Dare to be Stupid

I fell in love with dinosaurs early. Awkward and overweight, I was a fan of the giant herbivores who could hold their own against the horrible meat-eaters. Triceratops, popular with friends, was obviously great but never one of my favourites. Stegosaurus though, the oddly-shaped, lumbering dork with a back full of plates and a spiky tail, now that was my guy.

I would team up dinosaur toys with “my guys”,¹ my small army of action figures, and spend hours coming up with scenarios I imagined these giant beasts must’ve gone through millions of years ago. But you know, with Donatello and some of the ThunderCats helping out. My stegosauruses (stegosauri?) were often the last minute heroes who showed up and saved the day from Skeletor’s army or a host of hungry theropods.²

So it hurt extra hard when I first heard stegosaurus may have been the dumbest of the dinosaurs.

First let’s clarify, we can’t actually know that for sure. They’ve been extinct for 150 million years so analyzing their behaviour is out of the question, but we have found a lot of stegosaurus fossils and this we do know. They had really small brains.³ Like, absurdly small.

A bigger brain doesn’t always mean more intelligence. Brain size is relative and usually proportional to body size, so it’s easy to understand why a sperm whale has a significantly larger brain than us but still won’t be doing algebra any time soon. They need a lot more brain just to control that much bigger body.

But a fully grown, adult stegosaurus, which could have weighted as much as 5,000 kg, but with conservative estimates placing them somewhere between 2,700 – 3,200 kg, had a brain weighing no more than 70 or 80 grams⁴. About the same size as a puppy’s. For comparison, an adult human brain is about 1.3 to 1.4 kg⁵, 18 to 20 times bigger. And we don’t weight no 3,000 kg either.

After noticing a large cavity in the stegosaurus’ spinal canal, famed 19th century paleontologist, O. C. Marsh, thought it may have housed a second brain or nerve center to control its back half. Since its brain was so small compared to its body, he didn’t think it could possibly control the whole thing.⁶

When I was a kid, the idea of dinosaurs as dumb brutes was already seen as outdated. Birds evolved from theropod dinosaurs⁷ (two-legged meat-eaters) and some species are among the smartest animals on Earth today. We believe their dinosaur ancestors may have had similar levels of intelligence so was Stegosaurus closer to them in smarts or did their tiny brains really mean they was real dumb? And how smart were dinosaurs, really?

Dinosaurs roamed our planet for a long time. In the fossil record, their bones are found in all three periods of the Mesozoic Era. They first appear in the Triassic but it wasn’t until the middle of the period and they were just one of multiple archosaur¹⁴ clades who saw success after the End-Permian extinction.¹⁵ It was in the Jurassic Period that dinosaurs really hit their stride and began their rise to dominate most land habitats on Earth. Stegosaurus existed in the late Jurassic when dinosaurs were already the dominate vertebrate lifeforms on land.

Dinosaurs fall into one of two clades, the ornithischians²² (bird-hipped dinosaurs) and the saurischians²³ (lizard-hipped dinosaurs). Stegosaurus was an ornithischian, like the hadrosaurs (duck-billed dinosaurs)²⁴ and ceratopsians (horned-face dinosaurs)²⁵, and had hips similar to birds in that their pubis bone is pointed down and backwards. This allowed them to have a larger gut cavity capable of holding expanded digestive organs. With this, they could eat some of the tougher plants that were off-menu for other herbivore dinosaurs.

In addition to longer digestive tracts and fermentation chambers (to break down cellulose-rich vegetation), many ornithischians, like the stegosaurus, had a beak which allowed them to chop vegetation efficiently.²⁶

Stegosaurus stenops is the type species but there are three stegosaurus species recognized: S. stenops, S. ungulatus and S. sulcatus.

The stegosaurus is one of many species in the Stegosauria²⁸ clade, a group of quadrupedal herbivores that all had dorsal plates and a weaponized tail, but the shape of these would differ from species to species.

Stegosauria is divided into 2 families. The huayangosauridae, who arrived earlier and were more primitive and the more derived stegosauridae, from which stegosaurus was a member.

When the original stegosaurus fossil was found by O. C. Marsh, the paleontologist believed it was similar to a turtle, with the plates laying flat on its back, overlapping like the shingles on a roof. Stegosaurus means roof(ed) lizard and this is where its name comes from. Once more specimens were found, he realized his error and we began displaying the stegosaurus as we usually see them now, with two rows of alternating plates pointing up.

Within the hind limbs, the tibia and fibula were short compared with the femur. This suggests it could not walk very fast, as the back legs would have overtaken the front ones. Some references suggest a maximum speed of 15 – 18 km/h,³⁸ others go as low as 6 – 8 km/h.³⁹

• They were used as armor, although others dispute this as they were seen as too fragile and ill-placed to be of any real use defensively.
• Different stegosaurus species may have had varied plate arrangements to help with species recognition. They may have also differed between sexes of the same species.
• They were found to have “extreme vascularization of the outer layer of bone” so could have helped with thermoregulation like dimetrodon’s sail⁴² or an elephant’s ears.
• The extreme vascularization could also mean they were used for display and could change colours in certain circumstances like when attracting mates or feeling threatened.

At the end of its tail, the stegosaurus had four bone-piercing spikes that were made of the same ossified dermal tissue as their dorsal plates.

Known as the thagomizer,⁴³ early 20th century paleontologists debated whether it was used for defense or display, but more recent studies on multiple specimens have revealed a high rate of trauma-related damage to their tails. This is what we would expect to see if they were using them as giant spiked whips.

Adding to this theory, an Allosaurus fossil was found with a puncture wound in its pubis bone that matched the spikes from a stegosaurus’s tail.⁴⁴ Analyzing it revealed the wound became so infected that it eventually killed the Allosaurus. That’s so metal!🤘🏻

Ok enough stalling, let’s talk intelligence. Dinosaurs were around for a long time in many different shapes and forms. Their levels of intelligence would of evolved differently from one another over the 180 million years they existed on Earth. We can’t lump all dinosaurs into one category, because like mammals, birds, or any other large group of living organisms, each species within has evolved a brain suited for their specific lifestyle, crafted by natural selection and the fact that their parents were pretty similar to them and lived long enough to meet a partner, mate, and have a child. It was good enough for mother nature, so it was good enough for them.
🕹️(<- mother nature’s stamp of approval)

Animal intelligence isn’t easy for us to assess. Intelligence has levels, and is very complex. Less than a century ago, we though we were the only tool-users in the animal kingdom, but our fascination with wildlife and wanting to understand how they live has revealed our ignorance. Other primates like chimpanzees and orangutans are now known to use tools too, like small sticks to fish termites⁴⁶ or stones to crack nuts.⁴⁷ Other mammals like elephants and dolphins,⁴⁸ who we last shared a common ancestor with 95 – 100 mya,⁴⁹ have also been observed using tools in their environments. And don’t forget birds, the only dinosaurs left today. Crows are known for crafting tools like hooked twigs to retrieve insects⁵⁰ and woodpecker finches use cactus spines to extract grubs.⁵¹ Egyptian vultures sometimes use rocks to crack ostrich eggs⁵² and some species of parrots use tools to scratch themselves.⁵³ Since many bird species use tools today, it’s at least possible their dinosaur ancestors did as well.

And tool use isn’t the all of it. There is no surefire way to figure out an animal’s intelligence, but one proxy we use, as a possible way of comparing the intelligence levels of different species, is measuring their encephalization quotient (EQ).⁵⁴ An animal’s EQ is similar to its brain-to-body mass ratio, but its more refined than that, as it also considers allometric effects, the relationship of body size to shape, anatomy, physiology and behaviour. Basically, the EQ is the ratio of the brain weight of the animal to the brain weight of a “typical” animal of the same body weight. The basic equation of allometry is \(E = CS^r\), where \(E\) is the brain’s weight, \(S\) is the body weight, \(C\) is the cephalization factor, and \(r\) is the exponential constant.

Brain size alone, the stegosaurus was not working with much. Often said to have a brain the size of a walnut, this isn’t quite true, but it isn’t too far off either. It’s more like 6 walnuts.⁵⁸ And a human’s brain is about 112.5 walnuts.⁵⁹ We should measure more brains in walnuts, this is fun. Grizzly bears (29 walnuts), sperm whales (708 walnuts), Alaskan crabs (0.002 walnuts), North American beavers (0.4 walnuts), Harpy eagles (0.2 walnuts), moose (39 walnuts).⁶⁰

Their tiny brains and big bodies did mean the stegosaurids were pretty low in EQ compared to other dinosaurs we know. Only their ankylosaurian cousins and the tiny-headed behemoths of the Mesozoic, the sauropods, have a smaller value in the dinosaur groups I reviewed. Allosaurus, one of stegosaurus’ predators in the Late Jurassic, was a Carnosaur. They had an estimated EQ between 1.0 and 1.9, significantly higher than Stegosaurus’ 0.6. Just like today’s wolves are significantly more cunning and crafty than the deer and other wildlife they prey on.

The big smarty-pants of the age of dinosaurs seems to be the Dromaeosaurids (running lizards) and the Troodontids (tooth that wounds).

The Dromaeosaurids were small-to-medium sized feathered dinosaurs known for being fast and agile meat-eaters. Velociraptor and Deinonychus were both Dromaeosaurids. Species’ names in this clade often include the word “raptor” as they have many similarities with modern birds of prey.

The Troodontids score even higher in EQ and again, we believe they are another link in the dinosaur-to-birds chain that evolved millions of years ago. Archaeopteryx and the other avian theropods scored among the highest of all. As the missing-link dinosaurs that first lead us to the theory that birds may have come from dinosaurs, it makes sense that they could have been the smartest of the family.

Together, the Dromaeosaurids, Troodontids, and avian theropods like Archaeopteryx form a clade called the Paraves.⁶²

Body mass is another variable we aren’t always 100% sure about. There is a wide range of estimates when it comes to dinosaur mass. I found some estimates placing a fully grown stegosaurus somewhere over 5,000 kg,⁶³ but other estimates have them as low as 2,700kg.⁶⁴ And both could be true. One study estimated that the mass of some dinosaur species had a four-fold range. Hans Carl Larsson, an expert in ancient biodiversity at McGill University, noted “the broad ranges of body mass estimates, combined with the ambiguous ratio of endocranial volume occupied by the brain, present a high degree of uncertainty for [creating an] index of brain size”.

It seems most dinosaurs had brains no better than modern day reptiles. Hans Larsson found that Allosaurus had a cerebrum-to-brain-volume ratio that was very similar to non-avian reptiles, although Tyrannosaurus’ ratio was a bit more in inline with today’s birds. Since Tyrannosaurs are considered primitive or basal members of the coelurosaurs, this is seen as evidence that Coelurosauria marks the beginning of a trend in theropod brain enlargement. Troodon’s cerebrum-to-brain-volume ratio was 31.5% to 63% of the way to modern birds and Archaeopteryx was even closer at 78%. The branch of dinosaurs that eventually lead to birds seems to have had a significantly bigger cerebrum-to-brain-volume than many, or even all, of the dinosaurs that came before them.