Cretaceous Sea Serpent

A Mosasaurus hoffmannii descends into the ocean again after surfacing to breathe. Two Nyctosaurus fly past.
Mosasaurus hoffmannii is probably the largest mosasaur, with lengths of up to 18m. It was more robust built than it's relatives like Tylosaurus.
Nyctosaurus, a nyctosaurid pterosaur with a wingspan of 2m, is mostly known for it's enormous antler-like crest.