PLEISTOCENE ECOLOGY AND BIOGEOGRAPHY 401 



Before proceeding directly into a consideration of possible causes, 

 it appears essential to examine the nature of late Pleistocene extinc- 

 tion more closely [cf. Simpson (1953) for a general treatment of 

 phyletic extinction]. If it is no different from that of the early 

 Pleistocene or Tertiary there is little point in proceeding farther. 



Extinction without Replacement 



In addition to Equus, cited above, the following North American 

 genera, representing specific ecological life forms with respectable 

 Tertiary lineages, disappeared in the late Pleistocene or sub-Recent 

 time: the elephants and mastodons Mammuthus and Mammut, 

 the camels Tanupolama and Camelops, the Old World antelope 

 Saiga, the cervid deer Sangamo?ia, and Cervalces, the shrub-oxen 

 Symbos, Eucemtherium, and Preptoceras, the pronghorns Breameryx 

 and Stockoceros, the giant beaver Castoroides, and others. They re- 

 flect abandonment of grazing and browsing habitats by roughly three- 

 quarters of the mammalian herbivore fauna. In the West Indies four 

 genera of ground sloths and sixteen of hystricomorph rodents, also 

 herbivores, disappeared in sub-Recent time. 



The general rule that abandonment of life forms and the disap- 

 pearance of genera or subfamilies without replacement occurred only 

 in the late Pleistocene may have a single minor exception. Boro- 

 phagus, a New World equivalent of the hyaenid life form, is unknown 

 beyond the First Interglacial (Hibbard in Flint, 1957, p. 462). How- 

 ever, at no time since the extermination of the Upper Cretaceous 

 duck-bills and other herbivorous dinosaurs has there been un- 

 balanced extinction of equal magnitude. 



Fig. 5. Generic extinction rate curves for various mammalian orders. 

 The number of genera last recorded divided by estimated age for each 

 epoch is shown for three Late Cenozoic epochs: M = Miocene, 17 million 

 years; P = Pliocene, 11 million years; PI = Pleistocene, one million years. 

 Pleistocene extinction rates rose sharply in the primates, rodents, eden- 

 tates, fissipeds, notoungulates, liptoterns, proboscideans, perissodactyls, 

 and artiodactyls. There was no comparable rise in extinction rates among 

 the insectivores, rabbits, cetaceans, pinnipeds, and sirenians. Data on 

 extinct genera were obtained from Simpson (1945) ; orders with poor late 

 Cenozoic fossil records, as the bats, pangolins, and hyraxes, are not 

 included. 



