The Plesiosaur Fossil is a landmark in Endless Ocean. It is found embedded into the wall to the South of Whalebone Chasm in the Abyss with Frilled Sharks swarming around it. Located at coordinates F-6, it is near one of the entrances to Secret Cave.
When you focus on it using the A button, Katherine will chime in on the radio, saying:
“Hey! This is a fossil! I think it’s a type of Plesiosauria. This thing is in really great condition. It’s amazing that the entire body has been preserved.”
This fossil is no longer a fossil when it makes its appearance in Endless Ocean: Blue World. Instead, it appears as the ultra-legendary Sea Serpent, which in reality is most likely a member of the Plesiosauria family.