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Re: Flightless Birds (was Re: BCF ANDPDW)



        The only cruddy thing about George's "digest" postings is that we
live beneath the shadow of sending three replies before George's first ever
gets posted to the list. Oh well, at least he won't get booted for too many
messages...
At 05:31 PM 8/5/98 -0400, George Olshevsky wrote:
>What you want to know is why were, say, herrerasaurians and lagosuchians
>>replaced< by ceratosaurians, ceratosaurians by tetanurans, tetanurans by
>maniraptorans, and so forth. >If< this is indeed what actually happened. [...]
>Well, this isn't exactly what happened. [...] None of the more birdlike
>maniraptorans seems to have displaced tyrannosaurids or ornithomimids, for
>example.
        True enough. I would not say that these groups *replaced* previous
groups, nor does you "BCF" theory seem to require this. My point was that,
in order for these groups to radiate from flight to flightlessness, there
would have to be open niches, and/or some sort of competitive advantage
within the recently flightless clade. They don't have to replace the
established groups, simply exclude them from the niches they take over.
        You're hypothesis of gradually "improving" (I use obsolete
terminology to speed explanation, let's not get into the technicalities)
metabolism associated with each new radiation seems to fit the bill. I would
assert that I don't think this has been demonstrated, nor does it seem
sufficient by itself to allow these radiations. However, you have minimally
answered the challenge.

>The only discernible pattern is the timing of the appearance of the
>progressively more birdlike theropods, not what they may have evolved into
>after finding and occupying a congenial set of niches.

>Also, it seems as if at any one time, the dominant dinosaurian predators in a
>particular region are also the least birdlike
        This is an interesting observation. However, phylogenetically,
tyrannosaurs may be closer to birds than, say, bullatosaurs (vide Sereno's
most recent phylogeny). As far as morphology is concerned, to be the
"dominant predator" seems to require a lot of change, including increased
size. There seems to be evidence that much of the phenetic difference
between, say, tyrannosaurs and birds, is due to allometric growth and may be
over-emphasized within a deliberately phenetic context.

>Both BCF and BADD account for these successive theropod radiations with the
>same kinds of cladograms;
        To a certain extent, except "BCF" requires that the cladogram be
read more deliberately as a map of the evolution of the group rather than as
a graphic description of propinquity of descent.

>(BADD still likes saurischians, BCF doesn't; big deal).
        I doubt saurischian monophyly would cripple "BCF". :)

>BCF, these dinosaurs were in the trees for a long time, [...]
        Except the characters you map onto the tree. In a "BCF" model, you
map "arborality", "flight", etc. a lot lower on the tree, meaning that these
characters must be reversed higher up much more often. Example:
        Phylogeny [modeled on your theories]:
        (Lagosuchids, ("Phytodinosauria", ("herrerasaurs", (Ceratosauria,
(Carnosauria, (Ornitholestes, (Arctometatarsalia, (Oviraptorsauria,
(Deinonychosauria, (_Unenlagia_, Avialae))))))))))
        Let's look at two characters, "flight" and "arborality". I imagine
that the distribution of other characters is going to be pretty much the
same in "BCF" as it is in most folk's phylogenies. We'll assume that
arborality evolved at the base of the tree and flight evolved before the
common ancestor of Neotheropoda for "BCF". Under a conventional model, these
devloped in avialae. Let's assume that arborality was not present in at
least one taxon within each ingroup. Number of character transformations for
each model is given below:
        BCF     17      Conventional Model: 2
        I doubt you will find character weightings which can erase the 15
step difference.

----------------------------------------------------------------------------
Jaime A. Headden wrote:
>But suddenly, they can stand up to those marsupials and mammals that gave
them >such a hard time when they were smaller, psilopterid-like.
        Motive and method. I'm not sure, though. That nether world between
being too large to fly (!?!?) and too small to fight seems an awful hurdle
for selection. I wouldn't be surprized if there were either some geographic
isolation from larger predators or some depletion of the large predator
fauna when that occurred.
        
>This was a true case of secondarily flightlessness, but was it a
>reversal, or a convergence? Semantics, really.
        Unfortunately, as I pointed out earlier, semantic differences have
lead to enough frustration already. Let's be careful, if not for my blood
pressure, for George's.
        :)
        Wagner

        
----------------------------------------------------------------------------
    Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
                    "...To fight legends." - Kosh Naranek