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Of the vertebrates there are only 2 different body plans that make up the entire group. Fish and Tetrapods. All extant species of fish are vertebrates and as diverse as they are exhibit basically the same basic body plan between all variations of fish. Tetrapods are the same way, everything that has four limbs is a tetrapod and that includes all reptiles, amphibians, aves, and mammals. While there may be variations on the Tetrapod body plan such as adapting limbs into flippers or wings everything that has four limbs still is and always will be a tetrapod... even if evolution has reduced the number of limbs. One thing that evolution never allows is that an animal can never grow out of it's ancestry. Ever hear that saying "You can't choose your parents?", exactly like that. So even though whales adapted fins into limbs and then into flippers such that they are now completely confined to the ocean, and can't come back... they are still mammals, and still tetrapods and so on.