Ontogeny of Stringocephalus gubiensis and the origin of Stringocephalus

A growth series of Stringocephalus gubiensis, described here, reveals that the cardinal process of Stringocephalus originated from a bilobate structur e, and the growth of its dorsal median septum is highly related to cardinal process development. From this observation we conclude that Kaplex is, str ucturally and temporally, a suitable ancestor for Stringocephalus, with Bor nhardtina ancestral to Kaplex. Subfamilial classification of the family Str ingocephalidae is revised based on the presence or absence of median septa and the nature of hinge plates as well as on cardinal process formation. Th e Kaplexinae and Omoloninae are proposed as new subfamilies, with the Kaple xinae, including Kaplex and Erectocephalus, and the Omoloninae, including O molania, Kumbella, and Hemistringocephalus. Morphologic study of South Chin a and Western Canadian Stringocephalus reveals that late forms of the genus tend to have a higher dorsal septum and more complex cardinal process than the early forms. In view of functional morphology, these trends reflect fu nctional perfection of the cardinal process build-up and the lever system f or improving the efficiency of opening and closing the shell.