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part of a gastropod shell possibly

If you go east or north of Riyadh you are in the Cretaceous, if you go west or south, you are in the Jurassic. I found an Jurassic ammonite (not a particularly good one) south of Riyadh many years back.

@bill-heim I see what you mean about the ammonite, but its the ring in the center with no spiral ridges and the glossy appearance that make me think this is the top of a conch shell, likely a modern one, that has been worn.

It was found nowhere near a beach.

It was only shiny because I had just washed it.

I am not saying that is an ammonite, I was saying that I (me)found an ammonite there.

Somebody suggested it’s a rudist of some kind.

rudist seems the most likely option. also, whether or not you found it by a beach does not necessarily change much, since there are loads of marine fossils to be found inland, even in the highest mountain ranges.