[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: "Flight theory has legs"
On Sat, Jul 12, 2003 at 04:38:50PM -0700, Jaime A. Headden scripsit:
> Graydon (firstname.lastname@example.org) wrote:
> >All of which is very *pedal*, though; those locomotor modes are
> >extensions of flat surface locomotion.
> As in chameleons, fully quadrupedal. Their ancestors, as indicated by
> agamids and other iguanians, are sprawling-climbers. A similar functional
> consideration is given to the prolacterifom? *Megalancosaurus* (Renesto,
> 1994, 1999), with a nearly veritcal leg stance despite having an ankle and
> wrist indicating ancestry with sprawlers [based on some in prep work].
But you're -- if you're arguing for a scansorial orgin for birds -- not
just arguing for climbing, you're arguing for _leaping_.
Chameleons have lots of specialized climbing adaptations for precise
movment on narrow branches, and they move slowly.
> >I think they're plausible as slow climbing postures, but implausible
> >as a typical locomotor posture because they involve a very limited
> >range of the available range of motion in the limbs.
> I cannot see how speed is relevant here. Locomotion is locomotion.
If you're looking at a scansorial origin for birds, they have to be
leapers or parachuters or something of that character, no? That
requires the boing-boing-jump sort of tree dwelling, rather than step
... step ... step ... sorts of tree dwelling that you get with
chameleons or sloths or whatnot.
> the two- and three-toed sloths are "lethargic" animals that are
> nonetheless very functional and capable in their environment, and even
> slower on a tree trunk or the ground; their ancestors were graviportal
Sloths move around in trees just fine, yes. What they *don't* do is
jump or parachute. They're essentially cryptic animals -- lots of
camoflage, very slow locomotion, low metabolism. That's not a
particularly plausible niche to get a flapping flyer out of, since
flapping necessarily involves a high energy output.
> >Something that did that a lot would have different limbs.
> Would you care to elaborate on these features?
Dromeosaurs have knees about at the same level as their pubic boots;
this essentially takes the femur completely out of utility in that
climbing posture, because it will always be maximally or near maximally
forward to get the legs folded up enough to allow the manual claws to
grasp the tree.
If the dromeosaur was fully scansorial, spending all its time in trees,
it would have some kind of limb adaptation to either swing is legs out
at the hips or to have much shorter legs that allowed it a fuller range
of motion with the primary muscle groups. (This is one of the neat
things about the reported '4 winged' forms; those sprawling hip
adaptations are what one would expect in something scansorial.)
> That a quadrupedal, long-clawed, grapsing animal has not been found
> does not mean that it is not possible to be found at Liaoning from
> known fossil material. Many Old World monkeys and several New World
> ones, as well as lemuriforms, have limb organizations similar to those
> posted at various times on this list. Other climber-walkers with
> functional grasping/locomotory adaptations in the limbs include the
> jungle cats, esp. jaguars, renowed for their prowess in the lower
> canopy and on the ground.
Cats are all four limbs with sharp claws, though; you're talking about
something that has terrestrial pedal unguals and scansorial manual
unguals. The monkeys and lemurs have hands _and often feet_ that can
close around the branch; are there any theropod hand morphologies that
allow that? (I understand not, but could easily be mistaken.)
> >You did, but you're talking about manual ungual substrate contact;
> >there isn't any difference other than stress load between walking up
> >a tree trunk and walking on the ground.
> Actually, considering the functional adaptations in the phalanges of
> many primates and birds, as in work from Yoder, scansorial primates,
> analyses of the scansorial *Eomaia*, etc., the best data is not in the
> claw but the praeungual hand. Opposing digits, elongate penultimate
> phalanges, curved penultimate phalanges, on occassion and rather
> telling the presence of an extensor and/or flexor tubercle on the
> dorsal/ventral phalangeal margins (respectively), occur in scansorial
> birds, primates, sciuromorphan rodents, and *Eomaia*, along with some
> other reptiles that have other features relating to an arboreal life
> style. In climbing animals, the claws are either strongly curved, or
> at the expense of this the phalanx is bowed. There are other
> considerations. One of my functional perspectives on flight origin
> tends to involve the features relating to a climbing function, and
> I've talked on this off and on over the last three years or so.
Wouldn't this require you to find that the feet of the
putatively terrestrially-clawed fossil forms aren't flat?
The point I was after is that a proto-bird that's scansorially
quadrepedal is going to be capable of being terrestrially quadrepedal,
and that there is no least hint of that anywhere that I'm aware of.
> >We can, too, but we have very different shoulder anatomy with lots of
> >brachiating ancestors in it.
> We have a rotatable scapula, a free strut-like clavicular struture, a
> condylar proximal humerus with a free glenoid margin. Hotzin chicks,
> in spite of lacking these features, do just fine.
Hoatzin chicks are very cool things, but they don't brachiate, and
they're relatively slow clamber-climbers, no?
email@example.com | Uton we hycgan hwaer we ham agen,
| ond thonne gedhencan he we thider cumen.
| -- The Seafarer, ll. 117-118.