Early Ontogeny of the Siphuncle and Shell in the Early Carboniferous Rayonnoceras (Actinocerida) from Arkansas, USA

Abstract

The apical end of Rayonnoceras shell forms a cup-shaped shell primordium with cicatrix. The morphology of the cicatrix is similar to that in recent Nautilus. Shell primordium is flattened ventrally where its margin forms a bilobed structure, which is absent in recent Nautilus and Carboniferous "Orthoceras". The first segment of the siphuncle is large, pear-shaped and in contact with the shell apex. The first endosiphuncular annular ring fills the entire first siphuncular segment, with the exception of the narrow central canal. The endosiphuncular rings consist of radial lamellae, each of which is composed of crystallites with a feather-like arrangement. Orthocerids, actinocerids, oncocerids and Nautilus have cicatrix. The cicatrix is not developed in bactrites, ammonoids, belemnites, Spirula, and probably also in sphaerortho-cerids. The cicatrix seems to indicate a special mode of embryonic shell secretion.