Stegosaurus lived in the Late Jurassic period around 155 to 145 million years ago. It is among the most easily recognized dinosaurs, due to the distinctive double row of kite-shaped plates on its back and the long spikes on its tail. The armor was necessary as it lived with such meat-eating theropods such as Allosaurus and Ceratosaurus. The use of the plates are still in dispute today. Some say it`s for mating porposes, others to ward off predators.
The quadrupedal Stegosaurus is one of the most easily identifiable dinosaur genera, due to the distinctive double row of kite-shaped plates rising vertically along the rounded back and the two pairs of long spikes extending horizontally near the end of the tail. Although large animals at up to 9 metres (30 ft) in length, the various species of Stegosaurus were dwarfed by their contemporaries, the giant sauropods. Some form of armor appears to have been necessary, as Stegosaurus species coexisted with large predatory theropod dinosaurs, such as Allosaurus and Ceratosaurus.
The hind feet each had three short toes, while each forefoot had five toes; only the inner two toes had a blunt hoof. The phalangeal formula is 2-2-2-2-1, meaning that the innermost finger of the forelimb has two bones, the next has two, etc. 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 skull was small in proportion to the body. It had a small antorbital fenestra, the hole between the nose and eye common to most archosaurs, 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 Marsh 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 grams (2.8 oz) contributed to the popular old idea that all dinosaurs were unintelligent, an idea now largely rejected. Actual brain anatomy in Stegosaurus is poorly known, but the brain itself was however small even for a dinosaur, fitting well with a slow herbivorous lifestyle and limited behavioural complexity.
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.
Discovery and speciesEdit
Several different Stegosaurus species have been found.
- Stegosaurus armatus: This was the first type of Stegosaurus to be found. Over thirty different skeletons have been discovered by scientists. This type had four tail spikes and small plates. At 9 meters (30 ft), it was the longest species of Stegosaurus.
- Stegosaurus stenops: Named by Marsh in 1887, it was discovered near Cañon City, Colorado, in 1886. This is the best known species of Stegosaurus, mainly because its fossils make at least one complete skeleton. It had large, broad plates and four tail spikes. S. 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 m (23 ft).
- S. ungulatus: Named by Marsh in 1879 from remains recovered at Como Bluff, Wyoming, it is only known from a few backbones and armor plates. It is probably the same as S. armatus.
- S. sulcatus: This is another partial skeleton. It is probably the same as S. armatus.
- S. duplex: This animal is probably the same as S. armatus. It was also named by Marsh in 1887,. Its fossils were found in 1879 by Edward Ashley at Como Bluff, Wyoming.
- ?S. seeleyanus: Probably the same as S. armatus.
- ?S. (Diracodon) laticeps: Named by Marsh in 1881 from some jawbone fragments.
- ^ 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.
- ^ a b Marsh OC (1887). "Principal characters of American Jurassic dinosaurs, part IX. The skull and dermal armour of Stegosaurus". American Journal of Science 3 (34): 413 – 417.
- ^ Marsh OC (1879). "Notice of new Jurassic reptiles". American Journal of Science 3 (18): 501-505.
- ^ Marsh OC (1881). "Principal characters of American Jurassic dinosaurs, part V". American Journal of Science 3 (21): 417 – 423.