The Australian specimens are described in recent editions of the journals Biology Letters and Palaeontology with one type of plesiosaur known as Umoonasaurus demoscyllus. It was about 2.4m (7.2ft) long and had crests on its head, perhaps for display or mating purposes.
The other species, Opallionectes andamookaensis, grew to about 5m (16ft) long and had small needle-like teeth.
"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 fossils, which were found at an Opal mine, included several skeletons and a complete skull of Umoonasaurus, and a partial skeleton of Opallionectes and they are thought to be of young reptiles, suggesting that the lake was a breeding and nursery ground.
Scientists believe adults that lived in the sea returned to the shallow inland lakes to reproduce and raise their young.
Australia was much colder when these plesiosaurs were alive, and the inland ocean would have frozen over in places during the winter. It is believed that the creatures might have evolved mechanisms to cope with the extreme climate, such as a faster metabolic rate.
