Palpedrals and Supraorbital Ossifications

["Jaime A. Headden" <qilongia@yahoo.com>]

  Pete's remarks on this particular bone have sparked
some interest I have previously entertained when I got
Lee's paper on *Pawpawsaurus* which preserves
purported palpedrals.

  In ornithischians, there appear to be four types of
palpedrals:

  Basal ornithischians through ornithopods have the
arch-shaped palpedral which would probably form a full
arch of bone around the orbital rim, but is only
semi-ossified. A tendon continues the arch,
apparently. Only *Agilisaurus louderbacki* has a
completely ossified palpedral arch. This is shorted
relative to skull length and orbital fore/aft length
in dryomorphs. It also appears to be firmly appressed
to the skull in these taxa, and it is rare to find one
without the bones attached to closely disassociated.
[data from Weishampel and Witmer, 1990, Sues and
Norman, 1990, and Norman and Weishampel, 1990, in the
Dinosauria]

  Stegosaurs appear to lack palpedrals at all, but
sport, along with ankylosaurs, a series of
supraorbital bones fused into a dorsal "shield" for
the eyeball. Looking at *Stegosaurus*, I see the outer
two s/o/o's look like very thick palpedral/tendon
ossifications, but are clearly distinct, so there may
be two separate ossifications, instead of
unidirectional or prograssive ossifications, which can
be hypothesized for *Agilisaurus*. The same condition
can be hypothesized for pachycephalosaurs. [data from
Coombs, Weishampel and Witmer, 1990, and Galton, 1990,
in the Dinosauria; Peng, 1990; and Coombs and Galton
elsewhere]

  Ankylosaurs, on the other hand, have skulls that are
so nearly completely covered by subdermal ossifcations
and fused skull plates that not even Coombs or
Maryanska have been sure if they were there. However,
*Euoplocephalus* and *Pawpawsaurus* exhibit a new type
of ossification that was apparently "jointed" with the
skull in such a manner as to "close over" the orbit
like a door. In *Euoplocephalus*, this may have acted
as a garage door, sliding within the orbital rim to
completely cover the orbit. *Pinacosaurus* is
purportedly similar to Pawpaw and Euoplo, but I
haven't seen it, so couldn't comment. These bones have
smooth inner surfaces, so would not likely have
supported musculature or tendons. [Coombs and
Maryanska, 1990, in the Dinosauria; Maryanska, 1977;
and Coombs, various dates; Maryanska and Osmolska,
1974]

  Psittacosaurs have very loose palpedrals, and skulls
are often foudn lacking them, while other have them,
sometimes just one. A lachyrmal foramen opens just
ventral to the palpedral, and this suggests to me that
these bones were mobile, and could adjust at will.
Previous suggestions of "sunglasses" are workable for
these taxa, but this is just an idea. [lots of refs,
including Sereno, 1990, in the Dinosauria, and in
Dinosaur Systematics]

  Functionally, I can see two uses for the first and
fourth, the arched and loose, types: Pete's hypothesis
of structures for musculature bracing owing to reduced
width of the frontals suggests the eyes were rather
spherical, to fit in the orbits, and also quite large.
Additional space for the occular muscles would be
required, and the braces and surface for them as well.
Psittacosaurs have wider frontals, and so based on
this hypothesis, would not need arch-palpedrals. The
palpedrals are triangular, and possibly mobile, and
could adjust the supraorbital flesh, but for what
reason I have no idea; dorsoventral movement may be
possible, but the bones would have to be examined to
test this. I think enough skulls are available in
America (the AMNH, or one) to allow ready access
within the country to permit this.

  The locked form of the stegosaur
supraorbitals/palpedrals suggests they may have braced
something, and may likely have supported dermal
structures; head knocking or wrestling stegosaurs
would spare the near-fatal likelyhood of tail-matches
in these prickly titans. Ankylosaur eye shields is as
good an idea as any, and I doubt there's any reason to
doubt it, worked out by Cambpell, Lee, Coombs, and
others in the past two decades.

  My comments,


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