Pneumatic Basicrania

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

Hehehe,
   I sent an email in reply to Ekaterina Amalitzkaya, but forgot
to send it to the list. Here is most of my comments, as best as
I can remember:

  The parabasisphenoid complex in theropods tends to be fused,
resulting in any pneumatic feature in one bone (the
parasphenoid) being caried over into the other (basisphenoid)
but seldom the reverse; i.e., whenever the parasphenoid is
pneumatic, so is the basisphenoid, but the basisphenoid can be
pneumatic with a simple parasphenoid.

  In theropods, there are really three or so features related to
the pneumatic para-basisphenoid, as seen in *Erlikosaurus* and
*Tyrannosaurus* (=tyrannosaurids) as brought up by Ekaterina;
the condition is rather more extensive, so I'll take a short
overview here and try to explain:

  1. Basisphenoid, pneumatization: absent, [plesiomorphic
     condition to theropods], 0; minimal with slight
     foramination of the ventral table or plateau [generally the
     condition in most tetanuran theropods], 1; extensive with
     channels, large foramina, and internal camellae,
     basisphenoid expanded bulbously.

  2. Parasphenoid, pneumatization: absent, [plesiomorphic
     condition to theropods], 0; forms internal camellae from
     basisphenoid, 1 [this is the condition in some theropods
     where there are foramen on the external surface that have
     been invaginated by basicranial diverticula of the cranial
     airsacs]; deep pocket formed from foramen caudally [the
     ornithomimosaur and troodontid "bulla"], 2.

  3. Basisphenoid recess, condition: deep and narrow,
     plesiomorphic condition to theropods], 0; shallow and broad
     [derived condition, tyrannosaurids for the most part, also
     called the basisphenoid plateau or something similar
     (sorry, my refs are not here, I've been moving!)], 1;
     recess shallow, forming a sulcus or small channel, or
     absent, 2 [this is the condition in birds, so is considered
     the dervied state].

  Up until about *Allosaurus*, the condition in theropods is
plesiomorphically "0" across the board.

  *Allosaurus* has several lateral pneumatic fossae formed from
  cranial diverticula on its skull, and especially has features
  relating to minor pneumatization of the basisphenoid.

  Tyrannosaurids possess a broadly pneumatized basisphenoid but
  shallow recess, and the basicranium ventrally has a quadrate
  form because of this.

  Ornithomimosaurs and troodontids have a minimally pneumatic
  basisphenoid, but have a deep pocket formed into the
  parasphenoid that inflates it caudally behind the rostrum or
  cultriform process. This is the "bulla." In ornithomimosaurs,
  the pocket is open and dorsally connects to the pineal
  foramen. In troodontids, it is closed to a foramen, and there
  is no contact with the pineal foramen. The condition of a
  bulla in both is interesting, with troodontids perhaps being
  slightly more derived in having this caudal pocket (as in
  ornithomimosaurs) closed off. Troodontids also lack a deep
  basisphenoid recess, which preserves as a sulcus and is
  effectively absent. They share this with birds.

  Dromaeosaurids have a deep basisphenoid recess, but have only
  a minimally pneumatic basisphenoid, as above; the parasphenoid
  is only partially pneumatic.

  *Erlikosaurus* has a pneumatic basisphenoid, but the condition
  of the basisphenoid recess and parasphenoid are unknown, and
  this portion is missing in the skull. The basisphenoid is
  extensively pneumatic, bulbously expanded, and formed into
  many little camellae, as in tyrannosaurids and oviraptorids.

  Oviraptorids have a bulbous basisphenoid, pneumatically
  camellate, without a basisphenoid recess, and minimal
  parapshenoid pneumatization. *Chirostenotes* is known by a
  skull, but the braincase is not complete and largely consists
  of the occipital plate, opisthotics, basitubera, and not very
  much else, so the conditions described above is largely
  unknown.

  There are other forms of pneumatization in the skull: 4. the
basipterygoid processes are dorsally invaginated with pockets in
*Itemirus*, dromaeosaurids, ornithomimosaurs, and troodontids;
5. The lateral braincase wall will have an extensive otic recess
in birds, oviraptorids, *Erlikosaurus*, troodontids, and
dromaeosaurids; 6. Pneumatic paroccipital processes are known in
dromaeosaurids, birds, ornithomimosaurs, tyrannosaurids,
*Allosaurus,* and troodontids.

  These features when applied to a phylogeny of theropods (say,
Sereno, 1999, or Holtz, 2000) indicates a Tetanurae and a
Maniraptoriformes, but for some reason oviraptorids and birds
and segnosaurs tend to group closer than dromaeosaurids do, and
tyrannosaurs are closer to birds than ornithomimosaurs are
(Sereno's Tyrannoraptora). However, it is based on pneumatic
features only, so there is caution in treating what Madsen and
Chure refer to as a highly plastic skull form, in *Allosaurus*
alone. Most dromaeosaurid and troodontid skulls share the same
conditions one among the others, but the case in
ornithomimosaurs is not well described, as in tyrannosaurids for
the most part, and only one oviraptorid braincase has ever been
described in any detail (Barsbold, 1977) which leaves the main
distribution rather sketchy.

  [Mind, as I am away from my refs, this is from memory <gasp!>
so any faults are mine and if anyone wants to elaborate, please do.]

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

__________________________________________________
Do You Yahoo!?
Get personalized email addresses from Yahoo! Mail
http://personal.mail.yahoo.com/