There’s no reason why you can’t preserve collagen tissue for millions of years. Place the tissue in an environment that precludes decay from bacteria, then let it dehydrate while encased in material that turns to rock. Move ahead millions of years... break open the rock and rehydrate. The tough part is preventing the decay, but I could imagine this happening under certain scenarios. Such as the tissue drying very quickly after death or the animal being buried within material such as volcanic ash that cuts off oxygen.
As to how such long term DNA preservation could occur, Asimov’s line comes to mind: “The universe is not only stranger than we think it is, it is more strange than we can think it is.”