Plesiosaurs were large marine reptiles with limbs shaped like flippers, very long necks and relatively small heads. The plesiosaurs that lived in the Eromanga Sea were mostly of an especially long-necked kind called elasmosaurs. Only two of the Australian plesiosaurs have official names (Crimoliasaurus maccoyi and Woolungosaurus glendowerensis) and the most impressive fossils are yet to be formally identified.

No animal that lives in the sea now has such a long neck, so we have nothing we can really compare it to for clues. Some plesiosaurs had necks longer than their bodies and their shape has often been described as a snake threaded through a turtle. Because of this odd shape, plesiosaurs were quite slow moving. But to catch fish they could move their head and neck rapidly like a snake striking.

They had curved teeth useful for catching fish, but not for chewing or crushing. It’s also likely that they caught and ate the squid-like creatures called belemnites that existed at the time. Some plesiosaur fossils have stones preserved in the stomach area, showing that they swallowed stones (called gastroliths) to grind up food in their stomachs. Rival males could have fought each other with their necks and both sexes might have courted each other with their heads out of the water.

Plesiosaur teeth have been found by Patricia and Thomas Rich in freshwater sediments, and they have interpreted these plesiosaurs to be freshwater. Cruickshank (1997) reports that several plesiosaurs, especially those from southern continents, have originated from non-marine sediments. And all modern secondary marine-adapted animals have freshwater as well as saltwater forms.

In Australia plesiosaur fossils are sometimes made of precious opal. One 4 metre specimen from the Andamooka opal fields, now on display in the South Australian Museum, is made up of 36 kilograms of opal, worth up to a million dollars. It was kept in a bank vault for some years until it was sold to the museum for a generous 25 thousand dollars by its discoverers, Molly and John Addyman.

An opalized fossil dubbed "Eric" was found near Coober Pedy in 1987 and was a Leptocleidine - a group of small-bodied pliosaurs two to three metres in length that lived in the Early Cretaceous of England (Leptocleidus superstes), South Africa (L. capensis), and Australia (Leptocleidus clemai).

They were most likely fish eaters and may have led a similar life to today's seals and sea lions. Leptocleidine pliosaurs seem to have prefered the shallow waters of lagoons or estuaries, and perhaps lived in freshwater lakes. Freshwater plesiosaurs (not-too-distant cousins of pliosaurs) are known from teeth in the southern Australian fossil sites.

Like turtles, they probably came ashore to lay eggs on sandy beaches. No nests have ever been found but if they laid eggs, they had to have laid them on land. There is no evidence at all that plesiosaurs gave birth to live young in the water.