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Re: Stegosaurus ungulatus and S. armatus



In a message dated 95-09-19 20:01:49 EDT, anarvaez@umd5.umd.edu (Amado
Narvaez) writes:

>I was admiring the Battat/Museum of Science in Boston Stegosaurus 
>ungulatus and decided to look up information about the creature in my 
>collection of reference materials. Lessem and Glut's _Dinosaur 
>Encyclopedia_ suggests that 1) the species S. ungulatus may actually be 
>the same as S. armatus, and 2) "there may have been four pairs of tail 
>spikes." I was surprised that there was any uncertainty at all about the 
>number of tail spikes. Most restorations confidently depict four pairs of 
>tail spikes. Can anyone explain for me why the uncertainty exists at all? 
>(I'm anticipating that the kids at my school are going to question the 
>accuracy of the 8 spikes of the Battat stego since for most of them there is

>one and only one Stegosaurus -- S. stenops, which has only 4 spikes

First of all, no specimen of any North American stegosaur has yet been found
with a full set of eight caudal spikes. On the Morrison quarry maps that I
have seen, there is one stegosaur tail with five complete spikes nearby and
portions of a sixth. Marsh's first restoration of _Stegosaurus ungulatus_ is
the famous one of 1891, showing the plates arranged in a single row along the
back and the tail spikes in _four_ pairs. Lull (1910: "The Armor of
Stegosaurus," Am. J. Sci., 4th ser., vol. 29(171): 201-210) agreed that the
holotype of _Stegosaurus ungulatus_ (YPM 1853) had "associated with the one
skeleton _no fewer than four pairs_ [emphasis mine] of spines and three odd,
sharp-edged, spine-like plates, one of which is so much larger than the other
two that it seems to imply that at least one intervening size is missing."
It's a bit difficult to interpret Lull's statement, because it is not clear
until you look at his figures what he meant by spines and spine-like plates.
The "spines," not the "spine-like plates" are the caudal spikes, and there
are, as he put it, "no fewer than" four pairs. That is how many he mounted in
the Yale specimen. All eight spikes were _not_ preserved in YPM 1853, but
there _were_ four spikes of different sizes, hence the argument pro four
_pairs_.

The uncertainty in spike number (and also dermal plate arrangement) arises,
of course, because almost all stegosaur specimens are found with much less
than a complete set of plates and spikes.

The species "_Stegosaurus" stenops_, which as far as I'm concerned belongs in
a genus different from _Stegosaurus_ (I've been using the name _?Diracodon_
for it), is well known to have had exactly two pairs of caudal spikes. The
differing caudal-spike count is one of several features that distinguish the
two genera.

The type species _Stegosaurus armatus_ seems to be different from
_Stegosaurus ungulatus_ in having taller, straighter vertebrae (among other
things), but until it is properly prepared, described, and illustrated, we
cannot be certain that this is not just individual variation.

G.O.