Ontogeny and morphology of the retrochoanitic septal necks of Agoniatites vanuxemi from the Middle Devonian Cherry Valley Limestone

Abstract

Retrochoanitic septal necks represent the primitive condition in the Ammonoidea. In this study, we examine the morphology of the septal siphuncular complex of specimens of Agoniatites vanuxemi (Hall 1879) from the Cherry Valley Limestone of New York State, USA. Specimens were embedded in epoxy, ground to a median plane and etched in 5 % HCl. The phosphatized membranes allow us to study the formation of the septal necks. Septal–siphuncular membranes occur next to the septal neck. They are adapical of the septum at the deepest point of the adorally projected ventrolateral saddle. Similar membranes occur on the adapical side of the septum on the dorsum in the deepest part of the adorally projected septa. The septal neck morphology on both the dorsal and ventral sides is well preserved. The septal neck begins in the same position on its dorsal and ventral sides, but projects farther adorally on the ventral side. By the third whorl, the adoral projection has doubled in size and is one-fourth to one-third the length of the chamber. This may be related to the deepening of the ventrolateral saddle through ontogeny. Auxiliary deposits are also present on the inside of the septal necks. They occur inside the dorsal and ventral sides of the septal neck at the adapical end. On some septa they appear to extend adapically beyond the end of the septal neck. A model for the morphogenesis of retrochoanitic septal necks is proposed based on the morphology observed in these specimens and previously published models.