The Purpose of the Plates of Stegosaurus (Stegosaurus Week)

The horns and frills of Triceratops.  The tube-like crest of Parasaurolophus.  The two crests of bone on Dilophosaurus.  The sail on Spinosaurus.  What function do these various bells and whistles that adorned these so-called "Terrible Lizards" serve?  For years, most paleontologists assumed that they were for the sole purpose of combat, be it against predators, or the inter-specific variety.  But now, more and more paleontologists are looking to birds to answer the question of functionality when it comes to these bony dinosaurian protuberances.

But what, specifically, about birds is it that is helping paleontologists figure out the purpose of these structures?  It all boils down to an interesting phenomenon called "sexual selection."  Most people are familiar with the term "natural selection."  Popularized by Charles Darwin, natural selection essentially states that animals that are unfit to survive and reproduce in a given environment will die, and will be unable to add their genes to the genepool.  (Certainly an oversimplified definition, but you get the picture.)  Sexual selection, on the other hand, is a mode of natural selection, and introduced by Charles Darwin, as well.  Sexual selection states that some individuals in a given population will be more likely to breed than other individuals will because they will stand out above the rest of the population.  There are many ways of doing this, and birds are but one example.  Horns and antlers are one instance: typically, if an animal has larger horns or antlers, they will be able to not only fend off predators better (i.e. natural selection), but they will be more likely to be able to fend off other males, and be more likely to be picked for the females (i.e. sexual selection).  In many animals, form overcomes functionality in this endless quest for a mate, especially on insular (or island) populations.  One of my favorite examples of this is the birds of paradise from New Guinea, as you can see in the video below.

That's all well and good, but how does that apply to the dinosaurs that we were talking about above?  Well, for years, paleontologists assumed that dinosaurs like Triceratops and its relatives were using their horns and frills to fight off predators.  Well, for Triceratops, that makes sense: with forward-facing horns and a two-inch thick frill, fighting off Tyrannosaurus doesn't seem that far out of the realm of possibility.  However, upon examination of many of the other relatives of Triceratops (collectively called ceratopsian dinosaurs), you can see that, perhaps, not all of these frills and horns evolved to fight off predators.  Below we have just one example.  The picture you see is of a skull that below belongs to a ceratopsian dinosaur called Einiosaurus.  As you can see, it does not seem anywhere near as well equipped for fighting off predators as Triceratops does.  For example, its frill has a pair of massive holes in it.  Furthermore, of its three horns, one points downwards, and two point towards the sky at about a forty-five degree angle.  Unless Einiosaurus was being attacked by giant woodchuck-like, burrowing dinosaurs, or being dive-bombed by Tyrannosaurs in F-14s (as seen in Calvin and Hobbes!), it is difficult to see how Einiosaurus might have defended itself against its predators using its frill and horns.  Another analogy I like to make is this: if you are a knight going into battle, you don't necessarily want to have a pair of giant holes in your shield, and your sword bent and pointing towards the ground.

So how does this all tie in to Stegosaurus?  Well, a same sort of discussion has centered around Stegosaurus for many years.  Were the plates used for defense?  Or were they used for something else?  First let's address the idea of defense.  IN THE PREVIOUS POST, we discussed the thagomizer, the group of tail spikes, on the rear end of Stegosaurus.  These tail spikes were almost certainly used to fend off enemies, and seemed to have done a very good job, too.  So, if you think about it, if you were to cover a stegosaur in these spikes, it would be almost impervious to attack, right?  Well, what's interesting is that, early in stegosaur evolution, many of these animals actually did have a lot more spikes than Stegosaurus did.  As a matter of fact, the plates of Stegosaurus are nothing more than heavily modified spikes!  Below, we have a few more primitive stegosaurs, all of whom demonstrate the fact that, prior to Stegosaurus, many of the plates were actually spikes!

So if the spikes were better than plates were at defending an animal against predators (which is the only logical conclusion that I think people can draw from the data at hand), then why did some of the stegosaurs change?  For many years, paleontologists thought that they had a pair of answers to this interesting dilemma.  The first was the idea that perhaps Stegosaurus used its plates as a thermoregulaton device.  If the animal was too cold, then it could turn its body so that its plates faced the sun, maximizing its surface area that was facing the sun, and enabling it to warm up quicker.  The reverse would have also worked: when it became to hot, Stegosaurus could turn perpendicular to the sun, minimizing the surface area that was absorbing the sun.  Another theory was that Stegosaurus could flush blood to the plates, turning them a brighter color.  This could have either frightened off enemies, or instead it could have been used to attract a mate.

These two ideas seem fairly good in theory: however, much like the skull of Einiosaurus, there are a few massive holes in this logic.  If Stegosaurus used its plates as a thermoregulatory device, why do close relatives of Stegosaurus have very different plate shapes, or sometimes fewer plates altogether?  If there was one design that these animals used to warm up or cool down, one would imagine they would all converge on the same design.  But they didn't, which casts some serious doubt on the whole thermoregulatory idea.

There are two theories that seem to hold the most water today.  The first one has the same general idea that the "flushing the plates full of blood" idea has: make yourself more noticeable, as these plates were very impressive looking structures.  And, since they alternated down the back (SEE THE FIRST STEGOSAURUS WEEK POST HERE), then a side-on look of Stegosaurus would have been a very impressive sight, indeed!  Other stegosaurs of the opposite sex would undoubtedly think so, and these plates probably served a large role in attracting a mate!  Predators might have thought that the side-on view was impressive, too, and this might have caused them to think twice about attacking Stegosaurus.  It also might have caused other members of the same species to back down, too, in cases where inter-specific combat might have otherwise come into play.  As Matt Mossbrucker, the director and curator at the Morrison Natural History Museum likes to say, "think a skinny kid in a puffy coat."

Finally, the plates might have helped stegosaurs to differentiate from one another.  This is a tactic often used in animals today (again, the birds of paradise and many other birds: see the last paragraph of our post on the cichlids of the Great African Rift Lakes HERE), and is thought to have been a tactic used by many extinct animals, as well.  For example, the various horns and frills of the ceratopsian dinosaurs (like Triceratops and Einiosaurus that we were talking about before) are now thought by many paleontologists to have been used to tell each individual species apart, and its possible that that is what the stegosaurs were doing, too. 

Want to learn more about Stegosaurus?  Well, check out the Homebase for Stegosaurus Week HERE to partake in more of the festivities!