Why Air Sacs?

["Jerry D. Harris" <jharris@dixie.edu>]

A novel, and very interesting, interpretation in this remarkably comma-free 
paper:

Farmer, C.G. 2006. On the origin of avian air sacs; pp. 89-106 in Milsom, 
W.K., Powell, F.L., and Mitchell, G.S. (eds.), Frontiers in Comparative 
Physiology. II. Respiratory Rhythm, Pattern and Responses to Environmental 
Change. Respiratory Physiology & Neurobiology 154.

ABSTRACT: For many vertebrates the lung is the largest and lightest organ in 
the body cavity and for these reasons can greatly affect an organism's 
shape, density, and its distribution of mass; characters that are important 
to locomotion. In this paper non-respiratory functions of the lung are 
considered along with data on the respiratory capacities and gas exchange 
abilities of birds and crocodilians to infer the evolutionary history of the 
respiratory systems of dinosaurs, including birds. From a quadrupedal 
ancestry theropod dinosaurs evolved a bipedal posture. Bipedalism is an 
impressive balancing act, especially for tall animals with massive heads. 
During this transition selection for good balance and agility may have 
helped shape pulmonary morphology. Respiratory adaptations arising for 
bipedalism are suggested to include a reduction in costal ventilation and 
the use of cuirassal ventilation with a caudad expansion of the lung into 
the dorsal abdominal cavity. The evolution of volant animals from bipeds 
required yet again a major reorganization in body form. With this transition 
avian air sacs may have been favored because they enhanced balance and 
agility in flight. Finally, I propose that these hypotheses can be tested by 
examining the importance of the air sacs to balance and agility in extant 
animals and that these data will enhance our understanding of the evolution 
of the respiratory system in archosaurs.

Mentioned previously on the list as an e-pub; this is the "official" 
version.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jerry D. Harris
Director of Paleontology
Dixie State College
Science Building
225 South 700 East
St. George, UT  84770   USA
Phone: (435) 652-7758
Fax: (435) 656-4022
E-mail: jharris@dixie.edu
and     dinogami@gmail.com
http://cactus.dixie.edu/jharris/

"Trying to estimate the divergence times
of fungal, algal or prokaryotic groups on
the basis of a partial reptilian fossil and
protein sequences from mice and humans
is like trying to decipher Demotic Egyptian with
the help of an odometer and the Oxford
English Dictionary."
              -- D. Graur & W. Martin (_Trends
                  in Genetics_ 20[2], 2004)