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




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









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