Sharks are neither mammals nor amphibians; they are classified as fish, specifically belonging to a group known as elasmobranchs. This group also includes rays and skates. Unlike mammals, sharks do not have lungs and do not breathe air. Instead, they extract oxygen from water through their gills.
Sharks have certain characteristics that differ significantly from mammals and amphibians. For example, sharks are cold-blooded, which means their body temperature is regulated by the surrounding environment, unlike mammals that are warm-blooded and maintain a constant body temperature. Additionally, sharks lay eggs or give live birth, but they do not undergo any of the complex life stages that amphibians do, such as transitioning from a tadpole to an adult frog.
In summary, while sharks share some similarities with mammals and amphibians, they are fundamentally different as they belong to the fish category.