Question

How has the skeletal system evolved from fish to reptiles to birds to mammals?

Answer 1

Vertebrate skeletal system is having segmental origin.

Explanation:

Almost all vertebrates from fish have segmented myotomes. Myotomes arise from embryonic segments.
During evolution of vertebrates from fish to mammals muscles are modified as per need.
Fish muscles are developed for undulating swimming locomotion.
Frog developed limbs but segmental development of muscles remains same.
Same thing is true for reptiles, birds and mammal.