@sarahtaber whales (including orcas) are, though, descended from the ancestor of modern cattle, so at some time in the past, (proto-)cattle not only did swim, but kept on swimming, and thus became pelagic.

Cannibalism, anyone?

https://baleinesendirect.org/en/discover/life-of-whales/morphology/les-ancetres-des-baleines/#:~:text=Their%20ancestor%20is%20most%20likely,the%20giraffe%20and%20the%20hippopotamus.