Extinction of Species:
Extinction
Pseudoextinction
Mass extinction: 
Cambrian/Ordovician, 505 my ago
Devonian/Mississippian, 360 my ago
Permian/Triassic, 245  my ago
Jurassic/Cretaceous, 144  my ago
Cretaceous/Tertiary, 65 my ago
The Causes of Mass Extinctions
Fossils and the GEOLOGIC TIME SCALE