"I don't know; I've never Kippled" (was RE: What is a Dinosaur? and semilunate carpal)

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Dinogeorge@aol.com
>
>
> In a message dated 9/6/01 1:20:34 PM EST,
> twilliams_alpha@hotmail.com writes:
>
> << I wonder if this feature, like the increasingly inflexible wrist of
>  tetanurines, was designed to prevent the forelimb and hand from
> "wobbling"
>  while gripping prey - in other words, providing a more stable and secure
>  grip while engaging struggling prey. >>
>
> No, it was "designed" to prevent wobbling while the animal was
> winging its
> way through the air. Wobbling wings don't work very well, and any
> anti-wobbling adaptations would be selected for in the evolution
> of flight.
> The anti-wobbling features of the forelimb were later inherited by the
> flightless descendants of the winged ancestral tetanurans and exapted for
> other purposes.

Glad to hear that either (or both) of you have working time machines so that
you can actually observe these body parts in use... :-|

In fact, what we can say is that design would prevent wobble.  Full stop.

It can be noted that said wobble-prevention is consistent with a) a more
stable and secure grip while engaged in prey acquisition and dispatch, b)
stable flight mechanism, c)-x) other alternative functional scenarios, not
addressed here (and perhaps not yet imagined by any human, but equally
fitting for the data).

This highlights why we DO NOT (or at least SHOULD NOT) use the Just-So-Story
approach to adaptation.  A particular biomechanical structure can fit
multiple different scenarios simultaneously.  What we then must do is find
other alternative methods of choosing between the various scenarios, if
possible (for a given set of evidence, it is sometimes not possible to
choose between various alternatives).

Some potential ways of assessing the different scenarios:
I) The old-fashioned method (aka the confirmation bias): Scenario A is more
appealing to me than B, and I *think* (read "feel") that it fits the data
better.
II) The serious biomechanic method (aka "quantify, dammit!"): When we
actually figure the mechanics of the situation by means of physics, Scenario
C is found to be possible, while Scenario D would not actually work, even it
it is intuitively appealing.
II) The phylogenetic method: Scenario E matches the known anatomy of the
immediate precursors and basalmost representatives of the clade
demonstrating the derived condition, while Scenario F does not.

There are some additional methods of assessment, depending on the type of
scenario in question (biogeographic, ecological, etc.).

Some thoughts for the day.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796