BANGKOK, Thailand — Scientists have identified two ancient reptiles that swam in icy waters off Australia 115 million years ago, researchers said. They are among the first of their kind to be found from the period soon after the Jurassic era.

The discoveries — dubbed Umoonasaurus and Opallionectes — belonged to a group of animals called plesiosaurs, long-necked marine reptiles that lived during the time of the dinosaurs.

Researchers, led by paleontologist Benjamin Kear and a team from the University of Australia and South Australian Museum, identified the new species after piecing together fossils from 30 individuals collected from an opal mine in the past 30 years.

The team's findings were recently published in both the international journal Paleontology and the online edition of Biology Letters, a periodical published by the prestigious Royal Society of London.

Umoonasaurus was a rhomaleosaurid, a kind of plesiosaur that was the "killer whale equivalent of the Jurassic" period, Kear said. It was distinguished by its relatively small size of less than 8 feet long and three crest-like ridges on its skull.

"Imagine a compact body with four flippers, a reasonably long neck, small head and short tail much like a reptilian seal," Kear said.

Opallionectes was also a plesiosaur but much larger — about 19 feet long with masses of fine, needle-like teeth for trapping small fish and squid. Its name means "the opal swimmer from Andamooka."

"It's a missing link between older forms of the Jurassic period found in England about 170 million years ago and the much younger ones found in Antarctica and Patagonia which are about 65 million years old," Kear said.

Both creatures lived in a freezing polar sea that covered what is now Australia 115 million years ago, when the continent was located much closer to Antarctica. Kear said he expected more discoveries, which together could open a window on a period that he said had been largely unexplored in Australia.

"We're just scratching the surface and recognizing these new species are here," Kear said. "These species fill a time frame not represented in the world. There are older (fossils) from Europe and younger (fossils) in North America and not much in between. We have the missing pieces in the puzzle."

Colin McHenry, paleontologist at the University of Newcastle, said the discovery "was not a revolutionary find that overturns our ideas of what's gong on" but would help scientists better understand the period from 90 million to 140 million years ago.

"These animals represent another chapter in the story we are trying tell of how the marine ecosystems evolved during the age of the dinosaurs," McHenry said.

"A lot of the focus is on the land-based systems where dinosaurs lived," he said. "But what people don't realize is that there were a lot of interesting animals that were living in the sea."