Question

If I chose Tyrunt is it still possible to get Amaura by using Rock Smash?

Answer 1

No,you are unable to get the sail fossil for Amaura by rock smashing.To get Amaura,you will need to trade from another game by reviving the Sail fossil then trade.The Pokemon fossils that are available by rock smash are
Helix Fossil
Dome Fossil

Claw Fossil
Root Fossil

Skull Fossil
Armor Fossil
Cover Fossil
Plume Fossil
Old Amber

Hope I helped! :)

Source:http://www.ign.com/wikis/pokemon-x-y/Fossil_Pokemon

Answer 2

No.

Quite simply. There is no other way to receive the fossil you didn't choose. Sorry. :( You can get many other fossils through Rock Smashing though.

Hope I helped. :)
Source: Serebii's ItemDex pages.