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Tucson Tales III.
Tucson Tales III. - More Birds of a Feather.
A few more issues in the bird-dino debate bear mention, the wrist debate
and the arboreal vs cursorial origin of flight. Most quotes come from:
NOVA, The Case for the Flying Dinosaur.
--- The wrist debate ---
If you've seen Jurassic Park, you may remember the scene where they uncover
a velociraptor skeleton and the ensuing quote: "look at that half-moon
shaped bone in the wrist, no wonder these guys learned how to fly" (modern
birds posess a similar half-moon bone in their wrists). Larry Martin would
dispute that analysis on two counts, anatomy and functionality.
- anatomy -
According to John Ostrom, the half-moon bone in the wrist of deinonychus
evolved from a certain bone(ulnare?) in more primitive diapsids.
Cladists tend to believe that this feature was passed on to birds from
their therapod ancestor and persists in the wrists of all flying birds
today. Larry Martin thinks that in birds, the half-moon bone evolved from
a different bone(third distal carpal?). If Martin is right, it means
that the half-moon bone is a convergent feature and implies separate
lines of evolution.
- functionality -
Ostrom described the rotation of the wrist in deinonychus as an _inward_
turning which would allow them to grab food and then put it in their mouth.
Martin says that that would be of no use to a bird. Birds rotate their
wrists _downward_ to tuck in their flight feathers and prevent them from
dragging on the ground, a motion of no use to any therapod. Since both
types of movements involve a rotation, half-moon shaped bones evolved to
facilitate a swivel in the joint, but these bones evolved separately and
for different purposes. They only _look_ alike, they do not represent an
inherited character from dinos to birds.
--- Arboreal(trees-down) vs Cursorial(ground-up) Origin of Flight ---
Martin believes that the proto-bird ancestor was a small tree-dwelling
reptile. Feather evolution involved the elongation of scales which would
have increased mobility in tree-tree or tree-ground movements. Gliding
would result in selection pressure for flight feathers(like Archaeopteryx),
and eventually lead to fully powered flight.
The "trees-down" scenario for flight origin has been hotly contested by the
cladists, who favor a cursorial or "ground-up" theory of flight origins.
Padian's scenario involves a small warm-blooded therapod already covered
in feathers for insulation. Longer feathers evolve to assist in the capture
of insects that this hypothetical creature pursued while running.
Larry Martin doesn't like the cursorial theory. He says "It's hard to see
what advantage you get from flying or gliding when your running because
either activity will result in at least an initial loss of airspeed, which
means if your pursued, you're more likely to be caught; and if your pursuing
something, you're more likely not to catch it". He adds that ground-up
flight would be too energetically expensive.
When presented with these arguements, Padian responded "That would be true
if we demanded that animals evolve all-out sustained flight all at once.
If you have animals that run along the ground leaping and flapping and then
getting back down to the ground and leaping and flapping, _that_ eliminates
that difficulty because you get the question of evolutionary improvement
[comment - Everybody at the Tucson discussions found this ridiculous :-)]
- Was Archaeopteryx arboreal? -
One thing nobody disputes is that Archaeopteryx could fly, due to the
asymmetry of the veins on either side of the feather shaft. This results
in an airfoil contour which produces lift. When birds become flightless
(e.g. Kiwi) they lose this feature. It's not something the evolutionary
process will endow you with if you don't fly or glide. How well Archie
could fly is another question. It lacked the large breastbone/coracoid
feature found in modern birds and may have had trouble taking off from
Archie also has wingclaws, similar to one modern bird, the hoatzin chick.
Since the hoatzin uses it's wingclaws to climb around in the trees, it's
reasonable to assume that Archie did the same. This is not the only
character of Archie that could be considered arboreal. Based on his
(first ever) reconstruction of an Archie skeleton from actual cast bones,
Martin has also concluded that it had a somewhat upright posture, more
similar to a primate than a dinosaur. This would serve to keep the center
of gravity close to a tree trunk while climbing. Alan Feduccia has recently
published a study in which he concludes that the foot claws of Archie most
resemble those of a perching bird.
- So why would do many cladists refuse to accept the notion of an arboreal
origin of flight? -
Here's a quote from Padian: "Climbing trees does not seem to be something
they (i.e. therapods) did very well...When you look at the anatomy of Archie
and the creatures to which it is closely related, you find that there isn't
a single arboreal character in the skeletons...There is no evidence that
birds really spent much time in the trees at all before the Tertiary
sometime, after the age of dinos".
[comment - Padian may have been so seduced by his cladistic models that he
ignores important evidence and misses this one by about 100 million years.
While he's essentially correct that therapods were built for a running,
terrestrial lifestyle, he seems to totally ignore arboreal characters
of Archie, and his imposing his expectation of a running therapod as a
End Tucson Tales III.