Ammonoidea

Ammonoidea (ammonoids; phylum Mollusca, class Cephalopoda) Subclass of cephalopods which generally have planispirally coiled, septate shells (see SEPTUM). Characteristically the shells are tightly coiled and planispiral, although some are coiled loosely or spirally; the protoconch is globular; the shells may be either involute or evolute. Some forms have marked ventral keels; ribs and nodes may also be present. The siphuncle is variable but mainly ventral in position. Sutures are often very complex. Camaral deposits are absent. The Ammonoidea were probably tetrabranchiate (four-gilled) cephalopods. They constitute the largest cephalopod subclass, with 163 families including the ammonites, in which the suture lines form very complex patterns; the ceratites, in which part of the suture line is frilled; and the goniatites, with relatively simple suture lines. They range in age from Devonian to Upper Cretaceous. All members are now extinct. See also APERTURE; APTYCHUS; FORAMEN; PHRAGMOCONE; and VENTER.