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Re: Digit Loss
On Fri, Jun 08, 2001 at 05:17:30PM -0500, Steve Brusatte scripsit:
> However, the odds are also against the trivial deletion of all of
> these digit-forming genes. There must have been a reason why
> theropods lost their digits, and this process must have taken millions
> of years, and required many mutations, not a few trivial ones (as
> George puts it). Is it because a wing was forming? This is where I
> don't necessarily agree with George so readily. I think the wing is a
> possible answer, but my gut tells me that it may have had to do with
> something else. What, I don't know.
How much lateral flexure is there in theropod necks?
This is easier to visualize with modern birds, oddly enough, but I note
that -- with more pronounced S curves allowed by their cervical
vertebrae -- modern birds have more dorsal/ventral flexure than lateral
flexure in their necks by some lots. This neck flexure is something I
understand to have been a general trend in theropods over their
evolutionary history; it's been getting more flexible over time.
If the basic theropod hand motion is 'pull to the midline of the body'
-- something supported by the fixed 'palm in' hand positions -- then
this one motion is useful to a climber (where it's 'pull *me* over the
branch I've just grabbed') and to a predator (where it's 'pull this
thing I've just grabbed to where I can bite it'.)
Maniraptorans appear to have exapted the 'pull to the midline of the
body' arrangement into slashing, making it the primary killing attack
rather than a precursor to a killing bite. Such slashing would
advantage a two stage slashing process with the main arm motion and a
second motion in the hand, since you get a compound lever affect and
harder slashing that way. (The rotary sword strokes in traditional
Japanese sword fighting do the same thing -- rotate the whole body to
make the sword stroke, and rotate the sword with the wrists; the cutting
edge hits faster and has greater affect.)
Once that sort of compound motion is taking place, stresses on the
digits will be very high; this is *not*, unlike the claws of extant
animals, a motion along the line of the long bones, it's going to stress
primarily perpendicular to them. It may be the case that a reduced
number of digits allowed the remaining digits to be positioned more
closely to the plane of primary stress in the overall arm motion of the
slashing stroke of the hand claws, reducing rates of hand injury and
increasing predatory and reproductive success.
To maintain the end is to uphold the means.