It's not quite Nessie, but it's close.

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. (See "Pictures: Largest 'Sea Monster' Skull Revealed?")

Young Mosasaurs "Rare as Hen's Teeth"

The new mosasaur was discovered in the waste dump of a coal mine in western Hungary. Scientists uncovered thousands of fossils belonging to several Pannoniasaurus individuals ranging from three feet (one meter) to 13 feet (4 meters) in length at the site. (Watch a sea monsters video.)

The smaller fossils, which belonged to juvenile mosasaurs, are a rare find, scientists say.

"We generally get the big guys," explained study co-author Michael Caldwell, a mosasaur expert at the University of Alberta in Canada.

"Finding young or even smaller-bodied versions is as rare as hen's teeth in the fossil record of mosasaurs."

The discovery of so many Pannoniasaurus specimens at one site also suggests the species was a true freshwater dweller and not just a marine mosasaur that occasionally ventured into rivers, the way some sharks do.

"What's really cool is we have a place where they were living, and living at all stages in their life cycle," Caldwell said.

River Dweller Was Top Predator

During the Upper Cretaceous period when Pannoniasaurus lived, the site was part of a chain of tropical islands situated in the middle of a massive freshwater "seaway" that separated Africa and southern Europe.

Pannoniasaurus lived in freshwater rivers that cut through the islands and flowed into the seaway.

The rivers were home to fish, amphibians, turtles, lizards, crocodiles, and dinosaurs—the remains of which have all been found at the coal mine site. With its large size, Pannoniasaurus was "probably the crown predator in the ecosystem," Caldwell said.

As far as alpha predators go, though, Pannoniasaurus was fairly benign. Its small, sharp teeth suggest it fed mainly on fish and perhaps amphibians and lizards. (Also see "Long-Necked Sea Reptiles Had Unexpected Diet, Fossils Show.")

"I doubt it was a gigantic predator," Caldwell said. "It was probably just catching fish and perfectly happy doing that."

Sea Monster Acted Like a Croc

Unlike other marine mosasaurs that swam using large flippers, the limbs of Pannoniasaurus still resembled legs, which may have been useful for occasionally clambering onto land.

Pannoniasaurus "could well have been amphibious," Caldwell said.

"I suspect they behaved a lot like modern crocodiles do today. They spent a lot of time in the water, but there's nothing wrong with crawling from river to river when things dry up or even using shallow water to bask and moderate their body temperature." (See alligator and crocodile pictures.)

Paleontologist Randall Nydam of Arizona's Midwestern University called the discovery of a freshwater mosasaur "quite important."

"I never even really thought that we would have any freshwater mosasaurs because they've been such a specific marine animal," said Nydam, who was not involved in the study.

When the Hungarian fossils were first announced, he added, many paleontologists thought they belonged to a large land lizard, similar to a Komodo dragon, until evidence for their aquatic origin soon became overwhelming.

"This is really quite an outstanding find," Nydam said.

Other Freshwater Reptiles Existed?

Caldwell added it's unlikely that mosasaurs were the only ancient marine reptiles to successfully adapt to freshwater.

"I'm sure there were freshwater plesiosaurs and freshwater ichthyosaurs"—giant marine reptiles that resembled dolphins, he said.

"We just don't have the evidence yet."