Mosasaurs were large lizards, for a long time assumed to be most related to the modern monitor lizards such as the Komodo dragon, Varanus komodoensis. Certainly they had a lizard-like shape, except that their legs were modified into flippers for steering and their tail was flattened and supported a vertical crescent-shaped fin, adaptations that presumably helped them to swim efficiently.
However, some recent work has suggested that mosasaurs were not as closely related to monitors as we’d thought, and in fact their closest living relatives are the snakes. This isn’t a completely new idea, having originated with the nineteenth century palaeontologist Edward Drinker Cope, who suggested that snakes had a marine origin and placed them together with the mosasaurs in a group known as the Pythonomorpha. But for the last century or so this idea had been largely discarded, palaeontologists generally believing that snakes had lost their limbs as an adaptation to a burrowing, rather than swimming, lifestyle.
As things stand now, there’s evidence to support both points of view, but what is certain about mosasaurs is that they were lizards, which distinguishes them from the other big groups of marine reptiles such as the nothosaurs, ichthyosaurs, plesiosaurs and pliosaurs. Indeed, they’re unquestionably the biggest lizards to have ever lived, some species of Mosasaurus for example measuring in at well over 15 metres. They were also a very characteristic part of Late Cretaceous marine faunas, predominantly inhabiting inland seas such as the famous Western Interior Seaway of North America and the Tethys Sea fringing the Eurasian, African and Indian continental plates. But while mosasaurs don’t seem to have favoured the open ocean, they were very widely distributed, their remains being known from places as far apart as New Zealand, Peru, Sweden and Japan. However, the specimens you’ll seen in fossil shops are usually ones from the Late Maastrichtian Khouribga Basin of Morocco.