There are no fossils in Twist Mountain. You have to go to the Relic Castle, west of nimbasa. In there will be a trainer who will give you a Cover or Plume fossil (your choice). the Cover fossil will get you Tirtouga, Plume fossil gets you Archon.
BlossomTail - Actually, there ARE Fossils in Twist mountain. You talk to a worker there, and he will give you a different fossil each day, after you've received your national pokedex. The fossils are from the other regions and can only be obtained here. When he's given out all his fossils, you cannot return for more.
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In twist mountain.
Twist Mountain is located right before you reach Iccirus City on Route 7.
In the middle of Twist Mountain when it's winter.
route 13, Twist mountain, only in winter
You can't! Mua-ha-haa!