Stegosaurus plates

Last year, I took a geology course focused entirely on dinosaurs. It was about as fascinating as you’d expect for someone who loved dinosaurs as a kid (and who didn’t?) and who now gets to to revisit the subject in detail as an adult. I’ve written a few posts as a result. One of the topics from the class that I found rather surprising was stegosaurus plates: why did they evolve, and what function did they serve?

stegosaurus walking in foreground with trees and other dinosaurs in background
stegosaurus with characteristic plates along its back

I initially assumed that the plates were used primarily for defense in some way. They look kind of.. spikey. Maybe it’s like a hedgehog and the spikes ward off predators? But the plates weren’t actually armor or defensive in any way – and upon closer inspection, this makes sense. They didn’t cover very much of the stegosaurus’ body and left most of it vulnerable on the sides and bottom, whether the plates laid flat or stood vertically. What defensive purpose could this serve?

Even more interesting, the plates were not made of solid bone connected to the rest of the spine and skeleton, but rather were lined with grooves that likely meant they contained rich blood vessels. This makes it even less likely the plates were for defense: why expose vulnerable blood vessels to an attacker?

The real function of the plates has been debated, and some paleontologists suggested that the plates were a visual display to recognize other members of the same species, or to attract mates. This is definitely one possibility, although not one I find particularly interesting. But the evidence for blood vessels has also led some to suggest that the plates helped regulate body temperature. In other words, they would have acted as radiators to help the dinosaur cool off when too hot, and as “solar panels” to absorb more sunlight over a greater surface area to help warm the dinosaur when it was too cold.

However, some additional research has demonstrated that while this temperature regulation theory was possible, it probably isn’t why the stegosaurus evolved the plates in the first place and it also likely didn’t have much of a significant effect. If the plates did play a role in helping cool or warm the animal, one obvious question would be why other dinosaurs didn’t evolve something similar. Furthermore, if the plates served this purpose and were really more about function, then they would not have varied so much across different species of stegosaurids. Instead, this makes it more likely that they were simply used for display.

I’d prefer for the functional (temperature regulation) hypothesis to be true, but it seems more likely that the plates evolved more for display. In the end, it’s a debate that remains unsettled. The two different suggestions are not mutually exclusive – both could be true, to some extent. Or perhaps there’s some totally separate explanation that hasn’t yet been proposed by anyone just yet.

Stegosaurus, American Museum of Natural History

Stegosaurus ungulates, National Park Service

Stegosaurus Plate Debate, Smithsonian Magazine

The Stegosaurus Plate Controversy