The Thagomizer
[ STEGOFACT No. 002 ]
The spikes on the end of a stegosaurus tail are called the thagomizer.
This is a real term, used by real paleontologists, in real museums.
It comes from a Far Side cartoon. Gary Larson drew it in
1982: a caveman lecturing with a pointer at a diagram of a stegosaurus
tail, saying
"Now this end is called the thagomizer …
after the late Thag Simmons."
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Paleontologists had no name for the arrangement, and this one was better than
having none. Kenneth Carpenter used it in a talk at the Society of
Vertebrate Paleontology meeting in 1993, and it spread from there. It
is now used by the Smithsonian and by Dinosaur National Monument.
One small correction to a thing people often say: it is an accepted
informal term, not a formal anatomical one. No committee ever voted on
it. It simply won.
HOW MANY SPIKES?
Stegosaurus stenops had four, in two pairs. Each one was
roughly 60 to 90 centimetres long.
You may have read that it had eight. Marsh said so, for
S. ungulatus. He was working from bones of more than one animal that
had been mixed together. Carpenter sorted this out and S. ungulatus is
now reconstructed with four as well.
They stuck out sideways and backwards, roughly horizontal — not
straight up. A great many old museum mounts and a great deal of old artwork
get this wrong.
DID IT ACTUALLY HIT THINGS?
This is about as well evidenced as dinosaur behaviour ever gets, and the best
single piece of it is one bone.
Specimen UMNH 10781 is a tail vertebra from an Allosaurus. It
has a hole punched clean through one of its wing-like side processes. The
hole is not a bite — there are no tooth marks — and it matches the
cross-section of a stegosaurus tail spike.
The edges of the wound show remodelled bone. It healed.
Something stabbed an Allosaurus through the tail with a spike, and the
Allosaurus walked away and lived long enough for the injury to grow
over.
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It went the other way too. A stegosaurus neck plate from the Cleveland-Lloyd
quarry in Utah has a U-shaped notch bitten out of it that fits the
front of an Allosaurus snout. There is almost no meat on a plate, so
this was probably a fight rather than a meal.
Of 51 stegosaurus tail spikes examined in one study, about
10% had broken tips where the bone had then grown back. The animals
broke their spikes and survived it.
The tail could swing. S. stenops tail vertebrae lack the stiffening
tendons that lock many other dinosaur tails rigid, and the joints look built
for side-to-side movement. The base of the tail was more restricted, since the
big plates were in the way — so the far end was the mobile part.
A WARNING ABOUT A NUMBER YOU WILL SEE
Somebody will eventually tell you a stegosaurus could swing its tail at
40 metres per second. Do not repeat it.
That figure comes from a careful computer model of Kentrosaurus,
which is a different and considerably smaller stegosaur. Nobody has run those
numbers for Stegosaurus. The modelling does suggest a stegosaurus would
hit harder, having two pairs of spikes rather than one — but that
is a reasonable expectation, not a measurement.
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