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Did Stegosaurus Have a Second Brain?

PALEODEX · 15 September 2026 · 6 min read

There really is a hollow in the hips of Stegosaurus, and it really is bigger than the space that held its brain. The second brain is the part that came later.

A black-and-white line drawing of a Stegosaurus skeleton seen from the side, with a small skull held low at the left, a deep ribcage, short front legs and much longer hind legs.
O. C. Marsh's skeletal restoration of Stegosaurus, from The Dinosaurs of North America (U.S. Geological Survey, 1896). Public domain, via Wikimedia Commons.

The story goes that Stegosaurus had a second brain in its hips to run its back end. Every piece of that story has a source, and the sources do not all say the same thing. Here they are, in order.

A small brain for a big animal

The small brain is the famous part, and it has a source. In his 1896 monograph The Dinosaurs of North America, Othniel Charles Marsh compared the brain of Stegosaurus with an alligator's, scaled to body size, and concluded that "Stegosaurus had thus one of the smallest brains of any known land vertebrate." That is a claim about proportion: a small brain for the size of the animal.

The chamber in the hips

The surprise was at the other end of the spine. In an 1881 paper in the American Journal of Science, Marsh wrote that he had found "a very large chamber in the sacrum, formed by an enlargement of the spinal canal." The sacrum is the part of the spine that sits inside the hips. "This chamber was ovate in form," he went on, "and strongly resembled the brain case in the skull, although very much larger, being at least ten times the size of the cavity which contained the brain."

Two details are worth keeping straight. The paper is signed at Yale College in January 1881, and a footnote says its facts were first read to the National Academy "in November last", so the chamber was known by late 1880 and described in print in 1881. And when Marsh reused the passage in his 1896 monograph, "at least ten times" became "at least twenty times". Both figures are his. This post uses the earlier one.

An engraved plate on cream paper with four drawings of fossil casts at the same scale: a long, lumpy cast lying on its side across the top, the same cast seen from above at lower left, a ring-shaped cross-section at lower right, and beneath it a much smaller, narrow cast of the brain.
Marsh's casts, all at one-fourth natural size: the sacral chamber from the side and from above, a cross-section, and the brain (bottom right). The Dinosaurs of North America (1896), Plate XLVI, figs. 1–4. Public domain, via Wikimedia Commons.

What Marsh actually wrote

Marsh named the chamber and hedged the name in the same sentence: "The remarkable feature about this posterior brain-case, if so it may be called, is its size, in comparison with that of the true brain of the animal." Further down the same page, he went past the hedge: "The physiological effects of a posterior nervous center, so many times larger than the brain itself is a suggestive subject, which need not here be discussed. It is evident, however, that in an animal so endowed, the posterior part was dominant."

So Marsh was not simply being careful. He called the chamber a nervous center and judged the back half of the animal to be in charge. What he never wrote is the phrase "second brain".

How it became a second brain

That phrase arrived later. One well-documented stop on the way is a newspaper poem. The Linda Hall Library traces Bert Taylor's "The Dinosaur" to the Chicago Tribune of 26 February 1903, where it joked about a dinosaur with "two sets of brains". The library notes that a Brachiosaurus vertebra at the Field Museum prompted the poem, and that it is usually retold as dating from 1912 and as being about Stegosaurus. The misremembered version is a fair picture of how the idea travelled.

By 1932 the second brain was a popular misconception. Wikipedia's Stegosaurus article cites the paleontologist Barnum Brown describing it, and disputing it, that year. We have not read Brown's text ourselves, so it is marked as secondhand in the sources.

The same space, in living birds

The best clue to what really filled the chamber is still walking around. Birds are dinosaurs, a point the University of California Museum of Paleontology makes without hedging, and birds have an enlarged space in the same part of the spine. There, the top of the spinal cord opens into a groove that, in the words of a 2020 study, "houses an egg-shaped glycogen body".

A glycogen body is a cluster of large cells packed with glycogen, the form in which animals store sugar. Its cells are a specialized kind of glia, the support cells of the nervous system, not the neurons that do a brain's work. A 2025 study of this part of the bird spine draws the line back to dinosaurs itself: "A similar structure was documented by Marsh in 1881 in Stegosaurus."

A grainy grey CT scan of a cross-section through a bird's backbone. Inside a dark ring, a green teardrop shape sits at the top between two pale blue lobes, with small pink and orange patches at the sides. A legend below labels them glycogen body, spinal cord, accessory lobe, dentate ligament and endocast.
A CT cross-section through a bird's spinal canal, with the glycogen body in green. Stanchak, French, Perkel & Brunton 2020, Integrative Organismal Biology, Fig. 2 (panel G), CC BY 4.0.

Best match, not proof

So did the chamber in Stegosaurus hold a glycogen body? In 1991, E. B. Giffin reviewed enlarged sacral canals across dinosaurs, mostly sauropods and stegosaurs, and concluded that a glycogen body like a bird's fit the evidence best. The paper is not online, so this post relies on two accounts of it that agree: a 2019 explainer on Sauropod Vertebra Picture of the Week, and the 2025 study quoted above.

That is a strong comparison, and it is still a comparison. Only the bone around the chamber survives, not what was inside it, so its contents are inferred from living birds rather than seen.

The birds leave a question open too. The 2020 study found "no consistent evidence that the glycogen body has a role in body metabolism", and experiments that starved birds to test the idea gave mixed results. Other jobs have been proposed for it, and none is settled, even in chickens.

Put together, the honest answer is short. A glycogen body is the best match for the Stegosaurus chamber, not a certainty, and nothing in the evidence makes it a brain.

Why not a balance organ? In birds, the same stretch of spine also has canals in the bone and small lobes beside the spinal cord that may work as a sense of balance (Stanchak et al. 2020). That idea is about different parts of the organ, not the glycogen body. Those canals have not been reported in any non-avian dinosaur, though it is not clear anyone has looked, so this post does not offer a balance organ as a candidate for what filled the Stegosaurus chamber.

Why this belongs in PaleoDex

Because the second brain shows how a fact drifts. The hole is real, Marsh's measurement is real, and his own words went further than caution. The phrase that stuck came later, and the best modern reading comes from animals that are still alive. Marsh has a much bigger part in the Bone Wars and turns up again in the Brontosaurus story, and the case for birds as dinosaurs is laid out in Are birds dinosaurs? For the wider catalogue, see the Fossil Dex, and for how we handle sourcing, about the data.

Sources

Every claim above traces to the sources below. Where we relied on someone else's reading of a source, we say so.

Deliberately not in this piece. The plates and the tail spikes. Each is its own question, and neither is the one this post asks.
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