While dinosaurs dominated the land, a variety of reptiles reigned in the seas, and includedand the . In the new study, scientists investigated another similar beast extinct carnivores known as , generally thought of as gigantic finned marine lizards similar and perhaps even related to present-day monitor lizards.
The mosasaur fossils were unearthed starting in 1999 from an open-pit mine in the Bakony Hills of western Hungary. The researchers discovered several 84-million-year-old specimens, ranging from small juveniles to adults 20 feet (6 meters) long that had limbs like a land-dwelling lizard, a flattened, crocodilelike skull, and a tail unlike other known members of the mosasaur family.
The fossils were discovered in areas that were once freshwater floodplains home to fishes, amphibians, turtles, crocodiles, terrestrial lizards, pterosaurs, dinosaurs and birds. This newfound reptile appears to be the first known freshwater mosasaur, an animal comparable to modern river dolphins now seen in the Amazon, Ganges and Yangtze rivers.
The new species is named Pannoniasaurus inexpectatus. "Pannonia" refers to the part of Hungary it was found in, "saurus" means lizard, and "inexpectatus" refers to the unexpected occurrence of this
mosasaur in freshwater environments. 
"The size of Pannoniasaurus makes it the largest known predator in the waters of this paleo-environment," said researcher Lszl Makdi, a paleontologist at the Hungarian Natural History Museum.
The evolutionary history of mosasaurs suggested by these new findings is remarkably similar to that of whales and dolphins, Makdi told LiveScience.
"To the best of our knowledge, the ancestors of mosasaurs and of some related reptiles moved from land to aquatic realms at least 100 million years ago. Whether these were marine or freshwater environments, it is [uncertain], however some finds in Japan suggests the latter," Makdi said.
"Then sometime between this event and 85 million years before present the age of our locality in Hungary some primitive members of the already-marine mosasaurs adapted to freshwater life. Pannoniasaurus represents these. This scenario is very similar to the evolutionary history of cetaceans whales, dolphins."
The researchers plan to investigate new Pannoniasaurus fossils as they pop up in the hopes of learning more about the beast's biology, such as how it moved and what it might have eaten, Makdi said.
The scientists detailed their findings online today (Dec. 19) in the journal PLoS ONE.
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