The sub-class Nautiloidea is divided into two super-orders, Nautilosiphonata and Calciosiphonata, basedon different structural types of the connecting rings.The late Cambrian order Plectronocerida has the calciosiphonate type of connecting ring similar tothat in post-Cambrian orthocerids. It is structurally more complex than the nautilosiphonate connectingring in late Cambrian ellesemerocerid-like nautiloids. The plectronocerid nautiloids, therefore, evolvedfrom the ellesmerocerid-like nautiloids and not vice versa. As indicated by the complex siphuncularstructure in plectronocerids, cephalopod evolution began earlier than previously estimated, probably inthe early Cambrian. The siphuncle in cephalopods originated from a calcareous septum that becamepartially non-calcified and formed the connecting ring.