Estonioceratidae

Estonioceratidae comprises a family of loosely coiled tarphycerids from the Early Ordovician with smooth or ribbed shells that diverge at maturity. Siphuncles are ventral, septal necks straight, connecting rings thick and layered, as in ancestral bassleroderatids. Lack a dorsal impression, produced in more closely coiled forms when subsequent whorls overlap the ventral side of the previous. Give rise to the Tarphyceratidae and possibly to some trocholids.

Genera include: Estonioceras Aethoceras Alakoceras Aphetoceras Clytoceras Eichwaldoceras Pycnoceras Shumardoceras Tragoceras