISBN 0-520-24209-2. 
  14. ^ Marsh OC (1881). "Principal characters of American Jurassic dinosaurs, part V". American Journal of Science 3 (21): 417–423. 
  15. ^ a b c Bakker RT (1986). The Dinosaur Heresies. William Morrow, New York. 
  16. ^ Galton PM (1981) "Craterosaurus pottonensis Seeley, a stegosaurian dinosaur from the Lower Cretaceous of England, and a review of Cretaceous stegosaurs". Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 161(1):28-46
  17. ^ a b Marsh OC (1880). "Principal characters of American Jurassic dinosaurs, part III". American Journal of Science 3 (19): 253-259. 
  18. ^ Bakker RT (1978). "Dinosaur feeding behavior and the origin of flowering plants". Nature (274): 661-663. 
  19. ^ Buchholz (née Giffin) EB (1990). "Gross Spinal Anatomy and Limb Use in Living and Fossil Reptiles". Paleobiology 16: 448-458. 
  20. ^ a b Buffrénil (1986). "Growth and Function of Stegosaurus Plates". Paleobiology 12: 459-473. 
  21. ^ Bakker, R (1986). The Dinosaur Heresies. Penguin Books, 229-234. 
  22. ^ a b Davitashvili L (1961). The Theory of sexual selection. Izdatel'stvo Akademia nauk SSSR, 538. 
  23. ^ Farlow JO, Thompson CV, Rosner DE (1976). "Plates of the dinosaur Stegosaurus:Forced convection heat loss fins?". Science (192): 1123-1125. 
  24. ^ Main RP, Padian K, Horner J (2000). "Comparative histology, growth and evolution of archosaurian osteoderms: why did Stegosaurus have such large dorsal plates?". Journal of Vertebrate Paleontology 56A (20). 
  25. ^ "Stegosaur plates used for identification". National Geographic News (25 May 2005). Retrieved on October 26, 2006.
  26. ^ Colbert EH (1962). Dinosaurs: Their Discovery & Their World. Hutchinson Press, London, 82-99. ISBN 1-111-21503-0. 
  27. ^ Czerkas SA (1987). "A Reevaluation of the Plate Arrangement on Stegosaurus stenops", in Czerkas SJ, Olson EC: Dinosaurs Past & Present, Vol 2. University of Washington Press, Seattle, 82-99. ISBN. 
  28. ^ McWhinney LA, Rothschild BM & Carpenter K (2001). "Posttraumatic Chronic Osteomyelitis in Stegosaurus dermal spikes", in Carpenter, Kenneth(ed): The Armored Dinosaurs. Indiana University Press, 141–156. ISBN 0-253-33964-2. 
  29. ^ Stegosaurus ungulatus Carnegie Museum of Natural History. Retrieved 26 October 2006.
  30. ^ Stegosaurs Jacobson, RJ. Dinosaur and Vertebrate Paleontology Information. Retrieved 26 October 2006.
  31. ^ Weishampel DB (1984). "Interactions between Mesozoic Plants and Vertebrates:Fructifications and seed predation". N. Jb. Geol. Paläontol. Abhandl.: 224-250. 
  32. ^ Colorado Department of Personnel website - State emblems

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