Re: Pro(to)avis

[Tim Williams <twilliams_alpha@hotmail.com>]

James Cunningnam wrote:

Jim, you may be interested in this (if you haven't seen it already):

Gatesy,. S.M. and Dial, K.P. 1996. From frond to fan: Archaeopteryx and the 
evolution of short-tailed birds. Evolution 50(5): 2037-2048.

Abstract: "Modern birds have extremely short tail skeletons relative to 
_Archaeopteryx_ and nonavialian theropod dinosaurs. Long- and short-tailed 
birds also differ in the conformation of main tail feathers making up the 
flight surface: frond shaped in _Archaeopteryx_ and fan shaped in extant 
fliers. Mechanisms of tail fanning were evaluated by electromyography in 
freely flying pigeons and turkeys and by electrical stimulation of caudal 
muscles in anesthetized birds. Results from these experiments reveal that 
the pygostyle, rectrices, rectricial bulbs and bulbi rectricium musculature 
form a specialized fanning mechanism. Contrary to previous models, our data 
support the interpretation that the bulbi rectricium independently controls 
tail fanning; other muscles are neither capable of not necessary for 
significant rectricial abduction. This bulb mechanism permits rapid changes 
in tail span, thereby allowing the exploitation of a wide range of lift 
forces. Isolation of the bulbs on the pygostyle effectively decouples tail 
fanning from a fan movement, which is governed by the remaining caudal 
muscles. The tail of _Archaeopteryx_, however, differs from this arrangement 
in several important respects. _Archaeopteryx_ probably had a limited range 
of lift forces and tight coupling between vertebral and rectricial movement. 
This would have made the tail of this primitive flier better suited to 
stabilization than maneuverability. The capacity to significantly alter lift 
and manipulate and flight surface without distortion may have been two 
factors favoring tail shortening and pygostyle development during avian 
evolution."

Ian Paulsen wrote:

>  What is the pouncing proavis hypothesis?

J.P. Garner, G.K. Taylor, and A.L.R. Thomas. 1999. On the origins of birds: 
the sequence of character acquisition in the evolution of avian flight.  
Proceedings of the Royal Society: Biological Sciences.  266(1425):1471-2954.

Abstract: "Assessment of competing theories for the evolution of avian 
flight is problematic, and tends to rest too heavily on reconstruction of 
the mode of life of one or a few specimens representing still fewer species. 
A more powerful method is to compare the sequence of character acquisition 
predicted by the various theories with the empirical sequence provided by 
cladistic phylogeny. Arboreal and cursorial theories incorrectly predict the 
sequence of character acquisition for several key features of avian 
evolution. We propose an alternative 'pouncing proavis' model for the 
evolution of flight. As well as being both biologically and evolutionarily 
plausible, the pouncing proavis model correctly predicts the evolutionary 
sequence of all five key features marking the evolution of birds."

Cheers

Tim