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Re: Reevolving bones?
In a message dated 97-05-06 22:18:44 EDT, email@example.com (Jonathan R.
Wagner) writes (quoting me):
<< Dinogeorge wrote:
>No, it's indeed not >exactly< like that of other four-toed dinosaurs. But
>derives from a four-toed dinosaur foot >much< more easily than from a
>three-toed theropod foot.
Have you read a single word I wrote? I was just addressing this:
cartilaginous metatarsal + similarity of MT I to that of theropods
suggests reversal more strongly than a convergence of features that
would make it look like a reversal, which you can't even explain...>>
What cartilaginous metatarsal I? Name one other vertebrate that has a
cartilaginous metatarsal I!
<< >For one thing, no reversals are needed.
As John just went to great lengths to point out, reversals are NOT a
As neither John nor you seems to understand, reversals >are< a problem, and a
<< >In deriving a segnosaur foot from a theropod foot you need something like
>a dozen simultaneous reversals to make it happen. Rejecting theropod
>in favor of prosauropod ancestry is more parsimonious.
Name them (I know you can). Has it occured to you that some of these
"reversals" might be part of the same reversal either developmentally or
<< >The theropod foot is remarkably conservative, being present virtually
>unchanged, except for the fusion of certain elements, in all groups from
>ceratosaurians to birds.
Tell that to Tom "The Arctometatarsalian Pes" Holtz!
So what if it had been conservative? If the animals were
experiencing selection pressure for a four-toed foot, I wouldn't be
surprized if the foot changed drastically...>>
Tom Holtz knows this as well as anybody on the planet. Now tell me what kind
of selection pressure would create a four-toed foot from a three-toed foot.
Is this the same kind of selection pressure that would cause the pelvis to
spread laterally? The claws to narrow and elongate?
<< >But feet aren't the only non-theropod aspect of segnosaurs. There's the
>problem of the segnosaur pelvis. It looks nothing like a theropod pelvis:
What kind of comment is this? Are you trying to elevate the level of this
discourse or something?
<< >preacetabular processes of the ilia flare way out laterally,
Featuring a hooked process also seen in some other maniraptoform
So are segnosaurs maniraptorans? Maniraptoriforms?
<< >the pubis is opisthopubic to the point of being fused to the ischium,
With lower pubic poduncle, as in dromaeosaurs, I believe. Also, the
ischium bears a triangular obturator process, which has migrated distally.>>
So are segnosaurs dromaeosaurs? What's a dromaeosaur, anyway?
<< >there is a postacetabular lump on the ilium,
This is a clear segnosaur apomorphy. Cladists should care.
<< >and the ilia are very tall dorsally.
Also some oviraptorids, caenagnathids, _Microvenator_, and perhaps
Acetabulum has a antero-dorsal lip, similar to _Microventor_ (and
So are segnosaurs oviraptorids? Ornithomimids? I thought they were
<< >does not resemble a maniraptoran carpus in any particulars, but it >does<
>resemble a prosauropod carpus in the number, shapes, and disposition of the
I do not recall prosauropods having semi-lunate carpal blocks...>>
They don't--and neither does _Alxasaurus_. And, for that matter, neither do
other segnosaurs: although they do exhibit fused carpal elements--just not a
"semi-lunate carpal block."
<< >And I find "cladistic infallibility" repugnant and dogmatic.
Give it a rest George. No one says it's infallable.>>
Cladists insist that their methodology must for philosophical reasons take
precedence over biogeography and functional analysis. Here's the cladogram,
filled with reversals and other dubious features. You're the functional
analyst-->you< figure out how these things evolved.
<< >Reversals are a red flag in phylogenetic analysis--a signal that
>have gone wrong in the analysis.
You have *NO* theoretical basis for making this claim.>>
Sure I do. Or is cladistics infallible after all??
<< >Although I agree that phylogenetic analysis must precede evolutionary
Stop the presses!>>
<< >As far as cladistics goes, I was discussing this problem a few weeks ago
>Mickey Rowe, and I suggested the following experiment:
Let's all flip to the systematics channel, shall we?>>
What kind of comment is this?
<< >Using a computer program that simulates evolution
Can we really do this? Forgive, but I rather thought that if we
could do this, I wouldn't have to go to grad school. >>
You obviously have no idea about what I'm talking about here, do you? Back to
square one, if you please.