The ammonitella: a model of formation with the aid of the embryonic shell of archaeogastropods

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Abstract
Recent archaeogastropoda secrete their first conch (primary shell) without producing increments of growth. This conch remains attached to the epithelium responsible for its production until completed. Only afterwards does tissue detach from the edge of the shell and the conch begins to function as a protective exoskeletion. After a potential swimming phase by the veliger larva, the organic primary shell is deformed by mechanical means through muscular tension from the inside and the outside of the conch. It then becomes mineralized by argonitic crystallites and, thus, functional for the use of a benthic animal. The embryonic conch of ammonites (ammonitella) is devoid of increments of growth. The inner lip (dorsal side) of the aperture became flattened after the evenly rounded primary conch had been secreted. The primary organic shell was mineralized by aragonitic crystallites from within. All these features of the formation of the ammonitella can be interpreted in the light of early shell formation of recent archaeogastropods.

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