Generally iNat is about currently living organisms and recently extinct ones, not fossils.
It’s not against the rules, but you have to indicate that there is no “recent sign of organism”, which will drop it to ‘casual’ grade. In addition, there is the possibility of making a place easy to find for illegal or irresponsible collectors, so if a fossil is uploaded it’s best to either have it be only of common ones, or to obscure the location.
This topic has been covered before, you might get some insight from some of the past posts
- https://forum.inaturalist.org/t/are-fossils-acceptable/40395
- https://forum.inaturalist.org/t/uploading-prehistoric-fossils/13457
- https://forum.inaturalist.org/t/fossils-in-rocks/5670
- https://forum.inaturalist.org/t/rock-fossil-crystal-inat/24333
- https://forum.inaturalist.org/t/how-is-it-decided-which-fossil-taxa-are-added/35505