Article: Siphuncular structures in the Devonian nautiloid Archiacoceras from the Eifel of West Germany

Archiacoceras is shown to have the most complex connecting ring among chambered Cephalopoda. The siphuncle of A. subventricosum consists of four-layered connecting rings, radiating longitudinal lamellae, and central plates. The siphuncle of an undescribed species consists of a connecting ring composed of six layers and of actinosiphonate lamellae without central plates. All ontogenetic stages of the lamellae, from budding to maturity, are observed. Based on similarities with extant chambered cephalopods, lamellae and connecting ring layers are interpreted as having functioned as structures to allow cameral fluids to be removed and, thus, to have formed part of the buoyancy regulating mechanism. The genus is re-established as endogastric. Observations of new features support the retention of Archiacoceras in the Oncocerida.