Australia was once home to ancient reptiles that swam in huge icy lakes, fossil evidence suggests.
Reconstruction of Umoonasaurus demoscyllus showing an adult with crest (top) and juvenile (bottom).
The large, carnivorous reptiles lived 115 million years ago, during the age of the dinosaurs, when much of the continent was covered in water.
Fossils of two new species of plesiosaur were discovered near Coober Pedy in South Australia.
Plesiosaurs, famed for their long necks, are said to resemble Scotland's mythical Loch Ness monster.
The Australian specimens are described in recent editions of the journals Biology Letters and Palaeontology.
One, known as Umoonasaurus demoscyllus, was about 2.4m (7.2ft) long and had crests on its head, perhaps for display or mating purposes.
"Imagine a compact body with four flippers, a reasonably long neck, small head and short tail, much like a reptilian seal," said the lead author of the two papers, Dr Benjamin Kear of the University of Adelaide.
The other species, Opallionectes andamookaensis, grew to about 5m (16ft) in length and had small needle-like teeth.
Some 30 fossils were discovered at an opal mine near the outback mining town of Coober Pedy.
They are made up of the mineral opal, which filled the spaces left by bones when the original fossil-bearing rock was dissolved away by acidic ground water.
A genus of extinct marine reptiles, having a long neck, a small head, a short tail, and four large paddling limbs
First appeared at the start of the Jurassic Period, and thrived until 65 million years ago
Lake or sea monster sightings are occasionally explained as plesiosaurs but scientific evidence points to them being extinct
The fossils include several skeletons and a complete skull of Umoonasaurus, and a partial skeleton of Opallionectes.
They are thought to be of juvenile animals, suggesting the lake was a breeding and nursery ground.
Scientists believe sea-dwelling adults returned to the shallow inland waters to breed and raise their young.
At the time, Australia was much colder, and the inland ocean would have frozen over in places during the winter.
Scientists believe the creatures might have evolved mechanisms to cope with the harsh climate, such as a faster metabolic rate. They were carnivorous, feeding on fish and squid.