Here is a picture of a shark tooth found in the New Albany Shale of southern Indiana, USA. This layer dates to the Late Devonian Period.
The New Albany Shale is desolate of fossils as the sea at that time was thought be a low oxygen environment. When you look at the shale it is mostly black thin layers that peel off. Sometimes it has a rusty colored coating on the layer and it can smell like sulfur. Bands or nodules of pyrite can be found it as well. When fossils are found it is mostly small brachiopods and fossilized wood. It is possible that Cladoselache shark fossils can be found in this layer as well. See this web posting.
So seeing a nicely shaped shark's tooth in the layer is quite a surprise. The fossil is on display at the geology department at Indiana University Southeast (IUS) of New Albany, Indiana as of August 2013. It was listed as being found in 2004 by Jessica Kinder and Dale Lind.