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Re: Conservatism of dinosaur brain cases [long]
WARNING: Long post. Do try to stick it out, I think it gets kindof
interesting towards the end (it talks more about evolution in
>Sorry, this is still argument from authority. What >>evidence<< is
>there that an inflated parasphenoid (wasn't it basisphenoid?)
>capsule couldn't have occurred more than once within Dinosauria?
Sorry, I have an authority complex :) .
It is referred to in _The Dinosauria_ as Basiphenoid, but Peter
BucHHoltz told me that Tom HolTz said that the structure on
ErliC/Kosaurus is the PARAspehnoid (or is homologous
thereunto)(pers. com. from pers. com. Objection, hearsay, your
Unfortunately, there will be no real hypothesis until someone actually
takes the plunge and does a postion of the Segnosauria
(Therizinosauria?) study in detail. Personally, I don't trust the
data in my own cladogram very far, since I have yet to avail myself of
the new drawings of Erlicosaurus or the Alxasaurus paper (in the
Canadian Journal of Earth Sciences, 1993?).
Until then, there is no *evidence* that the bulbous parasphenoid, the
semilunate carpal, the , or in Erli(c,k)osaurus is or is not parallel
(or convergent?), but the former is a unambiguous synapomorphy of
Bullatosauria, for which taxon the latter is plesiomorphic.
On another plane, last time I looked, Erlikcosaurus' lacrimal was
barely exposed on the skull roof (a theropod synapomorphy), and
ceratainly it is much less so than on Plateosaurus. I should note,
however, that this is not as important, in the broad view, as the two
characters previously mentioned.
>By the way, I've used the supposed conservative nature of braincase
>anatomy myself on occasion. Still doesn't make it right. And
Why isn't it right? I'd refer it to personal communication. Granted,
a full-blown study would be better.
>evolutionary changes in manual and pedal anatomy might well be
>virtually irreversible and thus weigh >>even more<< than one or two
>features of braincase anatomy.
I cannot speak to manual anatomy, since I have no knowledge of the
Senosaurian manus. As for pedal morphology, I don't know about you,
but mtIV of Erli'osaurus [from now on, I am going to choose one
spelling or another and use it] is very short, and I wouldn't be
surprised if it didn't take much weight.
You may know that I disagree with what I have gathered of you're
[Dinogeorge's] position on evolutionary reversals.
My first point would be that bone that is still there, even if
it's only present in ovo, is never lost, and if the need arises, it is
much more readily available than bone that isn't. Since the
theropodian fourth toe is more or less complete, redeveloping the
metatarsal to reach the ankle is not all that big of a deal. See next
Second, the strong evolutionary pressures which you allude to
as causing the "irreversability" of manual and pedal morphology I see
as being the fundamental causes of such reversal. The feeding,
motive, and reproductive superstructures of an animal are subject to
extreme innovation, which often makes their form less useful in
establishing phylogenetic relationships (see my Currie quote of the
origional post on this subject). As long as a reversal is
advantageous to the animal, or not harmful, and accompanied by a
useful change, that animal's genes will be favored.
That such reversals are rare is possibly the product of the
fact (which you are also alluding to) that they may involve some major
changes in the way the animal operates, or it may have something to do
with phlogenies (like my cladeogram) which concentrate too much on
superstructural characters and not enough on conservative items like
EXAMPLE: The pes of most living birds is funcitonally tridactyl, with
a proportionally long digit IV (passeriforms) and an unremarkable
digit II. The primative condition for these digits in the
Dromaeosauridae (Paul, _PDW_) is greatly reduced digit IV (well, maybe
we are more secure saying this is primitive to the Coelurosauria) and
short, hyperextendable digit II. Rigorous cladistic analysis (which,
contrary to Martin (ASOTV, Paleoworld), does not involve the use of
wooden silhouette models) has confirmed that birds are sister taxon to
archaeopterigians, which are a sister to Dromaesaurinae. The only
resolution to this is a reversal to the primitive archosaurian and
theropod (respectively) conditions.
The argument may be made that this was allowed to happen
because evolutionary pressures for pedal locomotion was no longer as
great as those for flight. And yet, if the pressures on pedal use (in
the case of passeriforms, for perching) were not so great, why would
the toe redevelop at all? I admit, it's not a strong example, but I
think it is an important consideration.
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