Freshwater Mosasaur Stirs Marine Reptile Relationships

If ever there were marine creatures that could justifiably be called “sea monsters”, the fearsome mosasaurs would fit the bill. These extinct aquatic lizards – cousins of today’s monitors and Komodo dragons – were the apex predators of the world’s Late Cretaceous seas from the coast to the open ocean, and they gulped down prey with frightening, expandable jaws. As Samuel Wendell Williston, the early 20th century’s foremost authority on ancient marine reptiles, put it:

It is certain that the mosasaurs were much more predaceous and pugnacious in their habits than were any other truly aquatic backboned air-breathing animals of the past or present. They were the ‘land sharks’ of the ancient seas, and probably the only ones among water reptiles that would be dangerous and offensive to man, were they all living today.

From the time the remains of these aquatic lizards were discovered in the 18th century to the present, mosasaurs have been considered as wholly marine reptiles. They slid into the sea and, over time, natural selection molded them into streamlined, paddle-finned, fluke-tailed cruisers like Platecarpus and Plotosaurus. But a collection of over a hundred isolated bones found in a Hungarian bauxite mine complicates that evolutionary outline. These lizards weren’t restricted to the seas, and sleek, oceanic mosasaurs may have evolved more than once.

Between 85 and 83 million years ago, what is now western Hungary was an island – one of many in an archipelago created by the Tethys Ocean. Freshwater rivers flowed across these patches of land, nourishing habitats that boasted a variety of prehistoric life. The deposits preserved at the Iharkút mine, in particular, have yielded fish, amphibians, turtles, crocodiles, pterosaurs, plants, invertebrates, and dinosaurs, including the first ceratopsian known from Europe. Isolated vertebrae hinted that there was a relatively large monitor lizard in the same habitat, too, but, as paleontologist Laszlo Makadi and coauthors report today in PLoS One, skull elements found in the same quarry indicate that the lizard bones actually belonged to many, many mosasaurs.