[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: Campbell's even crazier than a MANIAC? (archeopteryx



--- On Wed, 9/24/08, Tim Williams <twilliams_alpha@hotmail.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
> much. 

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 
minimum.

> 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 
stepwise evolution.

> No, the intermediary stage proceeding flight DOES NOT have
> to be gliding.  It might have been; but this cannot be
> assumed.

Agreed.

> 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...