Alligators have remained unchanged for millions of years and their ancestors date back to the time of dinosaurs. With their long tails, scales and giant jaws with sharp teeth, these predators are often a symbol of fear in the swamp. Because of their bad reputation, alligators were once hunted to the point of extinction. Now, alligators are recognized as important engineers who help maintain balance in the wetland ecosystem of Big Cypress National Preserve in Florida. Photo by National Park Service.