My background is in zooarchaeology, so I've spent a lot of time identifying and analyzing bones from archaeological sites. (Although I'm not good with fish and reptiles. Mostly mammals and birds where I'm at.)

You can tell by the pixels

They’re a fish boner

Sorry that was my best guess, either broken or eaten cuttlebone. But if it doesn't look like the others then it's probably from another type of sea creature, someone else is bound to know more (E: looks like someone does!)..