402 V. S. MARTIN 



Cenozoic Extinction Rates Rise in the Pleistocene 



Extinction rates in number of genera per million years for several 

 mammalian orders increased greatly at the end of the Cenozoic 

 during the Pleistocene (Fig. 5). The data from Simpson (1945) repre- 

 sent last appearances of genera in each of twenty mammalian orders. 

 To obtain extinction rates, one divides the number of genera last 

 recorded in each period by the geological estimate of elapsed time, 

 17 million years for the Miocene, 11 for the Pliocene, and 1 for the 

 Pleistocene. 



It is obvious that only certain orders exhibit a strong Pleistocene 

 effect. Those include the artiodactyls, proboscideans, marsupials, 

 edentates, rodents, perissodactyls, fissipeds, and primates. Certain 

 groups with moderately good late Cenozoic fossil records, as the 

 cetaceans and pinnipeds, appear unaffected. The extinct Pleistocene 

 genera of marsupials are all Australian and include large kangaroo 

 and phalangeroid herbivores. Within the primates, 8 of 14 extinct 

 Pleistocene genera were lemur-like forms from Madagascar. Among 

 the rodents, a rise in Pleistocene extinction rate can be attributed 

 in part to the extinction of insular genera, 15 of them West Indian. 

 Regarding the entire late Cenozoic record of mammals, extinction 

 rates rise from 25 per million years in the Miocene, 40 in the Plio- 

 cene, to 203 in the Pleistocene. From this we may believe that 

 Pleistocene extinction transcends that of the rest of the Cenozoic. 

 Was it caused by climatic change of the Glacial periods? 



Pleistocene Extinction Rates Rise in the Last Glacial Period 



Hibbard's valuable list (1958; also in Flint, 1957) of Pleistocene 

 mammals shows a considerable measure of extinction in the First 

 Interglacial, the Aftonian. However, with the single exception of 

 Borophagus, there is continual replacement of generic types until 

 the Wisconsin. 



Estimating the Glacial periods arbitrarily at 100,000 years each 

 and the interglacials at 250,000 years we obtain for North America 

 the following extinction rates, expressed as number of genera per 

 100,000 years: Nebraskan, 1.0; Aftonian, 6.0; Kansan, 5.0; Yar- 

 mouth, 1.2; Illinoian, 1.0; Sangamon, 0.0; Wisconsin, 31.0 (ter- 

 minal records from Hibbard in Flint, 1957, with addition of Floridian 

 Melbourne mammals). On this basis it is possible to conclude (1) 

 that the extinction rate in the Wisconsin was considerably higher 



