Given a scientific question and its correct answer, generate supporting facts for the answer. This supporting fact can be an explanation for the answer to the given question.

Question: In addition to five classes of fish, what other classes make up the species of vertebrates? Answer: amphibians, reptiles, birds, and mammals
There are about 50,000 living species of vertebrates. They are placed in nine classes: five classes of fish plus amphibians, reptiles, birds, and mammals.