What purpose did the plates serve?
Stegosaurus is a kind of dinosaur that had plates along its spine. Researchers all over the world are divided as to what purpose the plates served. Some paleontologists believe that they were used for defense. However, of late this theory has been disputed as evidence through research shows that most probably stegosaurus used the plates to regulate body heat and/or they were used just as ornamental displays.
Stegosaurus was an herbivore that lived around 200 to 145 million years ago in the late Jurassic period. The dinosaur was distinctive with its 17 plates running along its spine, which now the scientists know were bony structures that grew out from under the skin.
In the late 19th century, a paleontologist concluded that the bony plates of a stegosaurus were used for defense and protection. However, this has been disputed because the plates were not big enough to cover the sides of the dinosaur and this means that the sides would have been unprotected.
However, modern-day paleontologists took the fossils of stegosaurus' plates and conducted some research. They found that the plates had large blood vessels running right through them. This led paleontologists to believe that the plates, in fact, were used to regulate the temperature of the blood rather than helping to protect them.
This second conclusion was also not accepted as the blood vessels in the plates did not branch out anywhere in particular. In addition, there some stegosaurus that did not plates. Instead they had spikes which were small so they could not have a thermoregulatory role.
At the moment, a verdict on the role of the plates is still open and scientists are looking for more clues. However, the thermoregulatory function is seen as a secondary function rather than a primary one.
(Source: http://EzineArticles.com/1740195 )
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.
(Source: http://en.wikipedia.org/wiki/Stegosaurus )