Ammonitida

The Ammonitida comprise the advanced Mesozoic ammonoidea found in the Phylloceratina, Lytoceratina, and Ammonitina. The Ammonitina are the most diverse containing at least nine superfamilies. The Phylloceratina and Lytoceratina are more conservative with the Phylloceratina including a single superfamily and the Lycoceratina maybe four.

Siphuncles are prochoanitic, except in the very earliest growth stage, characterized by septal necks that point forward, toward the living chamber, rather than toward the rear characteristic of their nautiloid ancestors. In most the siphuncle runs along the outer rim, or ventral margin. Sutures are generally complex with secondary and adventitious elements.