Platecarpus ptychodon Mosasaurus Tooth ("Flat Wrist")
Boxed (and Comes labelled - not shown for photography) specimens of a type of Mosasaurus called Platecarpus ptychodon. These are Cretaceous age (Maastrichtian). They are to right on the 65 Million years old date. Many still argue about what date the end of the Cretaceous and start of the Palaeocene era actually was, this is the KT Boundary. Maybe experts believe there was a 300,000 year margin where either could have occurred to have happened to start or end a sequence of geological time. Either way these are one of the last species of Mosasaurus to exist before the final extinctions, however long that took in real time. In certainly was not overnight.
They are some of the easiest to Identify but the hardest to get hold of due to be far less common than the other species. You can tell these are Platecarpus by the well defined striations (uniform ridges running vertically down the side of the tooth crown).