It's disappointing that you can't really tell it's embedded in rock and not mud.
The local paper had an article a while back about some stumps that sand washed away from and left exposed... estimated at 8500 years old. That's probably two orders of magnitude longer than I would've guessed something could last buried in wet sand. Admittedly the world is not packed with things that have a taste for cellulose.
one that's come free from the rock. I want to call it shale, because that's what the loose sedimentary stuff was called where I grew up, but I have no clue about geology.
one-that's-come-free again, lifted from its hollow and turned over, which shows the detail better.
I saw in a book that Liracassis petrosa looks sort of like this. A web page suggests that's like thirty million years ago. But...don't trust me in the slightest here.
The reverse was flat stone with a bit of another shell embedded. I have no way to know how mineralized this was, but it may well have been a proper fossil.