What could explain the external form of whales and fish being similar but skeleton to that a mammal?

The external similarity between whales and fish is a classic example of convergent evolution, where unrelated species evolve similar traits due to similar environmental pressures.

Whales are mammals that have adapted to a fully aquatic lifestyle, while fish are vertebrates that have evolved to live in the water. Despite their vastly different evolutionary histories, both groups have evolved streamlined bodies, fins, and other adaptations that allow them to move efficiently through water.

The common external form of whales and fish is the result of natural selection acting on both groups to best suit their aquatic environment. The streamlined shape reduces drag and allows for fast swimming, fins provide maneuverability and stability, and a protective layer of skin or scales helps to reduce friction and prevent water loss.

However, the internal anatomy of whales and fish reveals their distinct evolutionary lineages. Whales have mammalian characteristics, such as warm-bloodedness, the ability to give birth to live young, and lungs for breathing air. Fish, on the other hand, are cold-blooded, lay eggs, and have gills for extracting oxygen from water.

So while whales and fish may look similar from the outside, their internal structures, evolutionary history, and physiology are profoundly different, reflecting their unique adaptations to their respective aquatic environments.