Definitely an oyster. If you could tell me where it was found I could give you a better idea on identification. Oysters look pretty much the same through long swaths of time.

Definitely not Porocystis. A site near Bandera has produced several small floating crinoids. This doesn’t look like any I have found from there but might be what it is. Does it have calcitic cleavage anywhere, or does it look more like limestone fill?

Agreed, a bit too blurry to tell. Might be a spine, or a scaphapod as well. Need more info to tell.

Updated.

I concur.