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How to get more teeth
1 Tooth - Every 300 Kills (Instead of 600)
2 Teeth - Every 10 completed challenges
4 Teeth - Completing every challenge (If I fail I always get 10, if I achieve it I always get 14)
1 Tooth - Per relic
Thank you Zuxun for the information.
On August 8th there was a silent update with the Extinction Teeth System. Now you earn almost three times as many teeth as you would normally. The highest Ive seen was 19 teeth in one game.