From National Geographic:
Ancient Sea Monster Found—First Freshwater Species Known
Crocodile-like predator roamed rivers 84 million years ago, study says.
The recently unearthed creature belongs to a family of ancient aquatic reptiles known as mosasaurs, which looked like crosses between crocodiles and whales. Mosasaurs lacked the superlong necks found in plesiosaurs, which the legendary inhabitant of Loch Ness is alleged to be.
Dubbed Pannoniasaurus, the creature is the first mosasaur thought to spend its entire life in freshwater.
"The evidence we provide here makes it clear that similar to some lineages of [whales], mosasaurs quickly adapted to a variety of aquatic environments," study leader László Makádi, a paleontologist at the Hungarian Natural History Museum, said in a statement
Illustration: A mosasaur swims an ancient river in an artist’s rendering by Tibor Pecsics
Article by Ker Than, National Geographic News; December 19, 2012