A new mosasaur species discovered in Hungary is the first known example of this group of scaled reptiles to have lived in freshwater river environments similar to modern freshwater dolphins, researchers said.
They were the biggest and most dangerous predators in the water and occupied a similar niche to freshwater dolphins of today.
The new species probably adapted to freshwater environments similarly to river dolphins, such as those now inhabiting the Amazon, Ganges and Yangtze rivers.
Researcher led by Laszlo Makadi from the Hungarian Natural History Museum, Hungary said the species lived about 84 million years ago, the largest specimens reached about 20 feet in length, and belongs to a family called 'mosasaurs'.
They were conventionally thought of as gigantic finned marine lizards, similar and perhaps even related to present day monitor lizards.
Researchers discovered several fossils of the new species, ranging from small juveniles to large adults that suggest that this species had limbs like a terrestrial lizard, a flattened, crocodile-like skull, and a tail unlike other known members of the mosasaur family.
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The fossils were recovered from an open-pit mine in the Bakony Hills of Western Hungary, which were once flood-plains.
According to the study, this is the first known mosasaur that lived in freshwater, and only the second specimen of a mosasaur to have been found in rocks that were not once deposited in the ocean.
"The evidence we provide here makes it clear that similar to some lineages of cetaceans, mosasaurs quickly adapted to a variety of aquatic environments, with some groups re- invading available niches in freshwater habitats. The size of Pannoniasaurus makes it the largest known predator in the waters of this paleo-environment," Makadi said in a statement.
Even in the modern world, scaly reptiles in the aquatic world are extremely rare. Only a few species live in the water, and even fewer, like marine iguanas and sea kraits, live in the oceans.
The study was published in the journal PLOS ONE.