Pictured: Sophie the Stegosaurus, at the Natural History Museum in London. Found in Wyoming, she is the most complete Stegosaurus skeleton known to date. Photo taken by myself in February 2018.
Before I saw Jurassic Park, and Velociraptor captured my interest, my favourite dinosaur was a very different animal: Stegosaurus. It’s one of the most familiar of all dinosaurs, so I don’t know where I was first introduced to it: whether it was in my Children’s Encyclopedia of Prehistoric Life, or a dinosaur magazine, or watching with sadness as it put up a brave but futile fight against a Tyrannosaurus rex in Disney’s Fantasia. (Such a fight could never have happened in real life, as T. rex didn’t exist until 85 million years after Stegosaurus died out.) I probably liked it for its bizarre, unique appearance, and because there was a sort of nobility to it: this giant, mild-mannered herbivore might have been lacking in the intelligence department – my books frequently told me it had a brain the size of a walnut – but if faced with a malevolent predator like Allosaurus, it was ready to defend itself with its impressive spiked tail.
First described in 1877, Stegosaurus is known from many fossils found in the western United States – and, surprisingly, a specimen in Portugal. It lived in the Late Jurassic Period – from 155 to 150 million years ago – alongside a wide variety of other dinosaurs, including giant sauropods like Diplodocus. Growing up to 9m long and weighing 7 tons, it was the largest member of its family, the Stegosauridae. Its brain, incidentally, is now known to have been rather larger than a walnut, though undoubtedly very small compared to the animal’s body. Stegosaurus‘s simple, plant-eating lifestyle didn’t require an especially large brain.
There has been debate over what sort of plants Stegosaurus was physically capable of eating, with its narrow snout, horny beak, and small teeth placed further back in the mouth. A 2010 computer analysis of the skull suggested that it only had a weak bite and would have been limited to softer plant material and thin branches. However, a similar computer modelling study in 2016 – based on the skull of Sophie, pictured above – concluded that Stegosaurus had a stronger bite force and could therefore have handled a wider variety of plants. Due to the physical limitations of its head and neck, it would have mainly stuck to low-lying vegetation, though it might have been able to rear up on its hind legs.
The name Stegosaurus means “roof lizard”, as its describer, Othniel Marsh, originally believed that its famous bony plates lay horizontally on its back. Palaeontologists now know from better specimens that the plates stood upright in two rows, probably in an alternating arrangement along the back. Exactly what the plates were used for is uncertain. Given that they were permeated with blood vessels, relatively fragile, and offered no protection from a side-on attack, they would have had limited usefulness as defensive armour. A popular theory from the books and documentaries of my childhood is that the plates were used for thermoregulation, allowing the blood running through them to either absorb or lose heat, depending on how the animal oriented itself relative to the Sun. More recently, however, palaeontologists have questioned just how practically effective the plates would have been in this regard. As the plates were such conspicuous features, they could well have had a display function; indeed, one study indicates that they may have been sexually dimorphic, with males having larger plates.
The function of the four long spikes which pointed sideways from Stegosaurus‘s tail seems more obvious. They look very much like a defensive weapon, which Stegosaurus could have made good use of, given that it couldn’t move very fast and had to watch out for large carnivores like Allosaurus. Indeed, an Allosaurus vertebra has been found with a hole punched through it – the size of which corresponds with a Stegosaurus tail spike. This set of spikes, which can also be seen in other stegosaurs, has been dubbed a “thagomizer”. This name originally came from a 1982 cartoon by Gary Larson, in which a caveman is teaching his fellows about the Stegosaurus tail, explaining that the dangerous thagomizer is named “after the late Thag Simmons”.
As to how two Stegosaurus managed to mate with all those plates and spikes – well, palaeontologists aren’t sure about that. One suggestion is that the female would have to lie down on her side to make things as simple as possible for the male, but until we can observe a living Stegosaurus, all we can really do is speculate.