Mosasaur
Mosasaurs (from Latin "Mosa" meaning the 'Meuse river', and Greek σαύρος "sauros" meaning 'lizard') are an extinct group of large marine reptiles containing 38 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 known as aigialosaurs in the Early Cretaceous. During the last 20 million years of the Cretaceous period (Turonian-Maastrichtian ages), with the extinction of the ichthyosaurs and pliosaurs, mosasaurs became the dominant marine predators. They became extinct as a result of the K-Pg event at the end of the Cretaceous period, about 66 million years ago.
Plesiosauria
While the Loch Ness monster is often reported as looking like a plesiosaur, it is also often described as looking completely different. A number of reasons have been presented for why a plesiosaurian identity is unlikely. These include the assumption that the water in the loch is too cold for a, for the sake of this argument, presumed cold-blooded reptile to be able to survive easily, the assumption that air-breathing animals would be easy to see whenever they appear at the surface to breathe, the fact that the loch is too small and contains insufficient food to be able to support a breeding colony of large animals, and finally the fact that the lake was only formed 10,000 years ago at the end of the last ice age, while the latest fossil appearance of plesiosaurs dates to over 66 million years ago. Frequent explanations for the sightings include waves, floating inanimate objects, tricks of the light, swimming known animals, and practical jokes. Nevertheless, in the popular imagination, plesiosaurs have come to be identified with the Monster of Loch Ness. This has had the advantage of making the group better known to the general public, but the disadvantage that people have trouble taking the subject seriously, forcing paleontologists to explain time and time again that plesiosaurs really did exist and are not creatures of myth or fantasy.