The Curious Case of Stegosaurus’ Plates

There are many questions in the world that may never be answered. What existed before the Big Bang? Is Bigfoot out there? What was the purpose of Stegosaurus’ plates?

Ever since Stegosaurus was discovered over 100 years ago, paleontologists have tried to decipher the function of its infamous back plates. Were they for defense? For attraction? A way for Stegosaurus to make itself look taller, perhaps? Though many solutions have been proposed, no consensus has been achieved to this day.

This uncertainty is due in part to the uniqueness of Stegosaurus’ plates amongst the stegosaur family. Most stegosaurids, such as the African Kentrosaurus or the Chinese Huayangosaurus, possessed dual rows of triangular plates along their backs that ran parallel to one another. These plates were relatively small and may have acted as a defense mechanism, repelling predators with their varying levels of sharpness. Additionally, most stegosaurids also possessed a pair of elongated shoulder spines, making them some of the most well protected animals of the Jurassic.

Stegosaurus was different, however. Its plates are quite broad and shaped like a leaf, contradicting the triangular appearance of its close ancestors. These plates were far larger than those of its relatives too, with some reaching over a meter in length. While the plates were arranged in two rows like other stegosaurids, they would have alternated positions, giving Stegosaurus a staggered appearance. On top of the plates, Stegosaurus also lacked the shoulder spike common in the family, making it the most distinct member of the bunch.

With the unique plate size, orientation and shape, it’s clear that Stegosaurus probably didn’t use them like its relatives did. So how were they used? The original hypothesis was that they were used as a defensive mechanism, acting as armour to protect Stegosaurus from large predators like Allosaurus or Ceratosaurus. This theory doesn’t hold up for a number of reasons. The plates are rather thin and likely wouldn’t have given that much protection. More importantly, the plates only lay atop the back of Stegosaurus, thus leaving its legs and belly completely unguarded. Couple this with the fact that the plates were filled with blood, as demonstrated by imprints of blood vessels on the fossils, and the theory of defense goes straight out the window.

The presence of blood vessels in the plates has spawned one interesting theory about their functionality: were they used for heating and cooling? Analysis by James Farlow and Yale University during the 1970’s showed that they could, at the very least. Using metal cylinders crafted into the shape of Stegosaurus, Farlow showed that Stegosaurus could pump blood to its plates both when hot as a way to cool down, and as a way to absorb heat from the sun when cold. While this theory is entirely possible, it doesn’t hold up either. Stegosaurus would have had layers of keratin surrounding its plates for protection, thus making heat regulation far more difficult. More importantly, while the plates of Stegosaurus may have been able to regulate heat, no other stegosaurid had such a problem. In fact, no other large herbivorous dinosaur, like Triceratops and Iguanodon, had need for such complex mechanisms to regulate body temperature. So why would Stegosaurus? In short, it probably didn’t.

As commonly happens with centuries old dinosaurs, there are always some fun theories that originated from back in the day. Take for instance how it was believed that the plates of Stegosaurus “sapped their vigour” and were signals that Stegosaurus was about to go extinct. Goodness, that’s almost as ridiculous as the theory that dinosaurs went extinct because caterpillars ate all of the world’s plants! Next!

One more reasonable theory is that the plates of Stegosaurus were something of a status symbol, the Jurassic equivalent of a fancy car or watch. Under this theory, the plates would act as a way to distinguish species and to identify the dinosaur’s sex. For males, the size of the plates may have been a means to establish dominance and potentially to attract mates, like the lion’s mane in the present. Some studies have gone so far as to say that males and females had different plate structures, with females having taller plates and males having wider ones. This study is based in the notion that Hesperosaurus, a smaller stegosaurid genus that lived at the same time in North America as Stegosaurus, is actually synonymous with Stegosaurus. While this may be possible, the remains of Hesperosaurus are known from different areas than those of Stegosaurus, which points to them being different species.

So, where does this leave us? The truth is that the plates may have served multiple uses. Nothing in nature is black and white, so to say that the plates didn’t have multiple uses is foolish. I believe that the plates were probably used to make Stegosaurus look bigger to deter potential predators, while at the same time, serving as a way to attract mates.

Who knows? Maybe the plates had fake eyes on them like a butterfly to scare the life out of predators. Maybe they were colorful like a peacock to make it stand out to the lady Stegosaurs. For now, we’ll never know.

Works Cited:

I do not take any credit for the images found in this article.

Header image courtesy of John Conway, whose art can be found at his website: https://johnconway.art/

Stegosaurids courtesy of Gregory Paul, found here

Allosaurus vs Stegosaurus courtesy of Sante Mazzei, found here

Stegosaurus art courtesy of Jonathan Kuo, found here

1. Prothero, Donald R. Story of The Dinosaurs in 25 Discoveries: Amazing Fossils and the People Who Found Them. Columbia Univ Press, 2021.
2. Benton, Michael J. Dinosaurs Rediscovered: the Scientific Revolution in Paleontology. Thames & Hudson, 2020.
3. Lucas, Spencer G. Dinosaurs: the Textbook. Langara College, 2018.