My cousin recently received some matrix from the Carlile Shale Formation of Grant County, South Dakota, USA. He has been carefully breaking it down to extract the hidden fossil shark teeth. The fossils date to the Cretaceous Period (Turonian). We have been trying to identify what they are. So far names I am considering are Leptostyrax, Cretalamna (Otodus), and Cretodus.
Info Sources:
http://www.thefossilforum.com/
http://oceansofkansas.com/KS-