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
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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.
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Source: Serebii's ItemDex pages.