

Re: Thou Shalt Not Climb!

Tim Williams (twilliams_alpha@hotmail.com) wrote:

<Jaime, out of interest, could you (briefly) describe what anatomical features
in the hindlimb distinguish "branch-sitting" from "perching".>

  I think a good deal of dinosaurs were capable of climbing, at least in the 50
kg range or so and less, but some seem better suited for sticking in there a
while, including having a flexible knee. The pubis being reversed allows the
leg to curl tighter and the animal to crouch down lower, as it reduces the
depth of the trunk. But more than any of this is the foot and the size of the
unguals and, in some dinosaurs' cases, the presence of an especially large claw
that could be used to stabilize its position. However, branch-sitting as I
termed it would simply be being able to grasp branches with the foot without
having the anatomical derivatives that indicate perching, such as the fully
reversed and elongated hallux. It also helps that the forelimbs were long, and
may not have been fully derived as "wings" in many of these "sittable"
dinosaurs. Shorter forelimbs are available for some ornithischians with highly
mobile digits and long hands, such as *Heterodonosaurus*, which also sports
extremely long and gracile toes and hallux, which likely would have allowed it
to climb and even sit in trees with ease (speculation, of course).

<Can you explain exactly what is meant by a "scampering forelimb"?  In other 
words, what characters in the forelimb suggest that it was used for scampering?
 How are these forelimb characters different from characters associated with
(say) predation?>

  In *Protarchaeopteryx* and a host of several non-Pygostylian birds, the
forelimb is not so excessively longer than the hindlimb as to be ridiculously
skewed to flight-enabled features and a reduction of the animal's time on any
substrate such as the ground or a branch. This long arm is close in length to
the hindlimb, close enough that it approximates the length of a foot-based
stride without having to require a rotary scapula. Arm based climbers such as
monkeys use the scapula to provide much of the directional component to
movement in the trees, while the feet are used for providing the force of
movement. This seems at odds with early birds, with robust but short feet, but
also robust and long arms. Together, but as in scansorial mammals, the forelimb
and hindlimb lengths come close enough to even that they could have worked in
tandem without much difficulty. Vertical climbing would be acheivable with
spread-armed, crouch-legged spurts, where as in monkeys the arms are
directional and the legs provide impetus, but when the substrate is less
vertical, the animal can raise from a crouch, but keeps the arms and legs in
contact with the substrate, and uses them both for balance and direction,
walking and stabilizing with (again) moneky-like efficiency. The onl problem
with this is the relatively inflexible tail in most birds, which seems more
suitable to a tail-feather array or a counterbalancing pole, seemingly best
used for control during turning while on the ground or in the air, but this
doesn't seem too much of a problem because the animals were not so habitually
arboreal, which their pedal versus manual anatomy tells us is not as likely
than that they were scansorial instead.

<BTW, I don't disagree with the idea that _Archaeopteryx_ and other basal 
avians could have (and did) climb up trees.  I'm just intrigued as to how the
anatomy of _Archaeopteryx_ supports this interpretation.>

  Contrary to some reports, some *Archaeopteryx* specimens show an expanded
distal end to the scapula, though not so much so, but still more than the
mid-shaft diameter. This is explicitly true in the Thermopolis specimen
(personal observation, at any rate). This, along with the apparent absence of
an ossified sternum (though evidence of a certilaginous one is still strong)
suggests some relative mobility in the arms both relative to one another, as
well as a potential for the arms to preserve greater degrees of movement more
derived arboreal avians do not exhibit. The unguals of hand and foot are nearly
of a size with one another, and with the slightly greater curvature of the
manual over the pedal (which both Yalden and Feduccia note), there is a
stronger grasping function in the former than the latter, while in the latter
one the claw curvature is suited for digging into substrate more in parallel
with direction of movement. The arm is simply not suited, or even capable of
orienting in this manner to assist in transite, yet still is extendible to a
degree that the manus is in perfect distanct to "hold" the branch while
walking, increasing balance in a non-arboreal animal in a risky environment.
This posture and movement is almost exactly performed this way in a variety of
monkeys, though I am not suggesting referring to birds in any way as "monkey
dinosaurs" or drawing direct comparions, only drawing parallels. It is
neccessary to use the claws as grasping elements, since the digits were not
mobile as in monkeys, but the large size of the unguals, even without invoking
predation, can be used to eforce how much of these features were retained
despite the loss of the apparent predatory anatomy of the skull in more derived
birds than Archie. Why retention of predatory movements, without any selective
improvement towards flight, and a retention of the limb proportions? My
suggestion is a trend towards arboreal lifestyles without flight being a
[strong] factor, but retention of terrestrial capabilities, and a squirrel or
monkey-like flexibility in movement along branches. Predation may be a
selective factor in getting the animals into the trees, but it need not be one
to keep them there, once new niches were available. Seed and fruit-eating can
be used to explain expansion of the gut and thuse increased reversal of the
pelvic anatomy, while aboreal existence lightens (and shrinks) the animals,
which may enable gliding for foraging.


Jaime A. Headden


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