The first mosasaur was discovered about 1780 on the Meuse River in Europe.  The Family Mosasauridae were not dinosaurs but lepidosaurs.  (Reptiles with over-lapping scales.)  The four foot mouth of a large mosasaur was filled with conical teeth that made the reptiles the dominant predators of the late Cretaceous Period continental seas.  Paleontologists have estimated that a large mosasaur could equal the bite power of a full-grown T-Rex!
The largest of the Mosasauridae was hainosaurus  which reached a length of 17.5 meters (57.5 feet.)  Mosasaurs were powerful swimmers that breathed air and were well adapted to life in the open sea giving birth to live young and feeding on just about any animal they encountered.  They swallowed their prey whole just as modern snakes do so specimens have been found with a variety of partially digested contents including pterosaurs.  Ammonite specimens from the late Cretaceous Period have been found with puncture wounds that just fit the tooth style and spacing of mosasaurs.
Some scientists believe that, based on anatomical studies, mosasaurs and snakes had a common ancestor.
Mosasaurs became extinct, along with the dinosaurs, at the end of the Cretaceous Period.
The fossils for sale here are from the Maastrichtian age phosphate beds of Khouribga, Morocco.
Our tooth identification references are:
"Les Vertébres Fossiles Des Gisements De Phosphates Par Camille Arambourg et Jeanne Signeux No. 92." 1952.
"The basal mosasaurid Halisaurus from the latest Cretaceous phosphates of Morocco by Bardet N. & Pereda Suberbiola X." 2001.

Mosasaurs recognized from the Maastrichtian phosphates of Morocco:
Globidense sp.
Halisaurus sp.
Leiodon anceps
Mosasaurus beaugei
Platecarpus ptychodon
Prognathodon sp.