On the other hand, does it make sense to swear never to kill dinosaurs from now on? You may think that it is meaningless, since there is no dinosaur to be killed even if you want to. But it does make sense still. Dinosaurs once existed, which means we surely had the opportunity to kill some in those lifetimes. As explained above, when such vow is made, one’s karma of ever having killed dinosaurs will be purified, but not those that involved killing of other beings. If we vow not to kill any sentient beings, our karma of killing all kinds of beings can be purified. If the vow is made for the sake of particular beings, our karma of killing those particular beings can be purified.

- Quote from The Right View, "Liberating Living Beings"