When the characters crosed the underground ocean, they saw a large herd of tanystropheus.
Tanystropheus was a large lizard with an incredibly long neck. They are shown as purpleish-brown.
When the characters were surrounded by bloodthirsty xiphonactis swarms, they used boards as bats to swat the fish away. However, they were outnumbered by the fish. Suddenly, a huge herd of tanystropheus appeared from the depths and devoured the xiphonactis, thus saving the characters.
trevor: they don't want us, they want the fish
The Tanystropheus have flippers like the Elasmosaurus, so it is possible that they creators made an error that Tanystropheus was suppose to be Elasmosaurus.