EVOLUTIONARY ASPECTS IN MAMMALS 



(previously also termed Edentata) the armadillos (Dasypodidae) , 

 have departed the least from the ancestral plan, and are a very 

 ancient group, probably originating in the Paleocene, some 60 

 million years ago. Both the anteaters and the sloths of today are 

 decidedly very specialized animals. The remaining Xenarthra have 

 left very scanty fossil evidence. To briefly complete our palfeon- 

 to logical starting ground let us, via Figure 1, remind you of the 

 remaining mammalian orders living today. Any paleontologist, and 

 perhaps many of you, will be horrified at this unscrupulous sim- 

 plicity and superficial treatment of mammalian descent. I feel, 

 however, that this may be sufficient in our present context and 

 prefer to return to more detailed descriptions only if they are advan- 

 tageous to the physiological considerations to follow. 



CETACEA 



/ 



RODENTIA CARNIVORA 



XENARTHRA \ / PRIMATA 



cHiROPTERA — -Ill;;\\ A--::!!!!--*- UNGULATA 



INSECTIVORA 



MARSUPIALIA 



t 



MONOTREMATA 



Figure 1. Schematic arrangement of the existing mammalian orders. 



Turning then to the physiological phylogeny of temperature regu- 

 lation, we should also list some starting points. First of all, it seems 

 reasonable to assume that a certain variance in the environmental 

 factors is necessary for the establishment of temperature regulation. 

 In other words, before regulation appears there must be something 

 to regulate against. This naturally leads us to believe that the grad- 

 ual establishment of temperature regulatory mechanisms must have 



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