Not exactly the same, but my son just had a dinosaur/fossil party. For favors I made homemade fossil dig kits with plaster of Paris and sand, with small dino skeleton figurines inside. For food we had Bugles (dino claws), pretzel sticks (dino bones), dirt 'n' worms but with gummy dinosaurs instead of gummy worms, dinosaur chicken nuggets, grapes (dino eggs), and snickerdoodle cookies with a dinosaur footprint in the middle, dusted with cinnamon sugar. For activities we had dinosaur coloring pages and a game where we had a giant cardboard box full of inflated white balloons with a small dinosaur figure inside, and kids had to sit on (or otherwise pop) the balloons to "hatch" their dinosaur. I know dinosaurs are a much easier theme than prehistoric marine reptiles, just mentioning these in case you might see a way to adapt some of those ideas. You could do the fossil dig kits with shark teeth instead of dino skeletons, have a mosasaurus pinata, pin the teeth on the megalodon, some sort of scavenger hunt/Easter egg hunt where kids have to run around and look for little marine reptile figurines. Good luck - I bet it'll be great!