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RE: Campbell's even crazier than a MANIAC? (archeopteryx
--- On Wed, 9/24/08, Tim Williams <email@example.com> wrote:
> _Archaeopteryx_ and some dromaeosaurs (like
> microraptorines) show some of these features - but these
> features are weak or incipient at best (e.g., elongation of
> the penultimate phalanges; certain modifications of the
> hallux; claw curvature). Overall, the morphology of these
> theropods is consistent with animals that ran on two legs,
> and had forelimbs with minimal prehensile ability. Some
> features indicate that they may have had some scansorial
> ability, which helped them venture into trees. But not
One point I have never seen emphasized re _Archaeopteryx_ lifestyle/evolution
is that cycads (at least the ones I am familiar with) are very easy to climb
and perch in. A basketball is easily "perched" in the rosette of a "sago palm"
(_Cycas revoluta_), and the trunk is basically a vegetative form of velcro.
It follows that a small cursorial biped could use very minimal climbing talents
to adopt roosting and/or perch-hunting behavior and thereby position itself for
a gravity-assisted path to flight, w/out benefit of "highly arboreal ancestry"
or a skeleton optimized for climbing.
> There are alternative models for the origin of avian flight
> that propose a terrestrial runner or leaper as the ancestor
> of birds. These "ground-up" models often get
> shoved aside, because of the claim that it's
> "easier" to become airborne from an elevated
> position (like a tree), because the animal doesn't have
> to fight against gravity.
If "easier" is defined as "a larger and more diverse feedstock of
'flight-candidate' organisms", then gravity-driven evolution _is_ easier. In
other words, any animal that might fall or pounce from a high place has the
potential to evolve at least passive flight. The same cannot be said of all
ground-dwellers, most of which would need to evolve leaping ability first, at a
> But what tends to get forgotten
> is that the hindlimbs of theropods were capable of
> considerable strength, which may have enabled the ancestors
> of birds to launch into the air without the need to climb up
> a tree to gain elevation.
True. But note the exponential increase in air speed culminating in terminal
velocity experienced by gravity-driven animals.
Consider a pouncing animal of size such that achieving terminal velocity has a
high probability of serious injury upon landing. Any drag-producing structure
or shape can then function to increase the height from which it can pounce,
conveying advantage. A straight-forward step-wise series of design
modifications can then produce an airfoil.
Leapers are unlikely to find any drag-producing structure advantageous, as leap
length and height will be compromised. It is certainly possible to imagine a
"foil" that could utilize lift to extend leap length or possibly control
trajectory, but step-wise evolution of such a structure from 'scratch' is not
straight-forward. (Nor is it clearly impossible!)
Still, the characterization of gravity-driven flight evolution as "easier" than
the various "ground-up" scenario's can be well defended from the perspective of
> No, the intermediary stage proceeding flight DOES NOT have
> to be gliding. It might have been; but this cannot be
> Personally, I think that the ancestors of birds might have
> spent some time in trees, and used their proto-wings during
> aerial descents.
Perhaps birds achieved the basic flight equipment package while roosting and/or
perch-hunting in cycads; in which case "some" time might be characterized as
"slightly more than half" on a daily basis...