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The Debunker: Did Big Dinosaurs Have a Second Brain?

by Ken Jennings

Did you know that the second week of March is Brain Awareness Week around the globe? You didn’t? You weren’t aware of your brain? Conscious of your consciousness? Well, get with the program. March is perhaps the brainiest month of the year—it’s also when we celebrate the 1879 birthday of famous smarty-pants Albert Einstein, and the 1946 beginning of Mensa intelligence testing. But it turns out people will believe just about anything they hear about what’s going up between their ears. We’ve asked Ken Jennings to fact-check some particularly lame-brained misconceptions about gray matter.

The Debunker: Did Big Dinosaurs Have a Second Brain?

Yale paleontologist Othniel Charles Marsh was one of the great dinosaur experts of the 19th century, naming pretty much every extinct lizard you see today in museum lobbies or children’s books: Allosaurus, Diplodocus, Triceratops, Apatosaurus. Much of his work centered on the dinosaur he called Stegosaurus, meaning “covered lizard.” In 1881, he made a cast of a Stegosaurus skull, and was astounded to find that the giant animal probably made do with a 3-ounce brain, no bigger than a lime. (Not quite the “brain the size of a walnut” from popular accounts, but close.)


He also noticed a large canal lower on Stegosaurus’s spinal cord, near its hips. The cavity left room for a brain much larger than the citrus-sized one in Stegosaurus’s noggin. Marsh speculated that perhaps dinosaurs had a simple secondary brain near their tails. Since it would take nerve impulses a while to get up and down a 30-foot dinosaur, you can see the appeal of a “junction box” halfway down the line.

But most of Marsh’s thinking about dinosaur smarts turned out, over the next century, to be pretty misleading. Yes, Stegosaurus has a small brain, but that doesn’t mean all dinosaurs are slow, lumbering brutes. It just means Stegosaurus was one of your dumber specimens. And as for the second brain—well, we now know that its location is all wrong to contain nerves. The current thinking is that the “second brain” cavity probably contained a glycogen body, an organ for energy storage that today’s birds still possess. But that’s not nearly as fun as a butt-brain, so it’s taking Marsh’s bad hypothesis just as long to die out as the dinosaurs themselves took.

Quick Quiz: Stegosaurus is the official dinosaur of what U.S. state, which has the bighorn sheep as its state mammal and the columbine as its state flower?

Ken Jennings is the author of Because I Said So!, Brainiac, Ken Jennings's Trivia Almanac, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at or on Twitter as @KenJennings.