Old Earth Ministries Online Dinosaur Curriculum
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Lesson 49 - Stegosaurus, Part 2
Stegosaurus was the largest stegosaur, 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 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.
|S. stenops adult and juvenile mounted as if under attack from Allosaurus, Denver Museum of Nature and Science (Picture Source)|
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 67 kilometers per hour (45 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 Allosaurus, Apatosaurus, Camarasaurus, and Diplodocus.
The most recognizable features of Stegosaurus are its dermal plates, which consisted of 17
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:
In the past, some palaeontologists, 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 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. Recent structural comparisons of Stegosaurus plates to 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 area 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 display, 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 "blush" which would add to the visual threat display. 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
Stegosaurus stenops had four dermal spikes, each about 6090 centimeters (23 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.
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 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.
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 gastroliths 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 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 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 Stegosaurus's 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 retriever. 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.
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 Colorado in 1982. To learn more, see the Wikipedia article Stegosaurus in popular culture.
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