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While I agree that the heavily abbreviated forelimbs of hummingbirds
compared to their other avian relatives are comparable in that degree (brevity)
to alvarezsaurids, I should note the differences (as with kiwis in my previous
post) are marked in their distinctions:
First, the deltopectoral crest is distal to the abductor crest, where they
are separated by a large L-shaped gap (the deltopectoral crest is
"hatchet-shaped," an aspect that recalls nyctosaurid pterosaurs); the dpc is
further set lower on the humeral shaft than a line drawn between the adductor
and abductor crests. This alters the dynamics of humeral motility quite a bit.
But let us not forget that the scapulocoracoid glenoid is bounded by a distinct
lip or "fhalf" which, despite the globular humeral caput, prevents the humerus
from abducting by any serious amount (Senter proposes a nearly horizontal
humerus, but this strongly depends on the scapular orientation and "normative"
humeral attitude, rather than glenoid shape). Strong humeral torsion (~45
degrees if I'm not mistaken) misaligns the proximal and distal ends, allowing
Senter's reconstruction to permit the palmar sides of the manus to face ventral
with slight torsion along the forearm, while the proximal humerus is
perpendicular across its proximal end to the sagittal.
Senter's reconstruction also allows for an adaptive explanation for the
distinct "notch" between abductor and dpc crests, but not for the pectoralis
musculation, which should "flap" the arms. If this reconstruction should be
correct, it would permit strong ventroflexion of the arm at the glenoid,
bringing the palms inward to face one another (but not touch), and the large
size of the adductor insertions indicate potential strong motion back out; lack
of long moment arms for these crests indicate this motion was not particularly
rapid, so i would not support rapid flapping of the limbs.
Jason's original suggestion, that of display, would fit the bill for the limb
anatomy as Senter reconstructs the posture, but the remaining functional
anatomy is left to beg the question Greg Paul implies: What of the sternum?
Regardless of the ridiculously undersized limbs, I suspect Senter's analysis
is the best available until the size can be incorporated into a testable model.
As such, i would support the scratch-digging in so far as the anatomy is useful
for precise tearing at hardened substrates, be they eggshells, harded and
packed earth, caked mud, or what have you.
It should be noted that any functional suggestion of habitus must take into
account that not all alvarezsaurids (with the same anatomy, mind) are found in
different biomes, from the muddy estuarine and recessive inland-sea-bordering
Horseshoe Canyon Fm, the arid but braided Djadokhta/Baruungoyot, the swampier
Nemegt, etc. This could mean that not only termite mounds but also treebark,
logs, dried riverbeds or banks, may all have been potential regions one could
find an alvie "digging." It needn't necessarily be an earthen mound.
Jaime A. Headden
The Bite Stuff (site v2)
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
"Ever since man first left his cave and met a stranger with a
different language and a new way of looking at things, the human race
has had a dream: to kill him, so we don't have to learn his language or
his new way of looking at things." --- Zapp Brannigan (Beast With a Billion
> Date: Thu, 21 Apr 2011 09:36:41 -0400
> From: firstname.lastname@example.org
> To: email@example.com
> CC: firstname.lastname@example.org
> Subject: Re: dino-lice
> In Patagonykus a fragmentary forelimb is known, and it is more robust
> than in Mononykus and Shuvuuia. In Chiappe et al. (Mesozoic Birds 2002)
> Figure 4.16 shows Novas' reconstruction, and the hand alone is something
> like 5 cm long (much longer than Shuvuuia's entire arm). The olecranon is
> particularly robust and, I agree, quite reminiscent of anteaters. Yet the
> olecranon only functions in extension, and the different arm orientation
> (splayed laterally, according to Senter), makes it hard to see a digging
> function for the olecranon.
> Yet, to me, the humeri in Alvarezsaurs look like nothing so much as
> hummingbird humeri. Along with the extreme reduction of the arm length ,
> the proximal deltopectoral crest, and the keeled sternum this suggests, to
> me, a function in high frequency flapping.
> I would just briefly make three last points.
> 1) The claw on the first digit is retained in alvarezsaurs as it is in all
> basal avialans, including clearly volant ones. No one suspects
> Eoenantiornis or Yanornis of using their wings to crack open termite
> mounds, do they? Claws are also retained in ostrich wings where they
> presumably have no function.
> 2) Please keep in mind, whatever scenario you favor, that Shuvuuia's arm
> is under 4 cm in length, total. It would not have been visible outside the
> body feathers even when fully abducted. Though the proportions - width to
> length - of the bones in Shuvuuia's arm are very robust, the size of the
> limbs relative to body size and ancestral condition are diminuitive. I
> think the forelimbs are clearly dwarfed, and the hindlimb in Shuvuuia,
> where the foot alone is much larger than the entire arm, could clearly
> deliver at least ten times more force to dig into termite mounds than the
> arm. Consider that Brush Turkeys, with no special foot adaptations, move
> tons of soil and leaf litter to build their nesting mounds.
> 3) Shuvuuia has the smallest arm (the size of your thumb tip), and
> Patagonykus has a larger one, among alvarezsaurs. Yet whatever goes for
> one goes for the other as far as retaining the morphology. If you don't
> believe that Patagonykus could have such a large olecranon without using
> it to dig, then you don't believe Shuvuuia would either. In Shuvuuia's
> case the forelimb must have been retained for a reason other than opening
> termite mounds or tough food items, because it couldn't function in either
> of those efforts.
> > Patagonykus is a bit more
> > convincing:http://en.wikipedia.org/wiki/File:Patagonykus.jpg
> > Those are pretty outsize olecranons on a biped, with a very robust
> > forelimb attached to them. However (and I'm assuming this mount is
> > halfway decent) the deltapectoral crest is right next to the glenoid,
> > which isn't how fossorial limbs are laid out (It even looks high for a
> > theropod, although I'm saying that without any measurements, so pinch
> > of salt). Deltapectoral crest right next to the glenoid = small
> > pectoralis moment arm = relatively large, fast but weak movements.
> > Which makes more sense in terms of display than in digging stuff up.
> > As Eike points out though, something like Patagonykus definitely looks
> > like it did something with its arms, and the elbow extensors at least
> > look to be in charge of some big levers, and operating a very solid
> > looking forearm and manus (whats left of it). Which does fit the
> > fossorial pattern.
> > So, if that mount is real, what has a fairly shortened but not that
> > robust humerus, with the major retraction muscle seemingly geared more
> > towards motion than economical force application, but has a very
> > robust, short lower pectoral limb apparently geared towards more
> > deliberate, forceful movements? And more importantly, has anyone got
> > any more measurements? I think we can agree Jason has made Shuuvia and
> > Mononykus exerting large forces with their pectoral limbs sound pretty
> > difficult.
> > On 21 April 2011 04:32, Jason Brougham wrote:
> >> 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, 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
> >>>> arms
> >>>> increased their power via leverage. I like to think it was termite
> >>>> mounds,
> >>>> which I believe have been discovered from that time. Display feather
> >>>> fluttering
> >>>> 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
> >>>> its
> >>>> 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
> >>>> arms
> >>>> 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
> >>>> could
> >>>> 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
> >>>> of
> >>>> feathers or ribbons.
> >>>> >>
> >> Jason Brougham
> >> Senior Principal Preparator
> >> Department of Exhibition
> >> American Museum of Natural History
> >> 81st Street at Central Park West
> >> 212 496 3544
> >> firstname.lastname@example.org
> Jason Brougham
> Senior Principal Preparator
> Department of Exhibition
> American Museum of Natural History
> 81st Street at Central Park West
> 212 496 3544