Stegosaurus – Late Jurassic (155-150 Ma)Y’all like dinosaurs?Me too, and in an effort to

Stegosaurus – Late Jurassic (155-150 Ma)

Y’all like dinosaurs?

Me too, and in an effort to talk about the more popular groups, today’s subject is Stegosaurus, the covered lizard himself. Stegosaurus is a staple of pop culture depictions of dinosaurs. Everyone who knows anything about dinosaurs knows Stegosaurus, and most of those people love it. The people who don’t aren’t worth listening to, in my opinion. Stegosaurus is a fascinating animal for a million reasons, but I’m going to try and narrow it down to my favorites.

Part of Stegosaurus’ reputation comes from how striking and easily-recognized it is. The deep, narrow torso, plates, and 3 foot long (!!!) tail spikes are immediately indicative that the animal you’re looking at is a Stegosaurus or something close to it. Even by dinosaur standards, this is an unusual body plan unique to the Stegosaurus family, and these traits have been endlessly scrutinized in an effort to understand what they were for. The plates were tall and flat, arranged in an alternating pattern on its back and numbering anywhere from 17 to 22. Well-preserved Stegosaurus plates are etched with branching grooves. Othniel Charles Marsh, the legendary paleontologist who discovered Stegosaurus, thought they were used as armor. But, the lack of coverage and overall fragility of the plates leads us to think they weren’t used for defensive purposes, but for display. The grooves housing blood vessels that could be flushed, reddening the thin layer of skin covering them. This could be used either as a mating display or as a warning to any predator trying to fuck with it.

The four parallel spikes on a Stegosaurus’ tail are informally called the “thagomizer,” named after a joke in a Far Side strip. It’s worth mentioning that this isn’t just a pop culture thing—scientists love calling it that. The thagomizer was a weapon, and a formidable one. Most big dinosaurs had hardened tendons in their tails, making them stiff and ideal for balancing their large bodies. Stegosaurus didn’t have these tendons, making its tail much more flexible than that of the average herbivore. It compensated with a sloping posture, with shorter forelimbs and long, thick hind limbs. These proportions aided in swinging its tail, allowing it to anchor itself with the back legs while pushing off with the front, giving the swing lots of power. Many preserved spikes have evidence of impact trauma, and an Allosaurus has even been found with punctures in the bone. As it happens, Stegosaurus spikes happen to fit perfectly into these punctures. So, unless the Allosaurus in question just kind of fell sideways onto a dead Stegosaurus or something, this all points to a weaponized tail. If it’s good enough for forensic scientists, it’s good enough for rockbonologists.

Is the idea that Stegosaurus had two brains still a thing? It was a thing when I was a kid, but I’m not sure if it’s fallen out of favor or not. This was something proposed by Marsh when he first described Stegosaurus, as an explanation for the big cavity in its hip region. Because Stegosaurus’ brain is so tiny, reasoned Marsh, a secondary brain in the hips added extra processing power. The problem is, this cavity isn’t unique to Stegosaurus—sauropods had it, as do a certain group of dinosaurs called birds. Now, in birds, the organ filling this space is a part of the spinal cord. It’s not a brain at all, but something called a glycogen body. This should be an easy win for us. If this organ is present in living animals, it was probably there for the related Stegosaurus too, and it probably did the same thing. That brings us to problem 2: Nobody knows what exactly glycogen bodies do in birds. They’re made of cells rich in glycogen, a vital sugar used for storing energy. Despite this, the glycogen body seems unrelated to normal glycogen functions, and is, essentially, a big question mark.

While we’re on the subject of brains, a lot of people think dinosaurs were stupid, and a lot of that comes from studies of Stegosaurus’ brain case, which was super tiny. Only the upper-back corner of the skull was devoted to brain room. That being said, stop calling Stegosaurus stupid—it’s mean and he doesn’t deserve it. He had a tiny walnut brain, sure, but his brain was just the size he needed it to be to eat shrubs and swing his tail around. Herbivores, on the whole, don’t need to be quite as intelligent as carnivores, and Stegosaurus was good at what it did.

Also, nobody knows exactly how Stegosaurus had sex. Think about it.

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