1. Mammary glands: Mammals are characterized by the presence of mammary glands, which produce milk to nourish their young. Sharks, on the other hand, do not have mammary glands and instead lay eggs.
2. Hair or fur: Mammals have hair or fur that provides insulation and protection. Sharks, being fish, do not possess hair or fur.
3. Three middle ear bones: Mammals possess three middle ear bones (malleus, incus, and stapes) that amplify sound waves and transmit them to the inner ear. Sharks, on the other hand, have a different ear structure and lack these three middle ear bones.
4. Warm-bloodedness: Mammals are warm-blooded, meaning they can maintain a constant body temperature regardless of their surroundings. Sharks, on the other hand, are cold-blooded, meaning their body temperature changes with the temperature of their environment.
5. Diaphragm: Mammals have a diaphragm, a sheet of muscle that separates the chest cavity from the abdominal cavity and helps in breathing. Sharks do not have a diaphragm and instead rely on a different mechanism for respiration.
6. Live birth: Most mammals give birth to live young, with the exception of monotremes (platypus and echidna) which lay eggs. Sharks, on the other hand, lay eggs (except for a few species that give birth to live young).
7. Complex brain and social behavior: Mammals generally have complex brains and exhibit various forms of social behavior. Sharks, while having well-developed brains, lack the same level of complexity and social interaction seen in mammals.
These key differences distinguish sharks from mammals and place them in the class Chondrichthyes, which includes cartilaginous fish.