Step 15: Use the U-shaped lines on the left side as guides to draw the dinosaur's front flippers. Follow the path of the guides as you darken the lines but make the bottom part wavier. Add a few curved lines above the base of the flippers for the folds of skin. Darken the curved line on the right to create the mosasaurus' other front flipper.
Step 16: Use the U-shaped line on the right side as guides to draw the mosasaurus' hind flippers. Follow the path of the guide but make the top part thinner and add some curved lines at the base for the skin. Darken the last U-shaped line to create the other hind flipper.
Step 17: Draw a series of triangle- like shapes across the top part of the body to create the jagged spikes. Don't make the mosasaurus' spikes too tall. Keep them pretty close to the body. The spikes don't all have to look exactly the same. It's okay to have some size variation. Stop drawing the spikes at about the base of the tail. We'll continue drawing them in the next step. Darken the bottom guide to create the underside of the dinosaur's body.
Step 18: Continue drawing the triangle-like spikes across the top part of the body toward the mosasaurus' tail. When you get to about the middle part, cut across the tail with the spikes instead of following the top path of the guide. Continue adding the spikes on the right edge of the guide, and then cut across the tail to the left edge again. When you get to the left edge of the tail again, continue adding the spikes there. Drawing the spikes this way helps emphasize the flatness of the dinosaur's tail. Make the spikes near the end a bit bigger.
Step 1: Darken the remaining curved guide lines to the right of the body to finish the rest of the mosasaurus' tail.