Siphuncular structure in the Jurassic belemnitid Megateuthis (Cephalopoda: Coleoidea)

摘要

Each septum of the phragmocone in the Jurassic belemnitid Magateuthis is composed of a thick, lamello-fibrillar, nacreous layer and a thin, chitinous layer, here termed the additional chitinous layer, which covers the aboral surface of the septum. The nacreous and additional chitinous layers continue from the septum into the septal neck. The inner surface of the septal neck is covered by a thick, tubular, conchiolin layer that originates on the septal surface around the siphuncle. The connecting ring is composed of three conchiolin layers with high elasticity: the additional chitinous layer that fuses with the tubular chitinous layer and continues into the connecting ring, and an outer chitinous layer that is a non-calcified continuation of the nacreous layer of the septal neck and is often destroyed by diagenesis. As seen in higher magnification, the tubular layer consists of tightly packed tubes with chitinous walls that are oriented at right angles to the long axis of the siphuncle. At about half of its length, each tube divides into three branches that are oriented towards the siphuncular cavity. The siphuncular cavity contains diaphragms composed of flat calcareous sheets. The siphuncular structure in belemnitid Megateuthis is similar to that in aulacocerid Mojsisovicsteuthis? In extant Spirula, the inner layer of the septal neck is composed of calcareous pillars embedded in a chitinous substance, whereas, in Megateuthis, it consists of chitinous tubes. The chitinous tubular layer in Megateuthis continues into the connecting ring.