t 





of the dirk-toothed type of saber-toothed 

 carnivores in the Great Plains of North 

 America. He found that following the ex- 

 tinction of the creodont Machaeroides, the 

 role was empty until the appearance in the 

 late Eocene, some five million years later, 

 of the nimravids, probably representing 

 immigration from Asia. Several different 

 types of jaguar-sized nimravids alternated 

 as "top cat" in this role during the Oligo- 

 cene, but all nimravids went extinct in 

 North America about twenty-three million 

 years ago. 



There was another saber-toothed "cat 

 gap" for the next twelve milUon years or 

 so, until the immigration, probably via 

 Asia, of Barbourofelis, a lion-sized nim- 

 ravid. Barbourofelis lasted until near the 

 end of the Miocene, becoming extinct 

 about seven million years ago. Shortly 



after the extinction of Barbourofelis, an- 

 other dirk-toothed predator — Megan- 

 tereon — immigrated from Asia. A true 

 felid, Megantereon was an ancestor of 

 Smilodon, which thrived as recently as 

 12,000 years ago, during the late Pleis- 

 tocene. 



Why has the saber-toothed ecomorph 

 been so vulnerable to extinction, and why 

 is it not present today? Large carnivores 

 are, in general, highly susceptible to ex- 

 tinction. The skeletons of all sabertooths 

 suggest predators built for power, not 

 speed (the dirk-tooths were more power- 

 fully built than present-day lions or tigers). 

 That, in turn, implies a hunting style that 

 relies on ambush rather than the long 

 chase, a style associated in carnivores 

 today with a more-or-less solitary life style 

 and large individual home ranges. If the 



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