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Stegosaurs Osteoderms



Excuse me for coming late to this discussion.  Like most things
paleo, my information on Stegosauria and their varied osteoderms
is a couple of decades out of date.  But something stirred my
thinking in this area and I thought I would pass it along.

I'm not concerned with the plates.  I do think it it as least as
likely that the plates had some range of movement, as it is that
they carried some exotic form of resposive pigmentation, but my 
real concern at the moment is with the tail spikes.

As I was driving home this morning, I found myself behind a truck
that was pulling a chain cutting machine.  If you've never seen
one of these, they're quite impressive -- something like a huge
chainsaw.  This particular one was covered in rounded spikes about
a foot long.  The armature of the device was raised high in the
air, and as I looked up at these spikes, I started thinking of
how badly my car would be damaged if I ran into the machine.

Which led, of course, to thoughts about Stegosaurs.  

To state the idea quite bluntly: is it possible that some of
the tail spines of some Stegosaurs were mounted on the bottom 
surface of the tail?  

Mouting the spines on the top made sense when Stegosaur was 
reconstructed in the droopy dino style, with its tail dragging 
the ground.  But recent reconstructions show a stately, lighter 
beast with its tail held stiffly extended.  In such a position, 
downward pointing spikes might provide protection equal to, or 
better than upward jutting spikes.  This is especially true of 
the Stegosaurs that have extended femurs and shortened 
forelimbs like _Stegosaurus_ and _Kentrosaurus_.

Dredging my shallow pool of knowledge, I know that 
_Kentrosaurus_, with its large number of spines, has been
restored with the final pair at or near the terminus of the 
tail.  (By the way, I've never seen _Kentrosaurus_ restored 
to a more "modern" pose.  Should be an intimidating animal.)
There appears to be good evidence  -- perhaps the only 
strong Stegosaur osteoderm placement evidence -- for 
mounting the larger _Kentrosaur_ spines.  But is there actual 
osteo evidence for this placement of the final set of spines?

Looking at the only images of good old _S. stenops_ that
I have on hand, the placement of the plates seems to be
clearly at the dorsal crest (it also seems clearly, IMHO, 
that the plates formed as a single staggered row.  But 
that's another argument).  However, the tail spines are 
disarticulated.  I can't see anything in the tail vertebrae 
that suggests placement on either side, especially since
the final caudal vertebrae lack processes.

I would like to suggest that the paired spines, as seen in 
_S. stenops_ and _S. longispinus_ were paired not side by 
side, but above and below.  I would also expect them to be 
angled significantly toward the rear.  Other species without 
known pairs (I believe the spines known for _Huayangosaurus_ 
and some of the other Asian specimens all vary in size) may 
have carried these spines as single rows that "wrapped" 
the tail from top to base.  (However, when looking at an
animal with short rear limbs, like _Huayangosaurus_, it's
easier for me to believe that the tail was held on or near
the ground.)

For all the discussion of camo and cooling in the use of the
plates, the very nature of the spines suggests that they were
used as defensive, if not offensive, weapons.  An animal like
_S. stenops_, if it did indeed carry its tail in the pose
most commonly used these days, would have carried its tail
almost two meters above the ground (or more).  Unless we make 
the assumption that all predators of _S. stenops_ were both 
taller, and always attacked from above. Otherwise, all the 
upper armor was for naught.

Considering a possible lower placement for spines along the
tail might also explain some of the "extra" spines which
have been found with Stegosaurs worldwide, and have been
explained away (with little reason that I can see) as shoulder
or hip mounted spines.

All right, that's the whole rant.  Come tear me apart like
raptors going after an unarmored flank. <g>

Mark
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