Pore-canal network (“wrinkles”) in ammonoid shell wall (Cephalopoda)

Abstract

ABSTRACT The shell wall in ammonoids contains a pore-canal network similar to that detected recently in the shell wall of the Ordovician nautiloid Orthoceras. In the Jurassic monomorphic ammonoids Quenstedtoceras, Cosmoceras, Lissoceras and the Cretaceous heteromorphic ammonoid Ptychoceras investigated in this study vertical canals arranged in parallel rows, are partially preserved, although no horizontal canals are preserved as in Orthoceras. In Quenstedtoceras and Ptychoceras the vertical canals extend not only through the inner prismatic layer, as in Orthoceras, but also through the nacreous layer. Therefore both layers were secreted on the body-shell attachment band behind the apertural margin. In the juvenile shells, only the very thin outer prismatic layer formed the apertural margin. As in Orthoceras, the canals had walls of calcified organic fibres that were partially or entirely dissolved during diagenesis. The pore-canal network in the shell wall of Quenstedtoceras and Ptychoceras contributed to achieving neutral buoyancy. As indicated by the high numbers of mini-pores in the connecting rings, the siphuncle was employed for adjusting buoyancy during vertical migrations. The “wrinkles”, “furrows”, “pits”, “dots”, “ridges”, “Ritzsteifen” and “Runselschicht” in the “wrinkle layer”, previously described in the ammonoid shell walls, are remnants of the pore-canal network.