What Is a Stegosaur? Plates Nobody Has Fully Explained
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
The double row of plates along the back is among the most recognisable shapes in palaeontology and among the least settled. The plates were not attached to the skeleton, their arrangement was argued over for a century, and what they were for is still not agreed.
The plates and how they sat
The plates grew within the skin rather than attaching to the vertebrae, which is why they are frequently found scattered around a skeleton rather than in position and why reconstructing their arrangement took so long. Early reconstructions placed them flat against the body like roof tiles, which is what the animal's name refers to, and later ones stood them upright in a single row, in pairs, and finally in the alternating double row now generally accepted, supported by a specimen found with the plates preserved close to life position. They are not solid bone but contain extensive channels that carried blood vessels, and the surface bears grooves where vessels ran, which is the main evidence for the thermoregulation argument. They varied enormously in size along the body, largest over the hips and smaller towards the head and tail, and they differ between species and, in at least one case, between individuals in ways that have been interpreted as a difference between sexes, though that claim has been disputed on statistical grounds.
The competing explanations
Each proposal accounts for part of the evidence and none accounts for all of it:
- •Defence, which struggles because the plates are relatively thin, do not cover the flanks where an attack would come, and would break rather than deflect
- •Thermoregulation, supported by the blood vessel channels and by an early study on heat transfer, and weakened by the observation that related animals had spikes rather than plates and presumably needed to manage heat too
- •Display to rivals and mates, which fits the variation between species and the large size relative to any practical function
- •Species recognition, supported by the fact that plate and spike arrangements differ sharply between closely related species
- •Intimidation, with plates making the animal look larger from the side, which is plausible and difficult to test
- •Some combination, which is the position most researchers now take, with display the primary function and heat exchange a secondary consequence of the structure rather than its purpose
The tail spikes
The four spikes at the end of the tail are far better understood than the plates and are unambiguously weapons. They are thick, solid, up to a metre long in the largest species, and mounted on a tail whose vertebrae and musculature indicate substantial lateral mobility, so the animal could swing them hard. The evidence that they were used is direct: a substantial proportion of recovered spikes show damage and healed injury consistent with impact, and at least one predator vertebra has been found with a puncture wound matching a stegosaur spike in size and shape, which is as close to a documented encounter as the fossil record provides. Biomechanical modelling of the tail supports a powerful swing. The informal name for the arrangement, the thagomizer, entered technical use from a newspaper cartoon in 1982 and is now used seriously in the literature, which is an unusual route for a piece of anatomical vocabulary.
The brain and other corrections
Stegosaurs are the source of a persistent piece of misinformation, the claim that they had a second brain in the hip. The basis is a genuine enlargement of the spinal canal in the hip region, which is real and is present in many dinosaurs and in living birds. The interpretation as a brain was proposed in the nineteenth century and is wrong. The favoured explanation is a glycogen body, a structure present in birds at exactly that position whose function is not fully understood and which is not neural tissue in any meaningful sense. The related claim that the animal was exceptionally stupid based on brain size rests on comparisons that do not account for body size properly, and while stegosaurs did have small brains relative to body mass, so did most large herbivores and the inference to behaviour is weak. These corrections are worth making because both claims remain in circulation in popular material long after being abandoned by the field.
The takeaway
The plates grew in the skin rather than attaching to bone, which is why their arrangement took a century to settle, and their blood vessel channels support a heat exchange role while their variation between species points at display. Nobody has explained them completely. The tail spikes are unambiguous weapons, with healed damage on them and a matching puncture in a predator's vertebra. The second brain in the hip is a nineteenth-century error.