Ohalos 1:6

A person doesn’t convey impurity until he dies. Even if a person is cut up or in his death throes, he can still obligate and exempt from yibum (levirate marriage), and enable and disqualify from eating trumah. Similarly, domesticated and wild animals don’t transmit impurity until they die. If their heads were cut off, they transmit impurity even if they’re still convulsing, like a lizard’s tail, which convulses even though it’s no longer alive.

Ohalos 1:7

Whole limbs convey impurity regardless of their size – even smaller than an olive-sized piece from a corpse or an olive-sized piece of carrion, or a lentil-sized piece from a vermin can transmit their respective forms of impurity.