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

["Tim Williams" <twilliams_alpha@hotmail.com>]

I said:

> > << 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. >>

Tom Holtz replied:

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

Well, my TARDIS is on the blink at the moment, so I can't travel back in 
time and find out for sure.  I guess that's why I began my comment with "I 
wonder if..." rather than "I know in my heart-of-hearts that this is what 
happened...".

In other words, it was a hypothetical adaptive scenario.  Nothing more, 
nothing less.

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

These critters were predators (and I don't think that requires a first-hand 
observation.)  I don't think I'm going out on a limb (so to speak) in 
positing that certain modifications in the pectoral skeleton of a predatory 
animal *may* have had a predatory function.  Sure, I don't have empirical 
data to support this scenario over competing scenarios; but then again, I'm 
just brainstorming on the DML, not submitting to a scientific journal.

The difficulty (as I see it) is that ascertaining the biomechanical 
limitations or freedoms imposed or allowed by a novel feature does not 
necessarily tell you what the real-life animal used it for from day-to-day.  
Nor does an animal's position in a phylogeny tell us directly what it was 
used for; only that a novel adaptation was laid upon, or replaced, a 
pre-existing structure.  Of course, it can indicate what it was *not* used 
for; but after eliminating certain scenarios, what's left over can be open 
to broad interpretation.

For example, we might find that the orientation of the forelimbs and claws 
in tyrannosaurid is _consistent_ with their use in holding large prey; but 
how can a phylogenetic or biomechanical approach tell you _exactly_ what a 
tyrannosaurid forelimb was used for, without at least some intuition thrown 
into the mix?  As long as an intuitive explanation is placed within a 
biomechanical or phylogenetic context.  The first might tell you that 
tyrannosaurid arms were too short to fly with and too short to reach the 
mouth; the latter tells you (based on current phylogenies) that the short 
arms are a derived theropod feature and evolved from longer forelimbs.  It 
can't differentiate between whether tyrannosaurids used their forelimbs for 
(a) clasping prey; (b) males grasping females during copulation; or (c) both 
of the above.


> 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.

This is also a valid way of _proposing_ a scenario, as a prelude to 
evaluating it against other (competing) scenarios - by examination of the 
data.

I get the thrust of what your saying: intuitively-derived scenarios aren't 
worth a pinch of dingo scat unless they're consistent with biomechanical or 
phylogenetic (or biogeographical etc) data.  As I understand it, the notion 
that constraints placed on maniraptoran forelimb movement by successive (and 
cumulative) modifications of the pectoral skeleton possibly had a predatory 
purpose have been around for quite a while - and is supported by published 
biomechanical and phylogenetic evidence.

That's all.

> Some thoughts for the day.

One more from me: What's a "Kipple?"



Tim









_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp