Fossil Mosasaur 'Marine Dinosaur' Teeth with Roots in Matrix Group Cretaceous Period, 145-65 million years B.P. A trio of mosasaur Prognathodon sp. fossil teeth, each complete with tooth root and set in an oval matrix. 1.16 kg total, 12.1-13 cm (4 3/4 - 5 1/7 in.). From a Cambridgeshire, UK, collection. An extinct, large marine reptile with a snake-like body, long snout and paddle-like limbs. Found worldwide, they competed with other well-known sea predators of the Late Cretaceous-plesiosaurs and ichthyosaurs-for food, existing primarily on a diet of ammonoids, cuttlefish and fish. [3, No Reserve]