Re: The ground-nowhere hypothesis on the origin of bird flight

[Tim Williams <tijawi@yahoo.com>]

David Marjanovic <david.marjanovic@gmx.at> wrote:


> I keep reading on this list that the
> ground-up/trees-down dichotomy is outdated. It is outdated
> as a dichotomy -- the number of hypotheses is greater
> than two. However, reality does not need to lie in the
> middle.  Probably it lies somewhere else entirely.


That is true in an ecological sense.  However, the old ground-up/trees-down 
dichotomy boils down to the role of gravity in aerial behavior: Was the 
pro-avian working with or against gravity?  This is not really about whether 
the pro-avian lived in trees, or on the ground, because some models that favor 
a terrestrial pro-avian also propose a gravity-assisted path to flight - like 
'Pouncing Proavis'.  


So in the sense of the role of gravity, there can only be two hypotheses: the 
pro-avian is either using gravity to its advantage (e.g., Pouncing Proavis, or 
an arboreal glider), or it is actively fighting against gravity (e.g., leaping 
from the ground into the air to catch insects).  At any given moment, the 
pro-avian can either be working with or against gravity, not both.


However, it is theoretically possible that the role of gravity changed at 
different stages in the evolution of flight.  This is why the dichotomy is 
outdated.  Such a switch seems to be implied in the "stability flapping" model: 
initially the predator is using gravity to its advantage (to help pin down 
prey), but at some later evolutionary stage the theropod uses its wings to 
launch itself into the air, against the force of gravity.  If I understand the 
model correctly, that is.  (If I'm woefully misunderstanding this model, I 
apologise.)  I'm really looking forward to the papers.  I particular look 
forward to this statement being elaborated upon: "Selection for more efficient 
stability flapping provides a viable selection pathway to true powered flight." 
 How exactly?  What selective advantage would aerial locomotion be to a 
predator that wants to pin down terrestrial prey?  I'll happily W4TPs...



Cheers

Tim