Braincase blues [was: Re: Dermal coverings]

["Jonathan R. Wagner" <znc14@TTACS.TTU.EDU>]

George Olshevsky wrote:
>Also doesn't nullify my opinion that the description of the 
>Erlikosaurus skull is full of pesky and misleading little errors, such as 
>conflating parasphenoid and basisphenoid, [...]
        The following is based on my vertebrate paleontology class and from
Romer's _Osteology of the Reptiles_. I cannot vouch 100% for the currency of
this discussion, but I think it should hold up. Some of the topological
relationships are from memory (where noted) and may not be accurate, at
least not for all taxa).
        The rostro-ventral portion of the reptilian braincase is capped by
the basisphenoid. This is an endochondral, or replacement, bone, having
started growth as cartilige [sic] which was later ossified (as are most
other bones of the braincase). The basisphenoid is boxy in lateral view.
When viewed rostrally or ventrally, it is triangular, with the primary apex
directed rostro-dorsally. The lateral apices constitute the basipterygoid
processes, which connect the palate to the braincase. These are usually (in
dinosaurs) short, thick rounded processes, sometimes subangular in
longitudinal section. The anterior apex of the triangle passes SMOOTHLY INTO
THE PARASPHENOID ROSTRUM (emphasis added to make a later point with George).
In dinosaurs, the basisphenoid is often directed such that the flattened
center of the triangle is facing rostro-ventrally, with the basipterygoid
processes flaring caudo-ventro-laterally. The parasphenoid rostrum arcs
forward to an almost directly rostral orientation parallel with the roof of
the skull.
        The basisphenoid has a broad articulation with the basioccipital
caudally at or around the basal tubera. Caudo-dorsally (darn... I hate
rostral/caudal terminology...) the basisphenoid articulates with some bones
I forget, possibly the orbitospehnoid and the laterosphenoid (ossified only
in archosaurs). Within the lateral walls of the basisphenoid are the videan
canals, passages for the internal carotid artery, which pass rostro-dorsally
into the body of the bone. If I am not mistaken, the pituitary fossa passes
ventrally into this bone, and I believe cranial nerve VIII passes through
this bone, although as I recall that nerve never really exits the braincase.
        The parasphenoid is a dermal bone, having grown as bone rather than
replacing cartilige. It consists of two sections: the first part is the
parasphenoid rostrum, the elongate , scoop-like extension of the
basisphenoid. The second part is A SHEATH-LIKE COVERING OF THE BASISPHENOID.
This sheath covers the flattened rostro-ventral surface of the basisphenoid,
and is often indistinguishable from the latter bone. 
        he articulation between the basisphenoid and the parasphenoid
rostrum is also often obscure. the parasphenoid rostrum extends in an arc
rostro-dorsally and thence rostrally fromt the anterior apex of the
basisphenoid "triangle". The parasphenoid rostrum is a hollow tube which is
apparently open at the top, vaguely resembling a narrow chemistical scoop.
Dorsal to its base are twin openings for the median palatine artery. Lateral
to these are two thin pillars extending from the rostral wall of the
basisphenoid, the dorsum sillae [sic?], which, at least in some lepidosaurs,
sport the foramena for cranial nerve VI at their rostral extent.

        George's traditional exceptions to the the interpretation of
homology among the inflated parasphenoid of ornithomimisaurs and the
inflated basisphenoid of _Erlicosaurus_ are as follows:
        a)      They are in different bones
        b)      These bones are in different parts of the braincase (rostral
                and ventral)

        These can be countered by the following points:
        1)      These bones articulate; their topology with respect to the rest
                of the braincase is irrelevant.
        2)      The units of evolution* are structures (indeed, Whitmer points
                out in his big AoF monograph they are SOFT TISSUE stuctures),
                not bones. Bones can "flow" around structures, evolutionarily
                speaking... see the many different configurations of the AoF
                among archosaurs.
                        * morphological evolution, that is
        3)      Even disregarding the above, it could be suggested that it
                is the parasphenoid "sheath", not the basisphenoid, which is
                inflated in _Erlicosaurus_. It would then be the same bone that
                is inflated in both taxa.
        4)      In any case, homolgy is tested by phylogenetic reconstruction,
                not determined a priori of said reconstruction. Since homologous
                structures (i.e. structures inherited from a common ancestor)
                may change throgh evolution, it is circular to judge their
                homology a priori of analysis.

        Any errors in the above are mine alone, and may result in a spanking
for yours truly.

        Wagner
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     Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
  "Why do I sense we've picked up another pathetic lifeform?" - Obi-Wan Kenobi