DAWSON, W. R. 



achieved until after the appearance of effective temperature regula- 

 tion. 



The adaptive radiation of birds, in good part made possible by 

 the possession of homethermy, apparently began shortly after birds 

 first appeared. However, it did not proceed at a constant rate. As 

 far as can be determined from known fossils, the major flowering 

 of avian evolution occurred early in the Tertiary. By the end of the 

 Eocene most of the known orders of birds had appeared, and by the 

 end of the Miocene, most Recent families of birds were probably in 

 existence. Today thereareover 8,000 species of living birds, repre- 

 senting some 28 orders and 161 families. Six orders and 39 families, 

 not counting fossils of uncertain taxonomic position, are known to 

 have become extinct (Storer, 1960). All contemporary birds appear 

 highly modified for their respective adaptive niches, and none is es- 

 pecially primitive. Beddard (1898:160-161) concluded, "the few spe- 

 cially reptilian features in the organisation of birds have, so to speak 

 been distributed with such exceeding fairness through the class that 

 no type has any great advantage over its fellows;." In contrast, 

 mammals include both primitive and highly advanced types. Among 

 the former, the monotremes, though specialized in some respects, 

 have many of the structural features of therapsid reptiles (Simpson, 

 1959). 



BODY TEMPERATURE 



Central body temperatures of active birds are generally main- 

 tained between 38 C and 43 C, with the limits for individual species 

 being narrower (King and Earner, 1960). Perhaps utilization of this 

 band of temperatures resulted from a compromise between two un- 

 favorable ranges of temperature (Burton and Edholm, 1955). On one 

 hand, it was far enough above the rather moderate temperatures 

 which apparently prevailed in the Triassic and Jurassic (Brooks, 

 1949) so thatphysiological changes required to cope with minor fluc- 

 tuations in ambient temperature would be relatively small. On the 



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