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Stegosaurus

Stegosaurus, meaning “roof lizard,” is a genus of stegosaurid armored dinosaur from the Kimmeridgian to early Tithonian stages of the Late Jurassic Period (145 mya) in what is now western North America. It was first discovered from a bone bed in the Morrison Formation in Morrison, Colorado, and was first described in 1877 by Othniel Charles Marsh. These first fossils became known as the type species S. armatus.

Marsh initially believed the remains were from an aquatic turtle-like animal, which gave Marsh the idea to name it Stegosaurus (or roofed lizard); he believed the plates lay flat over the animal’s back, overlapping like shingles on a roof. For years, several subsequent fossil discoveries were considered stegosaurid bones, but were later reclassified. A confirmed stegosaurus specimen was discovered in Portugal in 2006, the first evidence that the animal was present in Europe as well.

The true origins of stegosaurus remains uncertain, as few fossils of primitive stegosaurs and their ancestors are known. Some stegosaurid fossils have been found in the lower Morrison Formation, indicating some stegosaurids existed several million years before Stegosaurus appeared itself, including Hesperosaurus from the early Kimmeridgian. The earliest stegosaurid, Lexovisaurus, is known from the Oxford Clay Formation of England and France, giving it an age of early to middle Callovian.

The extremely primitive stegosaurid genus Huayangosaurus from the Middle Jurassic of China (165 mya) predates Stegosaurus by some 20 million years. Even earlier was Scelidosaurus from the Early Jurassic Period of England (190 mya). The earliest known primitive stegosaurid was Scutellosaurus from Arizona (195-190 mya). Another possible bipedal primitive stegosaurid from around 195 million years ago was found in France.

The type species, S. armatus, is known from two partial skeletons, two partial skulls and at least thirty fragmentary individuals. At 30 feet in length, it was the longest species of Stegosaurus. There are two other known species of Stegosaurus: S. longispinus, known from one partial skeleton and was 23 feet in length; and S. stenops, known from one complete skeleton, also 23 feet long. Several other described species have been either reassigned to other genera or have been classified as dubious synonyms of the three known Stegosaurus species.

Due to its distinctive tail spikes and plates, Stegosaurus is one of the most recognizable dinosaurs, along with Tyrannosaurus, Triceratops, and Velociraptor. Stegosaurus is a large, heavily built herbivorous quadruped. It had a unique 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 spikes and plates has been the subject of speculation for decades. Both the spikes and plates were most likely used as defense mechanisms, however, the plates have also been proposed to play a part with thermoregulatory functions.

The most recognizable of the dinosaur’s plates, were the dermal plates, which consisted of 17 separate flat plates. These were highly modified bony scales similar to the scales of modern crocodiles. These dermal plates were not directly attached to the skeleton of Stegosaurus, instead arose from the skin. The largest of these bones were found over the hips and measured 2 feet wide and 2 feet tall.

There has been some debate on the arrangements of the plates, most notably by paleontologist Robert Bakker, who speculated that the plates were mobile to some degree. He suggested the plates could be flipped from one side to the other in order to present a predator with an array of spikes and blades that would impede an attacker’s ability to make an effective strike. Bakker suggested the possibly horned plates naturally sagged to the sides of the beast, covering the width of the animal along the spine, protecting from a side attack.

The function of the plates has been debated by others as well, with some paleontologists believing that the plates were actually too fragile to be used as a natural armor, and their placement were incorrect for defensive purposes, leaving the sides unprotected. Some researchers have recently speculated that the plates were used for thermoregulation (heat control). They noted these plates had blood vessels running through grooves and air flowing around the plates would have cooled the blood. Recent structural comparisons of Stegosaurus to alligator osteoderms seems to support the thermoregulatory theory.

But this theory has been questioned as well, since its closest relatives, such as Kentrosaurus, had more low surface area spikes than plates, implying that cooling was not important enough to require specialized structural formations such as plates.

Others believe the plates may have served to increase the apparent height of the animal, in order to intimidate any would-be attackers or to impress other members of the same species, such as a sexual display. However, since both male and female specimens seemed to have had these plates, the sexual display theory is not generally accepted. The blood vessels may have also played a part in visual display, with researchers theorizing that Stegosaurus would have pumped blood into its plates causing them to “blush,” perhaps making them look bigger. A 2005 study supports this theory.

There has also been debate about whether the tail spikes were used for display only, or perhaps used as a defensive weapon. Bakker noted the tail was likely more flexible than that of other dinosaurs, as it lacked ossified tendons, lending some credence to the notion of the tail being used as a weapon. However, this idea has not garnered much support after studies showed that the plates overlap so many tail vertebrae, that any tail movement would have been limited.

Bakker also suggested that Stegosaurus could have maneuvered its rear easily, keeping its hindlimbs stationary and pushing off with its powerful forelimbs, allowing it to nimbly swivel and deal a punishing blow with its tail.  A recent study has showed a high incidence of trauma-related damage to the tail of some specimens, lending weight to the idea that the tail spikes were indeed used in combat. Additional support for this idea was a punctured tail vertebra of Allosaurus into which a Stegosaurus tail spike fit perfectly.

Stegosaurus was the largest stegosaurid, reaching lengths up to 39 feet and weighing in at an estimated 5.5 tons. However, most researchers place Stegosaurus closer to 25 to 30 feet long. Stegosaurus was first considered to have been bipedal by Marsh. However, in 1891, Marsh concluded it was quadrupedal due to its heavy build. Despite being a quadruped, some believe it may have been able to rear up on its hind legs, using its tail for balance, and forage on higher foliage.

The long and narrow skull of Stegosaurus was small in proportion to the body. The low position of the skull suggests that Stegosaurus may have been more suited for browsing on low-lying vegetation. This interpretation is supported by the absence of front teeth, replaced by a horny beak. The placement of the jaws suggests that this animal had cheeks to keep food in its mouth while chewing.

A computerized analysis of the biomechanics of Stegosaurus’s feeding behavior was conducted in 2010, using two different 3D models of Stegosaurus teeth. Bite force was also calculated using the models and the known skull proportions of the animal as well as simulated tree branches of different size and hardness. The analysis suggested that Stegosaurus’s bite force was less than half that of a Labrador Retriever. This indicates that Stegosaurus could have easily bitten through smaller green branches, but would have had difficulty with anything over a half inch 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 the study.

Despite the animal’s overall size, its braincase was very small, about the size of a dog’s, and perhaps the smallest among dinosaurs. The fact that an animal weighing more than 4 tons having a brain weighing less than 3 ounces contributed to the popular belief that dinosaurs were unintelligent creatures, an idea that is no longer supported.

Stegosaurus is the State Dinosaur of Colorado (1982).

Image Caption: Reconstruction of a Stegosaurus skeleton in the Senckenberg Museum in Frankfurt am Main. Credit: EvaK/Wikipedia(CC BY-SA 2.5)

Stegosaurus


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