Huge 'Sea Monster' Ruled Ancient Rivers

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illustration of a mosasaur
Researchers discovered remains of a mosasaur species, Pannoniasaurus inexpectatus (reconstruction shown here), that lived 84 million years ago in freshwater floodplains. (Image credit: Tibor Pecsics)

Giant reptilian predators with a lifestyle comparable to modern freshwater dolphins may have made their home in ancient rivers, researchers say.

While dinosaurs dominated the land, a variety of reptiles reigned in the seas, and included dolphin-shaped ichthyosaurs and the Loch-Ness-Monster-like plesiosaurs. In the new study, scientists investigated another similar beast — extinct carnivores known as mosasaurs, generally thought of as gigantic finned marine lizards similar and perhaps even related to present-day monitor lizards.

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. [T. Rex of the Seas: A Mosasaur Gallery]

"The size of Pannoniasaurus makes it the largest known predator in the waters of this paleo-environment," said researcher László Makádi, 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, Makádi 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," Makádi 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, Makádi said.

The scientists detailed their findings online today (Dec. 19) in the journal PLoS ONE.

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Charles Q. Choi
Charles Q. Choi
Live Science Contributor
Charles Q. Choi is a contributing writer for Live Science and Space.com. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica.