It’s definitely from the Magantango! It has the classic orange fossil staining from the breakdown of the pyrite rich sediments. With my experience in the Mahantanho, i’m going to say it may be one of two things:
- What Eleanor (@egardner ) says, part of a poorly preserved brachiopod. It is too poorly preserved to tell the genus or make a positive I.D.
- A piece of pygidium from a small Dipleura. I am saying this, because of the tiny bumps on it. Some of the poorly preserved Dipleura pieces (which can be abundant in certain layers) often have those little bumps on them.
I have a page on the Mahantango:
However, I have yet to scan and post pictures of the fossils (including Dilpleura) found there.
Hope that helps!