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Body Size Evolution in Mesozoic Birds

J Evol Biol. 2008 Jan 9 [Epub ahead of print]
Body size evolution in Mesozoic birds.Hone DW, Dyke GJ, Haden M, Benton MJ.
Bayerische Staatssammlung für Paläontologie und Geologie, 
Richard-Wagner-Straße, München, Germany.

The tendency for the mean body size of taxa within a clade to increase through 
evolution (Cope's Rule) has been demonstrated in a number of terrestrial 
vertebrate groups. However, because avian body size is strongly constrained by 
flight, any increase in size during the evolution of this lineage should be 
limited - there is a maximum size that can be attained by a bird for it to be 
able to get off the ground. Contrary to previous interpretations of early avian 
evolution, we demonstrate an overall increase in body size across Jurassic and 
Cretaceous flying birds: taxon body size increases from the earliest Jurassic 
through to the end of the Cretaceous, across a time span of 70 Myr. Although 
evidence is limited that this change is directional, it is certainly nonrandom. 
Relative size increase occurred presumably as the result of an increase in 
variance as the avian clade diversified after the origin of flight: a 
progression towards larger body size is seen clearly within the clades
 Pygostylia and Ornithothoraces. In contrast, a decrease in body size 
characterizes the most crownward lineage Ornithuromorpha, the clade that 
includes all extant taxa, and potentially may explain the survival of these 
birds across the Cretaceous-Palaeogene boundary. As in all other dinosaurs, 
counter selection for small size is seen in some clades, whereas body size is 
increasing overall.

Guy Leahy