Re: hallux

[longrich@alumni.princeton.edu]

On Sunday, August 18, 2002, at 12:02 AM, DinoBoyGraphics@aol.com wrote:

> "I think  it's safe to say that a hallux-like structure is strong 
> evidence for
> arboreality in a lineage (although its absence does not preclude
> arboreality, e.g. sloths don't have opposable digits)."
>      Except that these are only superficial similarities. 
I'm not sure I get the argument here. The similarity between an 
ichthyosaur and a dolphin and a shark is "superficial" but also pretty 
significant in terms of inferring the behavior of one by looking at the 
other.

>  The adaptations of perching birds are really remarkably consistent.  
> The morphological alteration needed to turn a "normal" theropod foot 
> into that off a perching animal is much less extensive than that of 
> flight, so it's implausible to suggest that Archaeopteryx was a 
> perching animal yet had such weekly developed adaptations for 
> perching.  It's more reasonable to suggest that Archaeopteryx evolved 
> from perching ancestors and secondarily became more terrestrial, 
> perhaps when it invaded an archipelego environment.
>     Detailed morphology just does not bear out an archaeopteryx well 
> adapted to perching or climbing.  The claws are not shaped like 
> climbers,
>  its pedal phalanges are not curved like perchers,

	The morphology doesn't seem to imply an exclusively arboreal life, 
so in that sense its not well adapted to perching or climbing. However, 
the pedal proportions of Archaeopteryx seem to be intermediate between 
those of purely terrestrial and purely arboreal birds, and around those 
of birds that spend a lot of time both on the ground and in the trees. 
There is a fair amount of elongation in the penultimate phalanges, a 
modest degree of curvature in the pedal unguals- more than typical of a 
coelurosaur, except perhaps for the dromaeosaurs- plus a hallux, and a 
relatively short metatarsus with relatively long toes, which is typical 
of birds with grasping feet though hardly as well developed as many 
birds. Another interesting thing is the pedal proportions of digit II. 
In Zhao and Farlow's study, digit III and digit IV imply that 
Archaeopteryx and Confuciusornis are intermediate between highly 
terrestrial and highly arboreal; digit II seems to plot more closely 
with the arboreal forms, which is consistent with the idea that the foot 
was simultaneously specialized for terrestrial (III and IV) and arboreal 
(II) locomotion. Comparing Archaeopteryx and perching birds may have 
limitations- at least one major difference being the presence of large 
claws on manual digits I and III, such that even without any 
particularly remarkable specializations of the hindlimb, the animal 
might have been an agile climber.
	There's also the diversity problem though. Archaeopteryx is one 
genus, early birds were doubtless diverse and it may be problematic to 
generalize from Archaeopteryx to all primitive birds. Confuciusornis 
reveals this in that not all the specimens seem to have the same pedal 
proportions, some feet look more, or less, specialized for grasping and 
if you look at Zhao and Farlow you'll notice that they don't all come 
out in quite the same place in the arboreal-terrestrial split.

>  it's lack of an alula makes slow landings difficult to achieve,

	There are lots of ways to decrease the stall speed of an 
aircraft/bird. One is to use a trailing edge flap; the large tail of 
Archaeopteryx could have directed a sizeable amount of air downwards 
behind the wings and served this function. The staggered arrangement of 
the outer primaries in Archaeopteryx has also been suggested to have 
functioned as a series of leading-edge slots- that is, the functional 
equivalent of multiple alulas- by Norberg, and another possibility might 
be that this arrangement would produce a series of vortices which would 
stir up the air over the top of the wing and delay the onset of stall 
for a few more degrees like vortex generators used to delay stall on 
aircraft.
	The other thing is that since stall speed is the speed at which 
maximum lift equals weight, just putting more wing on the bird will 
reduce its stall speed, as will getting rid of extra mass. A hang-glider 
can have a stall speed of around 15mph, and it doesn't have any slots or 
flaps or anything, it's just got tons of wing area and little weight. A 
737 has a single-slot leading edge slat and triple-slotted trailing edge 
Fowler flaps- for a total of four slots and four flaps, an arrangement 
which can roughly double both the stall angle of the airfoil and the 
total lift of the airfoil- and it still would stall out around 100mph 
because it's got a high wing loading.
	It's not entirely clear how relevant the question of low landing 
speeds is anyway. Arboreal gliders actually tend to have pretty high 
wing loadings compared to powered fliers and show little in the way of 
specialized stall-delaying or lift-enhancing devices, but they still 
manage to land in trees safely all the time.

>  and the extremely long hind limbs put the center of gravity where you 
> don't want it, far away from the substrate. 
	Lemurs come to mind as extremely long-legged yet highly arboreal 
animals. Long limbs can actually be handy in at least a couple of ways, 
for grabbing one branch while holding on to the one you're on, or for 
leaping- long-legged leapers being able to maintain contact with the 
substrate for a longer distance while exerting force with the legs, and 
therefore exerting more work into the leap (work= force x distance). 
Chameleons walk along branches with the body held well away from the 
branch.

nL