The Plates Were Not a Roof
[ STEGOFACT No. 001 ]
The stegosaurus is named after a mistake.
Othniel Charles Marsh named it in 1877 from bones found near
Morrison, Colorado. He chose Stegosaurus — Greek stegos,
"roof", plus sauros, "lizard" — because he believed the plates
lay flat across the animal's back like the shingles of a roof. He also, at
first, took the whole animal for something aquatic and turtle-like.
Both ideas were wrong. The name stuck anyway. It is now among the most
famous dinosaur names in the world and it describes something that never
existed.
HOW THE PLATES WERE ACTUALLY ARRANGED
Marsh's 1891 restoration put the plates in a single row along the
spine. Also wrong.
Charles Gilmore corrected it in 1914: two rows, staggered,
so that no plate sits directly opposite another. That is still the accepted
arrangement.
We know this because of specimen USNM 4934, dug out of the Felch
Quarry in Garden Park, Colorado in 1886, which died and was buried with
its plates still very nearly in position. It has seventeen of them.
Across all known specimens the count runs from about 17 to 22 plates
and flat spines. Anyone who tells you there is one correct number is
overstating the case.
The plates were osteoderms — bone formed inside the skin. They
were not joined to the skeleton at all. In life they were sheathed in
keratin, the same stuff as your fingernails. We know this because a
Hesperosaurus plate has been found with that covering still preserved
on it.
SO WHAT WERE THEY FOR?
This is the part where I have to be honest with you: nobody knows.
Here is where the argument stands.
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ARMOUR
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Largely rejected. The plates are too thin to stop much, and they are on
top of the animal, where nothing was biting it. They protect the flanks
not at all.
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RADIATORS
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Weakened, but not dead. Farlow, Thompson and Rosner proposed in
1976 that the plates worked as heat-loss fins. Main and
colleagues in 2005 looked at the internal bone structure and
found nothing specially built for cooling — the same plumbing turns
up in ankylosaur armour. A CT study in 2010 landed on a careful
middle position: the plates probably did shed some heat, simply by being
large pieces of skin sticking up into the air, but that looks like a side
effect rather than what they evolved for. Your horns do this too.
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DISPLAY
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Currently the front-runner. But "display" splits into two camps that do
not agree: plates for attracting mates, or plates for
recognising your own species. There is no winner yet. Some
stegosaurs had no similar neighbours to be told apart from, which is
awkward for the second idea.
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BOYS AND GIRLS?
In 2015 Evan Saitta examined forty plates from a Montana quarry
and found two shapes with nothing in between: tall narrow ones, and wide oval
ones with up to 45% more surface area. Both came from animals that had
finished growing, so it is not simply a matter of age. He suggested one form
was male and the other female.
Two cautions, because you will not always be told them:
Those plates are not from Stegosaurus stenops. They are from a
close relative usually called Hesperosaurus mjosi. Copy that says
"stegosaurus plates come in male and female" is skipping that.
Saitta himself said he could not actually tell which morph was which sex.
Without eggs or the right bone tissue, nobody can. The reception from other
paleontologists was cool, and the argument is still going.
It remains one of the better cases anyone has made for telling male and
female dinosaurs apart, and one of the most argued over. Both things are
true at once.
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