Sea Monster, by James Gurney:
The giant mosasaur Tylosaurus grabbing a plesiosaur called Dolichorhynchops in a breach attack from below, similar to the way killer whales catch seals. A specimen of Tylosaurus was actually found with the skeleton of a “Dolly” inside.
He uploaded three posts about his process: 1, 2 and 3. Extremely recommended.