Where I live in Pennsylvania, the underlying bedrock is Triassic aged lacustrine deposists lain down when Pangea was splitting apart. Not many fossils are found but occasionally fish, reptiles, plants and dinosaurs are discovered. While exploring a building foundation excavation near my house I found some black shale which has shiny shells on the surfaces.
Above is a piece with both the front and back covered with the shells. It turns out that the shells are of a freshwater Conchonstracan called Cyzicus. It's an crustacean similar to ostracods but more closely related to shrimp. It been nicknamed the "Clam Shrimp" since it has two chitonous shells that enclose the body.
Here are some closer views of the shells that I took with my Zorb micro imager.
Here is a page with a nice anatomical picture and description plus some other fossils.
This page has links to a whole series of pages with more information about Conchonstracans.