My cousin has been working on some Carlile Shale Formation matrix. It was found in Grant County, South Dakota, USA. He has been carefully extracting the hidden fossil shark teeth. The fossils date to the Cretaceous Period (Turonian). The picture shows what appears to be a Cantioscyllium decipiens tooth fossil. Field of view (FOV) is about 3 mm.
Thanks to Kenny for the image.
Info Sources:
http://www.chalk.discoveringfossils.co.uk/5%20Cantioscyllium.htm
http://fossilsofnj.com/shark/c_decipiens.htm