Complex sentence: The term covers two families of animals, the Old World porcupines and New World porcupines, which both belong to the Hystricognathi branch of the vast order Rodentia, both are covered in quills, but still are quite different and are not closely related.
Split into simpler sentences: The term covers two families of animals, the Old World porcupines and New World porcupines. Both families belong to the Hystricognathi branch of the vast order Rodentia, both display similar coats of quills, but they still are quite different and are not closely related.