What Do You Call the Spikes on a Stegosaur Tail? A Cartoonist Decided
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
The arrangement of spikes at the end of a stegosaur tail had no name until a newspaper cartoon supplied one as a joke in 1982. Palaeontologists adopted it and still use it.
Where the name came from
Gary Larson drew a panel for his strip The Far Side showing a caveman lecturing others with a pointer at a diagram of the spiked tail, captioning it as named after the late Thag Simmons. Ken Carpenter, a palaeontologist at the Denver Museum, used the term in a presentation in 1993 while describing a specimen, and it spread through the field from there. It has since appeared in peer-reviewed papers, in museum labels and in textbooks. It has no formal standing, since anatomical nomenclature has committees and procedures and this went through none of them, and it is used anyway because the structure genuinely needed a name and had none.
What the structure is
The arrangement is consistent enough across the group to be described:
- •Usually four large spikes, in two pairs near the tail tip
- •Some species carry more, and one carried eight
- •The spikes are bone with a horny covering that made them longer in life
- •They project sideways and backwards rather than upward
- •The tail vertebrae there lack the processes that restrict movement
- •That gives the end of the tail unusual sideways flexibility
The evidence that it was a weapon
A structure of this kind might be display, and in this case the evidence for use in combat is unusually direct. Around one in ten known spikes shows damage and subsequent healing, which means the animal was alive when the injury happened and survived it, indicating repeated forceful use rather than accident. A tail vertebra of an Allosaurus has been found with a puncture wound matching a stegosaur spike in size and shape, and the bone around it shows infection, so the injury was inflicted on a living animal. Biomechanical modelling of the tail indicates it could be swung fast enough and with enough force to drive a spike deep into flesh and bone.
Which animals carried one
The arrangement is not confined to the single famous animal and the variation between relatives is informative. Kentrosaurus, from what is now Tanzania, carried spikes along much of its tail and further ones projecting from the shoulders or hips, giving a much spikier animal overall. Huayangosaurus, an early Chinese form, had spikes as well as plates along the back. Tuojiangosaurus and several others fall between. The general pattern across the group is that earlier members carried more spikes and fewer plates while later ones traded spikes for larger plates, retaining the tail arrangement throughout, which suggests the tail weapon was the part nothing was willing to give up.
What the plates were for instead
The spikes and the back plates are frequently discussed together and the evidence points in different directions for each. The plates are thin, are made of surprisingly light bone shot through with channels for blood vessels, and are set in the skin rather than attached to the skeleton, all of which makes them poor armour, since they protect nothing and would break. Suggested functions include regulating temperature by shedding or absorbing heat through the blood supply, which is now generally doubted on the grounds that the vessel pattern is more consistent with supplying a covering than with cooling, and display, either for recognising members of the same species or for competing within it, which is the current preference.
The takeaway
A newspaper cartoon in 1982 supplied a joke name for the tail spikes of a stegosaur, a palaeontologist used it seriously in 1993, and it entered the literature despite having no formal standing, because the structure needed a name. Around a tenth of known spikes show injuries that healed, and an Allosaurus vertebra carries a matching puncture with infection around it, which is direct evidence of use in combat.