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Re: Reevolving bones?



Dinogeorge wrote:
>No, it's indeed not >exactly< like that of other four-toed dinosaurs. But it
>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...

>For one thing, no reversals are needed.
        As John just went to great lengths to point out, reversals are NOT a
problem.

>In deriving a segnosaur foot from a theropod foot you need something like half
>a dozen simultaneous reversals to make it happen. Rejecting theropod ancestry
>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
funcitonally?

>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...

>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: the
        Horse droppings...

>preacetabular processes of the ilia flare way out laterally, 
        Featuring a hooked process also seen in some other maniraptoform groups.

>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.

>there is a postacetabular lump on the ilium,
        Who cares?

>and the ilia are very tall dorsally.
        Also some oviraptorids, caenagnathids, _Microvenator_, and perhaps
_Harpymimus_.
        Acetabulum has a antero-dorsal lip, similar to _Microventor_ (and
some oviraptorids?).

>does not resemble a maniraptoran carpus in any particulars, but it >does<
>resemble a prosauropod carpus in the number, shapes, and disposition of the
>elements.
        I do not recall prosauropods having semi-lunate carpal blocks...

>And I find "cladistic infallibility" repugnant and dogmatic.
        Give it a rest George. No one says it's infallable.

>Reversals are a red flag in phylogenetic analysis--a signal that something may
>have gone wrong in the analysis.
       You have *NO* theoretical basis for making this claim.

>Although I agree that phylogenetic analysis must precede evolutionary
>hypothesis
        Stop the presses!

>As far as cladistics goes, I was discussing this problem a few weeks ago with
>Mickey Rowe, and I suggested the following experiment:
        Let's all flip to the systematics channel, shall we?
       
>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.
----------------------------------------------------------------------------
      Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock TX 79409
      "The cost of living hasn't affected its popularity." - Unknown