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Well, we all have Mr. Paul's characteristically dismissive opinion on record.
I will try to keep an open mind on the subject, but please allow me to
describe why I am highly skeptical that alvarezsaurs dug into termite
mounds with their pectoral limbs.
The hand of Shuvuuia deserti, according to Suzuki et al. 2002, is slightly
more than one centimeter long, including ungual and phalanx. Thus the
entire arm was probably something like 2.5 centimeters in maximum reach
from glenoid to the point of the ungual. That's probably less than the tip
of your thumb. The sternum of Mononykus is about 2 centimeters long
(according to figure 4.15 in Mesozoic Birds (2002).
Termite mounds are commonly described as having a hardness comparable to
concrete or earthenware pottery.
Can Mr. Paul or anyone name an extant animal with a limb 2.5 centimeters
long and the pectoral muscle mass proportionate to a 2cm sternum that can
penetrate concrete? Even if it was a 2cm SPHERE of solid pectoralis fibers
with a 3cm lever arm I don't think it could exert enough newtons of force
to do so. I am aware that anteaters, pangolins, and varanids, with claws
and pectoral skeletons two or three orders of magnitude more robust do
open termite colonies, but is there any tiny bird or lizard that can
penetrate concrete? Mr. Paul is right that Shuvuuia's pectoral musculature
is extreme, but it is actually extremely TINY relative to a termite mound.
If alvarezsaurids were tearing open bark or fallen logs, can anyone
explain why their arms got smaller and more reduced as they specialized in
this? What was the adaptive advantage that was conferred to the tiny -
armed offspring of an ancestral animal like Haplocheirus in competing to
tear open insect colonies? How does a 3 centimeter long arm function
better than Haplocheirus' 30 centimeter long arm and 6 centimeter 1st
ungual (comparable to a pangolin?) in doing that job?
In my humble opinion reason dictates that an animal which evolves to tear
things apart with its arms will not have these arms become so reduced that
they can't reach the ground, can't reach one another, and can't be held
within the range of vision, and might not even be visible outside the
animal's body feathers.
Furthermore reason dictates that a keeled sternum CAN function in
fluttering display feathers, since that is precisely what does the trick
in living birds of paradise and myriad other bird species.
On the other hand, having a 1.5 cm row of display feathers doesn't sound
so eye - catching either.
> On 21 April 2011 02:11, <GSP1954@aol.com> wrote:
>> The extreme musculature indicated by the ossified keeled sternum and big
>> olecranon process combined with an enormous claw leave little doubt that
>> alvarezsaur arms where for tearing something apart. The shortness of the
>> increased their power via leverage. I like to think it was termite
>> which I believe have been discovered from that time. Display feather
>> does not explain any of the features.
> The big olecranon is interesting (anyone done any mechanics on it?
> compared it quantifiably to fossorial mammals?), especially with, as
> GSP says, the short outlevers. But the very large disparity between
> the limb lengths does mean it would have to be squatting on whatever
> it was digging, or pressed right against it. And it undeniably already
> has two large, powerful pelvic limbs to dig with.
> Hmm. Problem with the feather fluttering (which does go some way to
> explaining large pectorals) is that the lever ratio should be the
> other way around at the elbow. Although a short humerus decreases the
> moment arm at the shoulder, which equals more rapid movement (although
> you'd need to actually measure the distance from the glenoid to the
> deltapectoral crest, could be relatively lengthened). Problem with
> digging is that the rest of the body doesn't really fit, and things
> with relatively enlarged, muscular hindlimbs that need to be do a lot
> of heavy work would probably use their hindlimbs.
>> In a message dated 4/20/11 2:46:08 PM, email@example.com writes:
>> << The problem with imagining Mononykus tearing open conifer cones with
>> arms is the same as picturing it digging into insect mounds. It couldn't
>> reach the ground from a standing position. Its arms, fully extended,
>> could not even reach the knee.
>> Present company considered I'll engage in a little a priori speculation
>> and say that it seems unlikely that there would have been selection
>> pressure toward shorter, reduced, arms in your scenarios. If shorter
>> went along with clawing behaviors then we are imagining an animal that
>> would have to squat down on top of an anthill and then dig under its
>> belly, as a storm of angry hands poured out over its body. The arms
>> have maintained their ancestral, long, state and functioned just fine in
>> opening insect colonies, bark, or pinecones.
>> According to Senter(2005) Mononykus also could not hold anything between
>> its hands.
>> Selection pressure could have driven shorter arms with larger muscular
>> processes if there was selection for high frequency fluttering of a fan
>> feathers or ribbons.
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