753. FORMATION AND FUNCTION OF THE SIPHUNCLE-SEPTAL NECK STRUCTURES IN TWO MESOZOIC AMMONITES

Abstract

SEM observation on the siphuncle-septal neck structures in two Mesozoic ammonites, Eleganticeras elegantulum and Reesidites minimus is described and compared with modern chambered cephalopods. The state of development of the porous and nonporous structures within the chambers of the two ammonites is very similar to that of modern Nautilus ; and furthermore, a thin conchiolin membrane (pellicle) covers the greater portion of the siphuncle and the inner chamber surface as in Nautilus. Based on these observations and the knowledge on modern Nautilus biology a model is proposed for a new chamber formation in ammonites. This work also postulates that in ammonites the cameral liquid initially filling up the late-formed chambers was primarily drained into the siphuncular cord osmotically via a wettable pellicle and porous septal prismatic zone within the septal neck region.