system
Given the following post, respond as if you're commenting on Reddit:
human
Why did whales develop horizontal tails and fish develop vertical tails?

As the title states. Was it because of feeding habits, or because of predator/prey or just ancestry? Anyone able to shed some light on this for me?
gpt
Exactly. Whales and dolphins are descended from herbivorous land mammals, probably hippo-like. Land mammals, like us, have differently shaped spines - not like fish spines at all. Because of the way our spines work, it is more efficient for them to move vertically than horizontally. Get in a pool and try it both ways. 

Whale tails and fish tails are different because of convergent evolution. Like bird wings and bat wings. Not due to ancestry. That's why sharks (who share common ancestors with fish) have vertical tails, like they would.