To cause an extinction event, it's not enough to just kill the dinosaurs (as I learned when I used thunderclouds to kill them.) Instead, you have to initiate a catastrophic event. There aren't that many of those in the game. Here are some suggestions.

• FLOOD
• CTHULHU
• BLACK HOLE
• PLAGUE
• HURRICANE
• LHC - (short for Large Hadron Collider.) Activate it to create a black hole.