Figure 1.

Morphological evolution of externally-shelled cephalopods during the most intense phase of the "Devonian Nekton Revolution" in the Early and early Middle Devonian. Reconstructions of the loosely coiled ancestors of the two lineages under consideration and two representatives of each of the lineages leading to and comprising the Auguritidae and Pinacitidae. The reconstruction of the soft-body are largely speculative: 10 arms are based on the knowledge that both plesiomorphic coleoids have ten arms and the sister group of the Ammonoidea + Bactritoidea + Coleoidea, the Nautiloidea, is known to begin with ten arms buds in early embryonic development [201]. The orientation can be reconstructed from the shell morphology [58]. Presence of a hood and of a camera lucida-style eye as in Recent Nautilida is speculative [158]. The position of the eye and the hyponome are deduced from the position of the ocular sinus and hyponomic sinus, respectively. The reconstructions are not shown at the same scale.