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.
Vertebrate fossils of various sorts are found in the phosphate beds there, though it’s the mosasaurs that interest us here. Some specimens are partially articulated, mosasaur jaws in particular being particularly sought after by collectors, of which more will be said later. It’s the teeth though that you’ll usually find on sale.
Identifying single teeth to species is level is tricky because mosasaur species are defined on many more characters than just the shame and size of their teeth. Nonetheless, the majority of fossil mosasaur teeth appear to come from just a handful of species. The commonest are probably those from a Liodon species such as Liodon anceps, mosasaurs with an extremely wide distribution from places as far apart as Britain and New Zealand. The Moroccan specimens are usually identified as Liodon anceps, though other Liodon species have very similar teeth indeed. Be that as it may, typical Liodon anceps teeth are stout cones, curving slightly in a backwards direction, and obviously compressed from side to side. Such teeth are the default for mosasaurs, curved enough to grab hold of fish, but strong enough to crush ammonite shells.
Another common identification is Platecarpus ptychodon, a species that had teeth that tend to be much more slender in shape than those of Liodon, though otherwise similar in terms of size and the gentle backwards curve. Whereas Liodon are widely distributed, Platecarpus seem to have been largely confined to North America, and reports of this species from elsewhere are tentative at best. One interesting thing about Platecarpus is that their gut contents have been preserved in some instances, and these indicate that they fed primarily on fish, something their long, sharp teeth would be particularly well suited to. So whereas Liodon were generalist predators, Platecarpus would seem to have been more specialised as fish-eaters.
Globidens was a genus of mosasaur that had become even more specialised, with a robust set of jaws equipped with round, blunt teeth well suited to crushing the shells of molluscs. Several Globidens species are known, including at least one from Morocco, and these very distinctive teeth do occasionally get traded.
Mosasaurs from other genera are known from Morocco, including various species of Prognathodon and Mosasaurus, but these can be a bit tricky to identify, generally looking rather like the Liodon teeth described earlier. There’s also been a bit of to-ing and fro-ing so far as scientific names go, for example some experts referring to Liodon anceps as Prognathodon anceps instead. So short of having a mosasaur expert identify them, you usually have to take these names on trust. But one area where you don’t have to rely solely on trust is in judging the quality of your specimen.
The brown enamel part of a mosasaur tooth is usually exactly what it purports to be, but the root of the tooth, and even more so any attached jaw bone can be faked. Indeed, experts agree that most of the teeth with roots, and virtually all the mosasaur jawbones, sold online and at fossil shows are fakes to some degree, and the serious collector will bear that in mind when shopping. Sometimes all the collector has done is work the matrix back create the root below the genuine tooth, but higher value fakes are made using loose teeth, glue, and a sort of cement made from modern animal bone and the original limestone matrix that can be worked into something resembling bone.
Such pieces are attractive enough but the serious collector probably won’t want one. Real mosasaur bone has a distinctive colour and texture that can be recognised easily enough, especially when compared against a fake, but casual collectors will have a hard time recognising a fake when offered for sale at a show or online. Unless the price is low enough you don’t mind if it is a fake, or you’re patronising a trustworthy vendor who offers a guarantee of authenticity with every sale, then the best approach is to have an experienced vertebrate palaeontologist or fossil collector come shopping with you before spending any money!