Stegosaurus: Beast of the Week

This week we will be looking at one of the most instantly recognizable dinosaurs.  Let's check out Stegosaurus! 

Stegosaurus was a plant-eating dinosaur that lived in what is now the Western United States, including Wyoming and Colorado, during the Late Jurassic Period, between 155 to 150 million years ago.  Some Stegosaurus remains have also been found in Portugal.  The genus name, Stegosaurus, actually translates to "roofed reptile" because its iconic plates were at first believed by scientists to have laid flat on the animal's back like shingles on a roof.  As adults, most Stegosaurus hovered in size at around twenty to twenty five feet, but some individuals could have grown to about thirty feet long from beak to tail.  When alive, it would have coexisted with other famous dinosaurs, including Apatosaurus, Brontosaurus, Camarasaurus, Ceratosaurus, Torvosaurus, and Allosaurus.

Life reconstruction in watercolors of Stegosaurus stenops by Christopher DiPiazza.  Since its center of gravity was over the hips, it may have been easier for Stegosaurus to rear up on its hind legs than most quadrupedal dinosaurs, especially if it used its strong tail as a third support.

Stegosaurus is most well-known for its plates, which varied slightly between species, but more or less were diamond-shaped, depending on the species.  The speceis, Stegosaurus ungulatus, had more narrow and pointier plates.  Stegosaurus stenops had plates that were wider and more rounded.  On average, Stegosaurus possessed seventeen to nineteen of these impressive plates running down its back.  The evolutionary function of these bony structures remains a mystery but paleontologists have come up with a few ideas.  When first discovered, it was believed that these plates served as armor, but it was soon realized that in life, they were arranged sitting erect on the animal's back which wouldn't do much good for physical protection from predators.  Furthermore, the plates of a Stegosaurus are extremely thin and actually quite delicate!  A predator, like Allosaurus or Torvosaurus, would surely have had no problem biting right through them.

Stegosaurus stenops skeleton on display at the American Museum of Natural History in New York.

A second, possably more likely idea for these plates, was to help control Stegosaurus' body temperature.  Upon close inspection, they were found to have had many blood vessels in them in life.  If a Stegosaurus wanted to warm up in the morning, the blood in these plates could have been heated by the sun and then circulated to the rest of the body.  If the Stegosaurus wanted to cool off in the afternoon, the heated blood would cool off slightly when closer to the outside air while in the plates, and like before, be circulated to the rest of the body, cooling the animal down.  Many other animals use this method of temperature control, by using large ears like elephants and rabbits, or reptiles with sails or extendable ribs.  It is plausible that Stegosaurus' plates were also display adaptations, having possibly been brightly colored to impress potential mates or intimidate rivals.  

Stegosaurus stenops skeletal mount on display at the London Museum of Natural History.

The position of Stegosaurus's plates has also been the subject of some debate over the years.  Like I stated earlier, originally the plates were believed to have laid flat on the dinosaur's back, like shingles on a roof, for protection.  It was later realized that these plates belonged erect  growing off of the back.  The first version of this idea showed two rows of paired plates but the more recent idea is that the plates were still in two rows, but alternating, not parallel.  Other members of the stegosaurid family, like Kentrosaurus, for instance, did actually have parallel paired plates, however.

Stegosaurus had actual protective armor too!  Right under the chin, extending down the throat of a well-preserved Stegosaurus specimen, many small pieces of bony armor were discovered.  These small chunks of armor would have been embedded in the dinosaur's neck skin and acted like chain mail, protecting the throat from biting predators.  This neck armor is called gular armor. ("Gular" means throat.)  

The actual skull of Stegosaurus was extremely small in comparison to the rest of the body.  The skull of Stegosaurus was narrow, and was tipped with a short beak, which the dinosaur used to clip vegetation.  This food then would have been processed with Stegosaurus' small teeth further down into the mouth, which were also adapted for clipping and shearing vegetation.

Skeletal mount of Stegosaurus showcasing the gular armor.

At the opposite end of the body, at the very tip of the tail, Stegosaurus possessed four long spikes.  Stegosaurus would have been able to swing its tail around with deadly accuracy to keep any potential predators at bay if it was ever attacked.  Since Stegosaurus' hind limbs were so much longer than its front limbs, its center of gravity was near its hips rather than closer to the rib cage like it is with many other quadrupedal dinosaurs.  This would have enabled Stegosaurus to use its front limbs to help it rotate its body around more rapidly than one would expect from an animal of that size, greatly increasing its tail-swinging range.  This makes sense since potential predators would likely be aiming for its head and trying to specifically avoid its tail.

Tail of Stegosaurus.

 The pelvis of Stegosaurus also possessed an odd hollow section which is still somewhat of a mystery to scientists.  At first, this area was believed to be the site of a swelling of nerves, which could have acted as a "second brain" to control the animal's back half, since the actual brain was so small.  Scientists now know this is untrue and that this area appears to be for storing glycogen, a kind of molecule which animals can use for energy.  Glycogen bodies, similar to the one found in Stegosaurus', pelvis can also be found in the hips of modern birds and other reptiles.

That is all for this week!  As always feel free to comment below or on our facebook page!  Have a particular beast you would like to see painted and reviewed?  Let me know and I will add it to the list!

References

Buchholz (née Giffin) EB (1990). "Gross Spinal Anatomy and Limb Use in Living and Fossil Reptiles". Paleobiology 16: 448–58.

Buffrénil (1986). "Growth and Function of Stegosaurus Plates". Paleobiology 12: 459–73.

Carpenter K, Sanders F, McWhinney L, Wood L (2005). "Evidence for predator-prey relationships: Examples for Allosaurus and Stegosaurus.". In Carpenter, Kenneth(ed). The Carnivorous Dinosaurs. Indiana University Press. pp. 325–50. ISBN 0-253-34539-1.

Czerkas SA (1987). "A Reevaluation of the Plate Arrangement on Stegosaurus stenops". In Czerkas SJ, Olson EC. Dinosaurs Past & Present, Vol 2. University of Washington Press, Seattle. pp. 82–99. ISBN.

Lull, R. S. "The Armor of Stegosaurus." American Journal of Science S4-29.171 (1910): 201-10. Web. 

Saitta ET (2015) Evidence for Sexual Dimorphism in the Plated Dinosaur Stegosaurus mjosi (Ornithischia, Stegosauria) from the Morrison Formation (Upper Jurassic) of Western USA. PLoS ONE 10(4): e0123503. doi:10.1371/journal.pone.0123503