At one particular site that the Mahantango Formation is exposed at I have good luck finding Goniatites. These Cephalopods were animals related to the modern Squid and Octopus that had coiled shells like Ammonites but preceded them in the evolutionary timeline.
This is a large, fairly inflated piece of a shell from an unknown genera.
Here is a smaller Goniatite called Tornoceras sp. I profiled a couple of other specimens that I'd previously found in this post. I asked William Kochanov, one of the staff members from the PA Geological Survey, about my tentative ID of those fossils and he agreed with my assessment so I'm fairly confident that this specimen is also Tornoceras.