The turtle is a fossil, so we assume it is extinct. There are no giant turtles in the world except in the Galapagos Islands. Thus the verb that fits in the second blank must be past.
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AnonymousexplanationTo answer the question you have to know the idiom "known to". Otherwise, you're lost.
AnonymousThanks for the explanation Clive.Clive? What has Clive got to do with it?