How is it decided that dolphins give birth to calves, and sharks give birth to pups?

How is it decided that some animals (cattle, bison, elephants, dolphins, etc.) give births to calves, others (sharks, rays, seals, rodents, etc.) to pups, and etc.? How is it decided which species holds the title? This has been bothering me for a long time, and I just can't seem to find the answer.