Ocean Zones, Dolphin Hands and Legs, and Dolphin-Whales

I have six scientific Magic Treehouse Research Guides. I was reading Dolphins and Sharks and I learned three things:

1. There were three zones in the ocean - the sunlit zone, the twilight zone, and the midnight zone. They are measured by how much light there is in the water.
2. I learned that dolphins - and this is really crazy - Dolphins used to have hands and legs and live on land! Zoologists studied dolphins and other ocean creatures. They x-rayed the dolphins' flippers and they saw bones that looked like hands. And when they x-rayed their tails they saw bones that looked kind of like legs. That's how they learned that dolphins used to live on land.
3. Dolphins are a small type of whale with teeth.

Since I learned that, it made me feel like a zoologist!