A brain reconstruction of the sauropod Ampelosaurus compared to a common walnut.
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Courtesy Lawrence Witmer.
A brain reconstruction of the sauropod Ampelosaurus compared to a common walnut.

Walnut the True Measure of a Dinosaur’s Brain

Ampelosaurus had a surprisingly small brain. All fifty feet of the dinosaur – from its pencil-toothed muzzle to the tip of its long tail – was regulated by a mass of tissues about the size of a walnut and a half. That comparison isn’t sloppy shorthand. Ohio University paleontologist Lawrence Witmer actually went to the trouble of comparing the hulking sauropod’s brain to an English walnut to test an old axiom and correct a journalistic mistake.

Most everyone is familiar with the idea that huge dinosaurs had brains the size of a walnut. But where did this unflattering comparison come from? Witmer, who has tried to get at the origins of the idea, says the earliest example anyone has turned up so far is in Jennie Irene Mix’s 1912 book Mighty Animals, which says that the famous Jurassic sauropod Diplodocus “had a brain that was not much bigger than a walnut.” The meme didn’t really catch on until 1945, though, when paleontologist Edwin Colbert said that the armor-plated Stegosaurus also had a brain the size of the edible seed.

Witmer was reminded of the old saying when news source LiveScience covered a PLoS One study on the brain of Ampelosaurus that he had just published with Fabien Knoll, Ryan Ridgely, and coauthors. The 70 million year old sauropod, the report said, had a brain “which was not much bigger than a tennis ball.” That comparison is a bit too generous. (And Witmer nor his colleagues recall giving the news source such a comparison.) As Witmer explained in a Facebook post, “A tennis ball has a volume of about 140 cc, whereas the brain endocast of Ampelosaurus is just 39.5 cc.” A better measure of the dinosaur’s endocast, Witmer proposed, is a walnut. The side-by-side comparison of the pantry staple and the paleoneurological reconstruction shows just how startlingly small the dinosaur’s brain was.

In the post, Witmer wrote that he and his colleagues were “proposing the walnut as the new official unit of measure of dinosaur brain size, based on our microCT-scanned 26.2 cc walnut as the standard.” Of course, this was just some playful paleontology. “It’s just a joke, because it’s historically been such a player in dinosaur brain-size comparisons,” Witmer says. Still, the Cretaceous crack underscores the perplexing truth that some of the biggest animals of all time had ludicrously small brains.

“After studying maybe a couple dozen species of sauropods (which is a crap-ton of species for a CT-based study, “crap-ton” being another favorite unit of measure),” Witmer explains, “we have yet to find a clade that shows marked expansion of brain size.” In fact, Witmer points out, sauropod brains may have been even smaller than expected, since dinosaur brains didn’t fully fill the braincase endocasts that have been reconstructed from fossil remains. “Suffice it to say that if you thought a walnut or two was pretty small for an endocast volume, just imagine how much smaller the actual brain was,” Witmer says. Determining how much brain was in the braincase is the subject of further study by Witmer’s student Ashley Morhardt.

So what does all this mean for the biology and behavior of the animals? Are small brains really indicators of dullard dinosaurs? That’s difficult to say, Witmer says, since he and other paleo-brain specialists “can only talk in general and comparative terms.” Still, Witmer expects that sauropods “were pretty simple beasts” whose behavior was “largely governed by instinct and generally stereotyped responses.”

That doesn’t mean that sauropods were biological automatons, as some early 20th century paleontologists presumed. In addition to footprint evidence of sauropods walking together, Witmer points out, “we find sauropods with display structures” that belie some sort of social interaction and behavior. Ampelosaurus itself had such adornments – ornamental structures called osteoderms that gave the dinosaur’s flanks and back  some imposing decoration. And “Somehow all those sympatric Morrison [Formation] sauropods species were able to sort out whom to mate with,” Witmer says. Just because sauropods were probably dim-witted doesn’t mean that they weren’t capable of socializing or other relatively complex behaviors related to interacting with their own kind.

Of course, there was so much Mesozoic diversity and disparity that it would be wrong to assume that all dinosaurs were only capable of reflexive, knee-jerk responses to the world around them. Sauropods are just one part of an emerging picture of dinosaur neuroanatomy and behavior. “The exciting thing about recent comparative cognitive studies is how surprisingly ‘smart’ are a wide range of birds and reptiles, not just corvids” such as ravens, Witmer says. A sauropod’s intellect might not have been very impressive, but “we might indeed have been inspired by some dinosaur species to utter ‘Clever girl.'”