Why Did Stegosaurus Have Plates on its Back?

stegosaurus
What was the function of Stegosaurus' plates? (Flickr).

If it weren't for its pointed, symmetrical, vaguely threatening-looking plates, Stegosaurus would be a completely unremarkable dinosaur--a bland, small-brained, second-tier plant eater like Iguanodon. Fortunately for its place in the popular imagination, though, the late Jurassic Stegosaurus possessed one of the most distinctive "do"'s in the animal kingdom, those double rows of tough, bony, roughly triangular plates that lined this dinosaur's back and neck.

(See also 10 Facts About Stegosaurus)

It has taken a long time, though, for these plates to be assigned their proper position and function--or, at least, to what most modern dinosaur experts today believe to be their proper position and function. In 1877, the famous American paleontologist Othniel C. Marsh coined the name Stegosaurus, Greek for "roof lizard," because he believed that this dinosaur's plates lay flat along the top of its torso, much like the armor of a crocodile. (In fact, Marsh was initially under the impression that he was dealing with a giant prehistoric turtle!)

A few years after this blunder--upon realizing that Stegosaurus was in fact a dinosaur and not a turtle--Marsh speculated that its triangular plates lined up sequentially, one after the other, across its back. It wasn't until the 1960's and 1970's that further fossil evidence was uncovered indicating that the plates of Stegosaurus were actually arranged in two alternating, offset rows.

Today, virtually all modern reconstructions use this arrangement, with some variation in how far the plates are tilted toward one side or another.

What Was the Purpose of Stegosaurus' Plates?

Unless further evidence comes to light--and Stegosaurus is already extremely well-represented in the fossil record, so any surprises seem unlikely--paleontologists agree about how Stegosaurus "wore" its plates.

The structure of these plates is also uncontroversial; basically, they were giant-sized versions of the "osteoderms" (protrusions of bony skin) that are found on modern crocodiles, and may (or may not) have been covered in a layer of sensitive skin. Crucially, the plates of Stegosaurus weren't directly attached to this dinosaur's backbone, but rather to its thick epidermis, which afforded them more flexibility and a wider range of motion.

So what was the function of Stegosaurus' plates? There are a few current theories:

1) The plates were a sexually selected characteristic--that is, males with bigger, pointier plates were more attractive to females during mating season, or vice versa. In other words, the plates of a male Stegosaurus were roughly analogous to the tail of a male peacock! (To date, unfortunately, we have no evidence that the size of Stegosaurus plates varied among individuals or between sexes.)

2) The plates were a temperature-regulation device. If Stegosaurus was in fact cold-blooded (as most plant-eating dinosaurs of the Mesozoic Era presumably were), it might have used its plates to soak up light from the sun during the day and dissipate extra body heat at night. A 1986 study concluded that the outer layers of Stegosaurus' plates were thickly lined with blood vessels, which helps support this theory.

3) The plates made Stegosaurus appear bigger to (presumably near-sighted) meat-eating dinosaurs like the contemporary Allosaurus. Stegosaurus adults with bigger plates would have been particularly unattractive to predators, and thus this trait was passed on to successive generations. This may have been an especially important consideration for newborns and juveniles, as an adult Stegosaurus would have been quite a mouthful, with or without plates!

4) The plates served an active defensive function, especially since they were only loosely anchored to this dinosaur's skin. When Stegosaurus listed to one side in response to an attack, the sharp edges of the plates would tilt toward its antagonist, which would presumably look for a more tractable meal elsewhere. Not many scientists subscribe to this theory, which has been advanced by the maverick paleontologist Robert Bakker.

5) The plates were covered with a thin membrane of skin, and were capable of changing color (say, to bright pink or red). This Stegosaurus "blush" might have served a sexual function, or it may have been used to signal other members of the herd about approaching danger or nearby food sources. The plates' high degree of vascularization, mentioned above in reference to temperature regulation, also supports this theory.

Stegosaurus Plates - The Mystery Persists

So what is the most likely answer? The fact is that evolution has a way of adapting specific anatomical features to multiple functions, so it may well be that the plates of Stegosaurus were literally all of the above: a sexually selected characteristic, a means to intimidate or defend against predators, and a temperature-regulation device. On the whole, though, the bulk of the evidence points primarily to a sexual/signaling function, as is the case with many otherwise puzzling dinosaur features, such as the long necks of sauropods, the huge frills of ceratopsians, and the elaborate crests of hadrosaurs.