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					| Extinction |  
					| Lecture 11 3/2/98
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					| |  | Last week we discussed how the diversity of life expands through evolution. | 
 |  | Today we will look at extinctions, where the diversity of life diminishes. | 
 |  | Extinctions are a common occurence in the geologic past. | 
 |  | Extinctions are the result of biologic and earth processes. | 
 |  | Mass Extinctions form the boundaries of geologic time units | 
 |  | Mass extinctions typically open environments for new groups to exploit and radiate into. | 
 | Major Mass Extinctions through Geologic Time | 
 |  | Cambrian-Ordovician boundary (510 MY) | 
 |  | Ordovician-Silurian boundary (440 MY) | 
 |  | Devonian-Carboniferous boundary (360 MY) | 
 |  | Permian-Triassic boundary (245 MY) | 
 |  | 90% of life forms become extinct | 
 |  | Triassic-Jurassic boundary (208 MY) | 
 |  | Cretaceous-Tertiary boundary (65 MY) | 
 | Causes of Mass Extinctions | 
 |  | Reduction in food resources | 
 
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