"Mere" bone? Are you joking? And what source would you accept? A scientific notebook from an alien paleontologist of 100 million years from now that happened to fall through a wormhole and into my lap? What source do you expect me to find?
I don't know if I even understand your question
. You want me to demonstrate somehow that bones - which fossilize and have demonstrably lasted across millions of years, even hundreds of millions of years - will not
be outlasted by:
- rebar-reinforced concrete - which already looks like rocks because concrete is just a mix of cemented rocks. After millions of years it's still going to look just like a rock. Most people already have a hard time telling a piece of concrete from rock now. And the rebar will just rust and react with everything over millions of years. Rebar-reinforced concrete isn't going to last across millions of years.
- cast iron - which would just look like a lump of rusted iron ore after millions of years in the rock cycle, if it remained in one lump, which is highly unlikely, since it is brittle and will break eventually. Even if a lump of "cast" iron (it wouldn't be "cast" anymore after millions of years) were found by future archaeologists or paleontologists, what would that prove?
- titanium, which oxidizes eventually at high temperatures, too; it's not indestructible, you know, and let's face it, it's not the most common material in our civilization. Even if a titanium bike frame lasted intact across millions of years, what do you think the odds of it being found are?