OK, so this is a tough one. I can say that it’s not a sea urchin spine, nor damsel fly, nor stick, but what is it? The picture is not detailed enough in resolution to give you a definitive answer, but I have an idea. Does it have horizontal lines (relative to the long axis) on one side and smooth on the other. If the answer is yes, I would say it is a single section of a stingray mouthplate assembly. Otherwise, I would have to see a better quality picture, and I would still probably not be able to help you. LOL