Mosasaur were large marine reptile family containing 40 genera in total. Their first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. Mosasaurs probably evolved from an extinct group of aquatic lizards. Mosasaurs breathed air, were powerful swimmers, and were well-adapted to living in the warm, shallow inland seas prevalent during the Late Cretaceous period. They became extinct at the end of the Cretaceous period, about 66 million years ago. This listing is for 1 tooth in matrix. You will receive one of the above or similar.