At what stage in bird evolution were gastralia and belly ribs lost? Why - was it just to reduce weight, or are there other plausible hypotheses?
Great question Lucy.
Gastralia were present in Archaeopteryx, confuciusornithids and in the enantiornithid Sinornis (they may or may not have been present in others); gastralia are not present in the hesperornithiforms or in the Neornithes (this information was compiled by Carrier & Farmer, 2000).
If non-avian theropods used their gastralia in a cuirassal breathing mechanism (as proposed by Carrier & Farmer in a variety of papers), it would make sense that the loss of the gastralia was in response to the evolution of the avian parabronchial and air sac system as the predominant method of respiration in more derived birds.
Of course, this being science there will be controversy - some may consider that the avian respiratory system was already established in earlier birds and possibly in some non-avian theropods. The problem is that there is insufficient evidence of soft parts in the fossil record to be certain about the lungs of basal birds.
Carrier & Farmer, 2000. http://www.biology.utah.edu/farmer/publ … logy26.pdf
leon Claessens and Pat O'Connor might dispute this a bit. They argue that avian style parabronchial and air sac system of breathing developed in archosaurs long before the loss of gastralia in avians. You can find several of their papers on Claessens web site here:
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