Aulacocerida

Aulacocerida is an order of primitive coleoid cephalopods, possibly derived from michelinoceraitids (Orthocerida) early in the Devonian, which in turn gave rise to the Belemnites.

Aulococerids are characterized by a straight, slim, conical shell with a large living chamber, simple aperture. The protoconch (initial, embryonic chamber) is egg-shaped and separated from the phragmocone by a constriction. The siphuncle is narrow and ventral, constricted at the septa and expanded in the middle of the chamber; necks are retrochoantiic, project backwards toward the apex. Nacre recovered from aulococerids as well as from belemnites resembles that found in recent Sepia and Spirula but differs from that of recent Nautilus.