Stegosaurus is a
genusIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of
armoredThe Thyreophora were a subgroup of the ornithischian dinosaurs...
stegosauridKnown 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....
dinosaurDinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
. They lived during the
Late JurassicThe Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.2 ± 4.0 to 145.5 ± 4.0 million years ago , which is preserved in Upper Jurassic strata. In European lithostratigraphy, the name "Malm" indicates rocks of Late Jurassic age...
period (
KimmeridgianIn the geologic timescale, the Kimmeridgian is an age or stage in the Late or Upper Jurassic epoch or series. It spans the time between 155.7 ± 4 Ma and 150.8 ± 4 Ma . The Kimmeridgian follows the Oxfordian and precedes the Tithonian....
to early
TithonianIn the geologic timescale the Tithonian is the latest age of the Late Jurassic epoch or the uppermost stage of the Upper Jurassic series. It spans the time between 150.8 ± 4 Ma and 145.5 ± 4 Ma...
), some 155 to 150 million years ago in what is now western
North AmericaNorth America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. In 2006, a specimen of Stegosaurus was announced from
PortugalPortugal , officially the Portuguese Republic is a country situated in southwestern Europe on the Iberian Peninsula. Portugal is the westernmost country of Europe, and is bordered by the Atlantic Ocean to the West and South and by Spain to the North and East. The Atlantic archipelagos of the...
, showing that they were present in Europe as well. Due to its distinctive
tail spikesThagomizer is an informal name for the distinctive arrangement of four to ten spikes on the tails of stegosaurid dinosaurs. These spikes are believed to have been a defensive measure against predators.-Paleobiology:...
and plates, Stegosaurus is one of the most recognizable dinosaurs. At least three species have been identified in the upper
Morrison FormationThe Morrison Formation is a distinctive sequence of Late Jurassic sedimentary rock that is found in the western United States, which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltstone and limestone and is light grey, greenish...
and are known from the remains of about 80 individuals.
A large, heavily built,
herbivorousHerbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
quadrupedQuadrupedalism is a form of land animal locomotion using four limbs or legs. An animal or machine that usually moves in a quadrupedal manner is known as a quadruped, meaning "four feet"...
, Stegosaurus had a distinctive and unusual posture, with a heavily rounded back, short forelimbs, head held low to the ground and a stiffened tail held high in the air. Its array of plates and spikes has been the subject of much speculation. The spikes were most likely used for defense, while the plates have also been proposed as a defensive mechanism, as well as having display and
thermoregulatoryThermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...
functions. Stegosaurus was among the least intelligent dinosaurs, having a relatively low brain-to-body mass ratio. It had a short neck and small head, meaning it most likely ate low-lying bushes and shrubs. It was the largest of all the
stegosauriaKnown 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....
ns (bigger than genera such as
KentrosaurusKentrosaurus is a genus of stegosaurid dinosaur from the Late Jurassic of Tanzania. Its fossils have been found only in the Tendaguru Formation of Tanzania, dated to the Kimmeridgian stage, between about 155.7 ± 4 Ma and 150.8 ± 4 Ma . Apparently, all finds belong to one species, K...
and
HuayangosaurusHuayangosaurus is a genus of stegosaurian dinosaur from the Middle Jurassic of China. The name derives from "Huayang", an alternate name for Sichuan , and "saurus", meaning "lizard"...
) and, although roughly bus-sized, it nonetheless shared many anatomical features (including the tail spines and plates) with the other stegosaurian genera.
Description
Averaging around 9 meters (30 ft) long and 4 meters (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 rounded back and the two pairs of long spikes extending horizontally near the end of its tail. Although a large animal, it was dwarfed by its contemporaries, the giant
sauropodsSauropoda , or the sauropods , are an infraorder of saurischian dinosaurs. They had long necks, long tails, small heads , and thick, pillar-like legs. They are notable for the enormous sizes attained by some species, and the group includes the largest animals to have ever lived on land...
. Some form of
armorArmour in animals is external or superficial protection against attack by predators, formed as part of the body , usually through the hardening of body tissues, outgrowths or secretions. It has therefore mostly developed in 'prey' species...
appears to have been necessary, as it coexisted with large predatory theropod dinosaurs, such as the fearsome
AllosaurusAllosaurus is a genus of large theropod dinosaur that lived 155 to 150 million years ago during the late Jurassic period . The name Allosaurus means "different lizard". It is derived from the Greek /allos and /sauros...
and
CeratosaurusCeratosaurus meaning "horned lizard", in reference to the horn on its nose , was a large predatory theropod dinosaur from the Late Jurassic Period , found in the Morrison Formation of North America, in Tanzania and Portugal...
.
The hind feet each had three short toes, while each forefoot had five toes; only the inner two toes had a blunt hoof. 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 no higher than 1 meter (3.3 ft) above the ground.
The long and narrow
skullThe skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
was small in proportion to the body. It had a small
antorbital fenestraAn antorbital fenestra is an opening in the skull, in front of the eye sockets. This skull formation first appeared in archosaurs during the Triassic Period. Living birds today possess antorbital fenestrae, but the feature has been lost in modern crocodilians...
, the hole between the nose and eye common to most
archosaurArchosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
s, 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 MarshOthniel Charles Marsh was an American paleontologist. Marsh was one of the preeminent scientists in the field; the discovery or description of dozens of news species and theories on the origins of birds are among his legacies.Born into a modest family, Marsh was able to afford higher education...
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
gramThe gram is a metric system unit of mass....
s (2.8
ozThe ounce is a unit of mass with several definitions, the most commonly used of which are equal to approximately 28 grams. The ounce is used in a number of different systems, including various systems of mass that form part of the imperial and United States customary systems...
) contributed to the popular old idea that dinosaurs were unintelligent, 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 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
Stegosaurus was the first-named genus of the family
StegosauridaeStegosauridae is a family of stegosauria, large thyreophorans. They lived longer than other Stegosaurs; while all Huayangosauridae and most of basal stegosaurs died out in Tithonian - Kimmeridgian, stegosauridae survived till Middle Cretaceous. They are usually characterized by triangular plates on...
. It is the
type genusIn biological classification, a type genus is a representative genus, as with regard to a biological family. The term and concept is used much more often and much more formally in zoology than it is in botany, and the definition is dependent on the nomenclatural Code that applies:* In zoological...
that gives its name to the family. Stegosauridae is one of two families within the infraorder
StegosauriaKnown 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....
, with the other being Huayangosauridae. Stegosauria lies within the
ThyreophoraThe Thyreophora were a subgroup of the ornithischian dinosaurs...
, or
armoredArmour in animals is external or superficial protection against attack by predators, formed as part of the body , usually through the hardening of body tissues, outgrowths or secretions. It has therefore mostly developed in 'prey' species...
dinosaurs, a suborder which also includes the more diverse
ankylosaursAnkylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. They are first known to have appeared in the early Jurassic Period of...
. The stegosaurs were a
cladeA clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
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
WuerhosaurusWuerhosaurus is a genus of stegosaurid dinosaur from the Early Cretaceous Period of China. As such, it was one of the last genera of stegosaurians known to have existed, since most others lived in the late Jurassic.-Discovery and species:...
from
ChinaChinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
and
KentrosaurusKentrosaurus is a genus of stegosaurid dinosaur from the Late Jurassic of Tanzania. Its fossils have been found only in the Tendaguru Formation of Tanzania, dated to the Kimmeridgian stage, between about 155.7 ± 4 Ma and 150.8 ± 4 Ma . Apparently, all finds belong to one species, K...
from
east AfricaEast Africa or Eastern Africa is the easterly region of the African continent, variably defined by geography or geopolitics. In the UN scheme of geographic regions, 19 territories constitute Eastern Africa:...
.
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
HesperosaurusHesperosaurus was a herbivorous dinosaur from the Kimmeridgian to Tithonian epochs of the Jurassic period , whose fossils are found in the state of Wyoming in the United States of America...
from the early
KimmeridgianIn the geologic timescale, the Kimmeridgian is an age or stage in the Late or Upper Jurassic epoch or series. It spans the time between 155.7 ± 4 Ma and 150.8 ± 4 Ma . The Kimmeridgian follows the Oxfordian and precedes the Tithonian....
. The earliest stegosaurid (the genus
LexovisaurusLexovisaurus was one of the first dinosaurs from mid-to-Late Jurassic Europe, 164.7 mya to be discovered. It was a stegosaur. Its fossils have been found in France and northern England....
) is known from the
Oxford ClayThe Oxford Clay Formation is a Jurassic marine sedimentary rock formation underlying much of southeast England, from as far west as Dorset and as far north as Yorkshire. The Oxford Clay is of middle Callovian to lower Oxfordian age and comprises 2 main facies. The lower facies comprises the...
Formation of England and France, giving it an age of early to middle
CallovianIn the geologic timescale, the Callovian is an age or stage in the Middle Jurassic, lasting between 164.7 ± 4.0 Ma and 161.2 ± 4.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the Oxfordian....
.
The earlier and more basal genus
HuayangosaurusHuayangosaurus is a genus of stegosaurian dinosaur from the Middle Jurassic of China. The name derives from "Huayang", an alternate name for Sichuan , and "saurus", meaning "lizard"...
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
ScelidosaurusScelidosaurus is a genus of quadrupedal, lightly plated, herbivorous dinosaur about long. It lived during the Early Jurassic Period, during the Hettangian to Sinemurian stages around 199.6 to 194 million years ago. Its fossils have been found in both England and in Arizona, in the United...
, from Early Jurassic England, which lived approximately 190 million years ago. Interestingly, it possessed features of both stegosaurs and ankylosaurs.
EmausaurusEmausaurus is a genus of thyreophoran or armored dinosaur from the Early Jurassic. Its fossils have been found in Germany. It is known from a skull and partial postcranial remains, although only the skull is known well. Armor includes conical scutes and tall, spiny elements.The type species,...
from Germany was another small quadruped, while
ScutellosaurusScutellosaurus is a genus of herbivorous dinosaur that lived in North America around 200 to 196 million years ago, in the Early Jurassic times....
from
ArizonaArizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
in the USA was an even earlier genus and was facultatively bipedal. These small, lightly armored dinosaurs were closely related to the direct ancestor of both stegosaurs and ankylosaurs. A trackway of a possible early armored dinosaur, from around 195 million years ago, has been found in France.
Discovery and species
Stegosaurus, one of the many dinosaurs first collected and described in the
Bone WarsThe Bone Wars, also known as the "Great Dinosaur Rush", refers to a period of intense fossil speculation and discovery during the Gilded Age of American history, marked by a heated rivalry between Edward Drinker Cope and Othniel Charles Marsh...
, was originally named by
Othniel Charles MarshOthniel Charles Marsh was an American paleontologist. Marsh was one of the preeminent scientists in the field; the discovery or description of dozens of news species and theories on the origins of birds are among his legacies.Born into a modest family, Marsh was able to afford higher education...
in 1877, from remains recovered north of
Morrison, ColoradoThe historic Town of Morrison is a Home Rule Municipality in Jefferson County, Colorado, United States. The population was 430 at the 2000 census...
. These first bones became the
holotypeA holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
of Stegosaurus armatus. Marsh initially believed the remains were from an aquatic turtle-like animal, and the basis for its
scientific nameBinomial nomenclature is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, although they can be based on words from other languages...
, 'roof(ed) lizard' was due to his early belief that the plates lay flat over the animal's back, overlapping like the
shinglesRoof shingles are a roof covering consisting of individual overlapping elements. These elements are typically flat rectangular shapes laid in rows from the bottom edge of the roof up, with each successive higher row overlapping the joints in the row below...
(
tileA tile is a manufactured piece of hard-wearing material such as ceramic, stone, metal, or even glass. Tiles are generally used for covering roofs, floors, walls, showers, or other objects such as tabletops...
s) 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, 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 Stegosaurus recovered from stratigraphic zone 1.
Valid species
| Occurrence data |
S. armatus
- Morrison Formation
The Morrison Formation is a distinctive sequence of Late Jurassic sedimentary rock that is found in the western United States, which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltstone and limestone and is light grey, greenish...
, ColoradoColorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
, WyomingWyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...
, and UtahUtah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...
, U.S.A.The United States of America is a federal constitutional republic comprising fifty states and a federal district...
S. stenops
- Morrison Formation, Colorado, Wyoming, and Utah, U.S.A.
S. longispinus
- Morrison Formation, Wyoming and possibly Utah, U.S.A.
|
Stegosaurus armatus, meaning "armored 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. This species had four horizontal tail spikes and relatively small plates. At 9 meters (30 ft), it was the longest species within the genus Stegosaurus.
- Stegosaurus longispinus, meaning "long-spined roof lizard", was named by Charles W. Gilmore
Charles Whitney Gilmore was an American paleontologist, who named dinosaurs in North America and Mongolia, including the Cretaceous sauropod Alamosaurus, Alectrosaurus, Archaeornithomimus, Bactrosaurus, Brachyceratops, Chirostenotes, Mongolosaurus, Parrosaurus, Pinacosaurus, Styracosaurus and...
and known from one partial skeleton, from the Morrison FormationThe Morrison Formation is a distinctive sequence of Late Jurassic sedimentary rock that is found in the western United States, which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltstone and limestone and is light grey, greenish...
in WyomingWyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...
. Stegosaurus longispinus was notable for its set of four unusually long tail spines. Some consider it a species of KentrosaurusKentrosaurus is a genus of stegosaurid dinosaur from the Late Jurassic of Tanzania. Its fossils have been found only in the Tendaguru Formation of Tanzania, dated to the Kimmeridgian stage, between about 155.7 ± 4 Ma and 150.8 ± 4 Ma . Apparently, all finds belong to one species, K...
. Like S. stenops, it grew to 7 meters (23 ft) in length.
- Stegosaurus stenops, meaning "narrow-faced roof lizard", was named by Marsh in 1887, with the holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
having been collected by Marshal Felch at Garden Park, north of Cañon CityThe City of Cañon City is a Home Rule Municipality that is the county seat and the most populous city of Fremont County, State of Colorado. The United States Census Bureau estimated that the city population was 16,000 in 2005. Cañon City is noted for being the location of nine state and four ...
, ColoradoColorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
, 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 adults and juveniles, one complete skull and four partial skulls. It was shorter than S. armatus, at 7 meters (23 ft).
Susannah Maidment and colleagues in 2008 proposed extensive alterations to the taxonomy of Stegosaurus. They advocated synonymizing S. stenops and S. ungulatus (sometimes considered valid; see below) with S. armatus, and sinking
HesperosaurusHesperosaurus was a herbivorous dinosaur from the Kimmeridgian to Tithonian epochs of the Jurassic period , whose fossils are found in the state of Wyoming in the United States of America...
and
WuerhosaurusWuerhosaurus is a genus of stegosaurid dinosaur from the Early Cretaceous Period of China. As such, it was one of the last genera of stegosaurians known to have existed, since most others lived in the late Jurassic.-Discovery and species:...
into Stegosaurus, with their
type speciesIn biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
becoming Stegosaurus mjosi and Stegosaurus homheni, respectively. They regarded S. longispinus as
dubiousIn zoological nomenclature, a nomen dubium is a scientific name that is of unknown or doubtful application...
. Thus, their conception of Stegosaurus would include three valid species (S. armatus, S. homheni, and S. mjosi) and would range from the
Late JurassicThe Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.2 ± 4.0 to 145.5 ± 4.0 million years ago , which is preserved in Upper Jurassic strata. In European lithostratigraphy, the name "Malm" indicates rocks of Late Jurassic age...
of North America and Europe to the
Early CretaceousThe Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
of Asia.
Nomina dubia (dubious names)
- Stegosaurus affinis, described by Marsh in 1881, is only known from a pubis
In vertebrates, the pubic bone is the ventral and anterior of the three principal bones composing either half of the pelvis.It is covered by a layer of fat, which is covered by the mons pubis....
and is considered a nomen dubiumIn zoological nomenclature, a nomen dubium is a scientific name that is of unknown or doubtful application...
. It is possibly synonymous with S. armatus.
- 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. 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, although others note that the material is non-diagnostic and likely synonymous with S. stenops.
- Stegosaurus sulcatus, meaning "furrowed roof lizard" was described by Marsh in 1887 based on a partial skeleton. It is considered a synonym of S. armatus. 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. Although named by Marsh in 1887 (including the holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
specimen), the disarticulated bones were actually collected in 1879 by Edward Ashley at Como BluffComo Bluff is a long ridge extending east-west, located between the towns of Rock River and Medicine Bow, Wyoming. The ridge is an anticline, formed as a result of compressional geological folding. Three geological formations, the Sundance, the Morrison, and the Cloverly Formations, containing...
, WyomingWyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...
.
- Stegosaurus ungulatus, meaning "hoofed roof lizard", was named by Marsh in 1879, from remains recovered at Como Bluff
Como Bluff is a long ridge extending east-west, located between the towns of Rock River and Medicine Bow, Wyoming. The ridge is an anticline, formed as a result of compressional geological folding. Three geological formations, the Sundance, the Morrison, and the Cloverly Formations, containing...
, WyomingWyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...
. It is known from a few vertebrae and armorArmour in animals is external or superficial protection against attack by predators, formed as part of the body , usually through the hardening of body tissues, outgrowths or secretions. It has therefore mostly developed in 'prey' species...
plates. It might be a juvenile form of S. armatus, although the original material of S. armatus is yet to be fully described. The specimen discovered in Portugal and dating from the upper KimmeridgianIn the geologic timescale, the Kimmeridgian is an age or stage in the Late or Upper Jurassic epoch or series. It spans the time between 155.7 ± 4 Ma and 150.8 ± 4 Ma . The Kimmeridgian follows the Oxfordian and precedes the Tithonian....
-lower TithonianIn the geologic timescale the Tithonian is the latest age of the Late Jurassic epoch or the uppermost stage of the Upper Jurassic series. It spans the time between 150.8 ± 4 Ma and 145.5 ± 4 Ma...
stage has been ascribed to this species.
Reassigned species
- Stegosaurus madagascariensis from Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
is known solely from teeth and was described by Piveteau in 1926. The teeth were variously attributed to a stegosaur, the theropod MajungasaurusMajungasaurus is a genus of abelisaurid theropod dinosaur that lived in Madagascar from 70 to 65.5 million years ago, at the end of the Cretaceous Period. Only one species has been identified...
, 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
HoplitosaurusHoplitosaurus was a genus of armored dinosaur related to Polacanthus. It was named from a partial skeleton found in the ?Barremian-age Lower Cretaceous Lakota Formation of Custer County, South Dakota. It is an obscure genus which has been subject to some misinterpretation of its damaged remains...
in 1902. Stegosaurus priscus, described by Nopcsa in 1911, was reassigned to
LexovisaurusLexovisaurus was one of the first dinosaurs from mid-to-Late Jurassic Europe, 164.7 mya to be discovered. It was a stegosaur. Its fossils have been found in France and northern England....
, and is now the type species of
LoricatosaurusLoricatosaurus is a genus of stegosaurid dinosaur from Callovian-age rocks of England and France...
.
Paleobiology
Stegosaurus was the largest
stegosaurKnown 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....
, reaching up to 12 meters (39 ft) in length and possibly weighing up to 5,000 kilograms (5.5 short tons). However, 7 to 9 meters (23 to 30 feet) was a more usual length.
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, 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. This has been proposed by Bakker and opposed by
CarpenterKenneth Carpenter is a paleontologist. He is the museum director of the USU Eastern Prehistoric Museum and author or co-author of a number of books on dinosaurs and Mesozoic life...
.
Stegosaurus did have very short forelimbs, in relation to its hind legs. Furthermore, within the hindlimbs, the lower section (comprising the
tibiaThe tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....
and
fibula) was short compared with the
femurThe femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...
. 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 kilometers per hour (4–5 mi/hr).
Tracks discovered by Matthew Mossbrucker (Morrison Natural History Museum, Colorado) suggest that Stegosaurus lived in multi-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. Stegosaurus may have preferred drier settings than other common Morrison Formation dinosaurs, such as
AllosaurusAllosaurus is a genus of large theropod dinosaur that lived 155 to 150 million years ago during the late Jurassic period . The name Allosaurus means "different lizard". It is derived from the Greek /allos and /sauros...
,
ApatosaurusApatosaurus , also known by the popular but scientifically deprecated synonym Brontosaurus, is a genus of sauropod dinosaur that lived from about 154 to 150 million years ago, during the Jurassic Period . It was one of the largest land animals that ever existed, with an average length of and a...
,
CamarasaurusCamarasaurus meaning 'chambered lizard', referring to the hollow chambers in its vertebrae was a genus of quadrupedal, herbivorous dinosaurs. It was the most common of the giant sauropods to be found in North America...
, and
DiplodocusDiplodocus , or )is a genus of diplodocid sauropod dinosaur whose fossils were first discovered in 1877 by S. W. Williston. The generic name, coined by Othniel Charles Marsh in 1878, is a Neo-Latin term derived from Greek "double" and "beam", in reference to its double-beamed chevron bones...
.
Plates
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. The largest plates were found over the animal's hips and measured 60 centimeters (2 ft) wide and 60 centimeters tall.
One of the major subjects of books and articles about Stegosaurus is the plate arrangement. The argument has been a major one in the history of dinosaur reconstruction. Four possible plate arrangements have been mooted over the years:
- The plates lay flat along the back, as a shingle
Shingle can refer to:*A flat covering element for a roof, including**Shake , a wooden shingle that is made from split logs**Roof shingle, a roof covering consisting of individual overlapping elements...
-like armor. 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.
- By 1891, Marsh published a more familiar view of Stegosaurus, 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), based on the arrangement of iguana
Iguana is a herbivorous genus of lizard native to tropical areas of Central America and the Caribbean. The genus was first described in 1768 by Austrian naturalist Josephus Nicolaus Laurenti in his book Specimen Medicum, Exhibens Synopsin Reptilium Emendatam cum Experimentis circa Venena...
dorsal spines.
- 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
The dinosaur renaissance was a small-scale scientific revolution that started in the late 1960s, and led to renewed academic and popular interest in dinosaurs...
' in the '70s). (The Stegosaurus in the 1933 film, King KongKing Kong is a fictional character, a giant movie monster resembling a gorilla, that has appeared in several movies since 1933. These include the groundbreaking 1933 movie, the film remakes of 1976 and 2005, as well as various sequels of the first two films...
, has this arrangement.) However, no two plates of identical size and shape have ever been found within the same animal.
- Two rows of alternating plates. By the early 1960s, this had become (and remains) the prevalent idea, mainly because the one Stegosaurus stenops 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.
In the past, some
palaeontologistsPaleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
, notably Robert Bakker, have speculated the plates may have been mobile to some degree, although others disagree. 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 fossilized 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.
The function of the plates has been much debated. Initially thought of as some form of armor, they appear to have been too fragile and ill-placed for defensive purposes, leaving the animal's sides unprotected. More recently, researchers have proposed that they may have helped to control the body temperature of the animal, in a similar way to the sails of the large carnivorous
SpinosaurusSpinosaurus is a genus of theropod dinosaur which lived in what is now North Africa, from the lower Albian to lower Cenomanian stages of the Cretaceous period, about 112 to 97 million years ago. This genus was first known from Egyptian remains discovered in 1912 and described by German...
or of the
pelycosaurThe pelycosaurs are an informal grouping composed of basal or primitive Late Paleozoic synapsid amniotes. Some species were quite large and could grow up to 3 meters or more, although most species were much smaller...
DimetrodonDimetrodon was a predatory synapsid genus that flourished during the Permian period, living between 280–265 million years ago ....
(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. Recent structural comparisons of Stegosaurus plates to
AlligatorAn alligator is a crocodilian in the genus Alligator of the family Alligatoridae. There are two extant alligator species: the American alligator and the Chinese alligator ....
osteoderms seems to support the conclusion that the potential for a thermoregulatory role in the plates of Stegosaurus definitely exists. This theory has been seriously questioned, since its closest relatives, such as Kentrosaurus, had more low
surface areaSurface area is the measure of how much exposed area a solid object has, expressed in square units. Mathematical description of the surface area is considerably more involved than the definition of arc length of a curve. For polyhedra the surface area is the sum of the areas of its faces...
spikes than plates, implying that cooling was not important enough to require specialized 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 or to impress other members of the same species, in some form of sexual
displayDisplay is a form of animal behaviour, linked to survival of the species in various ways. One example of display used by some species can be found in the form of courtship, with the male usually having a striking feature that is distinguished by colour, shape or size, used to attract a female...
, although both male and female specimens seemed to have had them. It has been suggested that, in addition to looking bigger, Stegosaurus pumped blood into its plates causing them to "
blushBlushing refers to the involuntary reddening of a person's face due to embarrassment or emotional stress, though it has been known to come from being lovestruck, or from some kind of romantic stimulation. It is thought that blushing is the result of an overactive sympathetic nervous system...
" which would add to the visual threat display. It is also possible that this blushing could have been used to attract mates. 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. Stegosaurus stenops also had disk-shaped plates on its hips.
Thagomizer (tail spikes)
There has been debate 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, that movement would be limited. Bakker 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. More recently, a study of 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. Additional support for this idea was a punctured tail vertebra of Allosaurus into which a tail spike fit perfectly.
Stegosaurus stenops had four dermal spikes, each about 60–90 centimeters (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.
"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 a Stegosaurus a temporary boost when it was under threat from predators. More recently, it has been argued that this space (also found in sauropods) may have been the location of a
glycogen bodyA glycogen body is an oval structure in the spinal cord of birds that is made of specialized cells that contain large amounts of glycogen. The function of this structure is not known, but it does not seem to be related to the normal function of glycogen in animals, which is the storage of energy...
, a structure in living birds whose function is not definitely known but which is postulated to facilitate the supply of
glycogenGlycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
to the animal's
nervous systemThe nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
.
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 having horizontal wear facets associated with tooth-food contact and a jaw probably capable of only orthal movements.
The stegosaurians must have been successful, as they became speciose and geographically widely distributed, in the late Jurassic. Palaeontologists believe it would have eaten plants such as mosses, ferns, horsetails, cycads and conifers or fruits and swallowed
gastrolithA gastrolith, also called a stomach stone or gizzard stones, is a rock held inside a gastrointestinal tract. Gastroliths are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. The grain size depends upon the size of the animal and the gastrolith's...
s to aid food processing (due to the lack of chewing ability), in the same manner used by modern birds and crocodiles. Low-level browsing on grasses, seen in modern
mammalMammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ian 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 behavior 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 meter above the ground. 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 meters (20 ft) above the ground.
A detailed computer analysis of the biomechanics of Stegosauruss 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 Stegosaurus were 140.1 N, 183.7 N, and 275 N (for anterior, middle and posterior teeth, respectively), which means that its bite force was less than half that of a
Labrador retrieverThe Labrador Retriever is one of several kinds of retriever, a type of gun dog. A breed characteristic is webbed paws for swimming, useful for the breed's original purpose of retrieving fishing nets. The Labrador is the most popular breed of dog by registered ownership in Canada, the United...
. This indicates that 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.
Popular culture
One of the most recognizable of all dinosaurs, Stegosaurus has been depicted on film, in cartoons, comics, as children's toys, and was even declared the
State Dinosaur of
ColoradoColorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
in 1982.
External links