My cousin has been extracting fossils from a piece of Carlile Shale Formation matrix. It was found in Grant County, South Dakota, USA. He has been slowly breaking it down to reveal the hidden fossil teeth. The fossils date to the Cretaceous Period (Turonian). The picture shows what might be a mosasaur fossil tooth. Each scale division is a mm.
Thanks to Kenny for the images.
Sources:
http://oceansofkansas.com/aug98mos.html
http://www.bioone.org/doi/abs/10.1660/062.114.0101