In every Pokemon game you get a chance to pick one of two fossil pokemon. Pokemon X and Y are no different. Between playing the first and second gym, you will get a chance to choose between jaw fossil, or sail fossil. You only get one per game.
If you choose the jaw fossil you will get Tyrunt, if you choose the sail fossil you will get amaura. Also females only have a 12% rate of coming from fossils, so you may want to save and retry the fossil if planning on breeding.
Amaura is a rock/ice pokemon, who is only obtainable by a sail fossil or breeding.
Amaura evolves into Aurorus.
Tyrunt is a dragon/rock pokemon, who is only obtainable by jaw fossil or breeding.
Tyrunt evolves into Tyrantum.