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

[Mike Habib <habib@jhmi.edu>]

> Still, a leaping animal would essentially begin with a ballistic  
> trajectory, and I have a hard time imagining an intermediate  
> structure that would be advantageous. I would think a leap would  
> lead to low drag structures and forelimb positions...
>
> ...I just cannot imagine evolution acting on anything aerodynamic in  
> a terrestrial ancestor without preexisting aerodynamic adaptations,  
> unless that ancestor is falling (be it a cliff or tree)

The catch here is that the ancestral condition for some groups, birds  
in particular, may include some sort of exapted foil.  Forelimb  
feathers are becoming increasingly well known for a range of  
maniraptorans outside Aves, including taxa which were clearly  
terrestrial.  Phylogenetically speaking, the current bracket places  
forelimb "proto-wings" at a node rather near the base of Maniraptora  
(perhaps even outside that).


don ohmes wrote:

> 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.

So, essentially, we might predict that a greater percentage of powered  
flyers would come from arboreal lineages than terrestrial ones.  In  
practice, this doesn't seem to have worked out - birds seem to have a  
mostly terrestrial ancestry (with some semi-arboreal members possibly  
present), and bats seem to have an arboreal ancestry, while pterosaurs  
and insects are somewhat equivocal at present.  All told, there is not  
a clear bias towards arboreal ancestry.  This could be because of the  
small sample (only four origins of powered flight), or it could be  
that terrestrial lineages have a greater chance of producing flyers  
than we might otherwise expect.  It could also be that derivation of  
powered flight is actually rare among arboreal lineages, even though  
gliders are rather common.  Quite a few of the living arboreal gliders  
would lose gliding efficiency through airfoil oscillation, so the  
actual 'flight-candidate' pool might be smaller than it appears.

Cheers,

--Mike


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280-0181
habib@jhmi.edu