Sunday, December 11, 2016

Stegosaurus stenops by cheungchungtat

Stegosaurus is a genus of armored dinosaur. Fossils of this genus date to the Late Jurassic period, where they are found in Kimmeridgian to early Tithonian aged strata, between 155 and 150 million years ago, in the western United States and Portugal. Several species have been classified in the upper Morrison Formation of the western U.S, though only three are universally recognized; S. stenopsS. ungulatus and S. sulcatus. The remains of over 80 individual animals of this genus have been found. Stegosaurus would have lived alongside dinosaurs such as ApatosaurusDiplodocusBrachiosaurusAllosaurus, and Ceratosaurus; the latter two may have been predators of it.

As the archetypal thyreophoran, Stegosaurus is one of the best-known dinosaurs, and has been featured in film, postal stamps, and many other types of media.

Stegosaurus was the first-named genus of the family Stegosauridae. It is the type genus that gives its name to the family. The Stegosauridae are one of two families within the infraorder Stegosauria, with the other being the Huayangosauridae. The infraorder Stegosauria lies within the Thyreophora, or armored 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.

Stegosaurus by Prehistoric Wildlife

These were a large, heavily built, herbivorous quadrupeds with rounded backs, short fore limbs, long hind limbs, and tails held high in the air. Due to their distinctive combination of broad, upright plates and tail tipped with spikes, Stegosaurus is one of the most recognizable kinds of dinosaur. The function of this array of plates and spikes has been the subject of much speculation among scientists. Today, it is generally agreed that their spikes were most likely used for defense against predators, while their plates may have been used primarily for display, and secondarily for thermoregulatory functions. Stegosaurus had a relatively low brain-to-body mass ratio. It had a short neck and a small head, meaning it most likely ate low-lying bushes and shrubs. One species, Stegosaurus ungulatus, is the largest known of all the stegosaurians (bigger than related dinosaurs such as Kentrosaurus and Huayangosaurus).

Stegosaurus remains were first identified during the “Bone Wars” by Othniel Charles Marsh. The first known skeletons were fragmentary and the bones were scattered, and it would be many years before the true appearance of these animals, including their posture and plate arrangement, became well understood. The name Stegosaurus means “roof lizard” or “covered lizard”, in reference to its bony plates. Despite its popularity in books and film, mounted skeletons of Stegosaurusdid not become a staple of major natural history museums until the mid-20th century, and many museums have had to assemble composite displays from several different specimens due to a lack of complete skeletons.

S. stenops skull cast, Natural History Museum of Utah. Author: Daderot

The most recognizable features of Stegosaurus are its dermal plates, which consisted of between 17 and 22 separate plates and flat spines. 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 hips and could measure up to 60 cm (2.0 ft) wide and 60 cm tall.

In a 2010 review of Stegosaurus species, Peter Galton suggested that the arrangement of the plates on the back may have varied between species, and that the pattern of plates as viewed in profile may have been important for species recognition. Galton noted that the plates in S. stenops have been found articulated in two staggered rows, rather than paired. Fewer S. ungulatus plates have been found, and none articulated, making the arrangement in this species more difficult to determine. However, the type specimen of S. ungulatus preserves two flattened spine-like plates from the tail that are nearly identical in shape and size, but are mirror images of each other, suggesting that at least these were arranged in pairs.

Many of the plates are manifestly chiral and no two plates of the same size and shape have been found for an individual; however plates have been correlated between individuals.

Back plate cast, Museum of the Rockies. Photo by Tim Evanson

Stegosaurus, one of the many dinosaurs first collected and described in the Bone Wars, was originally named by Othniel Charles Marsh in 1877, from remains recovered north of Morrison, Colorado. These first bones became the holotype of Stegosaurus armatus. Marsh initially believed the remains were from an aquatic turtle-like animal, and the basis for its scientific name, ‘roof(ed) lizard’ was due to his early 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 from 1877 to 1897. In 1878, Edward Drinker Cope named Hypsirhophus discurus, as another stegosaurian based on fragmentary fossils specimens from Cope’s Quarry 3 near the “Cope’s Nipple” site in Garden Park, Colorado. Many later researchers have considered Hypsirhophus to be a synonym of Stegosaurus, though Peter Galton (2010) suggested that it is distinct based on differences in the vertebrae.

Stegosaurus strikes out by palaeoartist Robert Nicholls This artist’s reconstruction shows a Stegosaurus striking a powerful blow to one of its predators, a Ceratosaurus.