Stegosaurus means “roofed lizard.” I never really understood why. The trundling herbivore bore an offset row of bony, triangular plates along the midline of its back, but this arrangement didn’t really look like a roof so much as a series of sails. Whether trying to keep off the rain or provide a barrier to serrated teeth, the plates didn't look like a very effective awning.
I had assumed that Stegosaurus was known more or less in its present form since the time of its discovery. Some things changed through the process of paleontology – like the number of tail spikes going from eight to four, the addition of pebbly bone armor under the throat – but these were relatively minor tweaks. What I missed was that the original vision of Stegosaurus was of an entirely different and unfamiliar dinosaur.
The notoriously cantankerous Yale paleontologist Othniel Charles Marsh named Stegosaurus in 1877. Instead of being an articulated skeleton with all the bones and ornaments in place, though, the initial find was more open to interpretation. Marsh had only a jumble of bones to start with, and, to his mind, the best arrangement of those peculiar plates was as an interlocking domed shell. The original Stegosaurus was basically a big turtle, carrying its bony roof on its back.
Marsh didn’t hold on to this original interpretation long. Stegosaurus Mk. 2 more closely resembled the dinosaur we know now, but with one important difference. After Marsh's initial description, additional finds included frighteningly-long spikes alongside other Stegosaurus bones. It wasn’t obvious where they should fit, so Marsh toyed with the idea that the piercing armaments belonged on the wrists. Threatened by a rampaging Ceratosaurus or Allosaurus, the formidable Stegosaurus could have reared back on its tail to present its foes with three-foot lances jutting from its arms. It was only the discovery of additional Stegosaurus, around 1887, that showed those weapons belonged at the tip of the tail.
At first, Marsh thought that Stegosaurus had eight spikes arrayed in two sets of four on either side of the tail. The skeleton of the Jurassic saurian on display at the Yale Peabody Museum of Natural History still has this arrangement, a monument to the history of science as well as life 150 million years ago. And, admittedly, the dinosaur's tail looks like one you wouldn't want to be on the wrong side of. There's even a pathological Allosaurus vertebra excavated from an eastern Utah bonebed that seems to have been pierced by a Stegosaurus spike, hinting that the highly-decorated herbivore really did employ that formidable weapon. In time, though, more complete finds confirmed that Stegosaurus had half the number of tail spikes that Marsh expected - not quite so visually striking as a set of eight, but clearly enough to send a pointed message to curious carnivores.
New finds have continued to alter our image of Stegosaurus since the time of the dinosaur's discovery. Despite the dinosaur's familiarity, though, there's still a great deal we don't understand about the Mesozoic celebrity. Stegosaurus remains one of the strangest dinosaurs of all. This was a massive, slow-moving herbivore with weak jaws and a row of decorations on its back that continue to inspire debate among experts to this day. The function and evolution of those plates alone have kept paleontologists going back to their inkwells to propose ideas involving everything from shedding excess heat to recognizing other Stegosaurus from afar. It's one thing to reassemble a skeleton and quite another to comprehend the life those osteological remnants represent. We know the shape of Stegosaurus, but we barely know its nature.