Conch Ultrastructure and Septal Neck Ontogeny of the Belemnite Conobelus (Duvaliidae) from the Valanginian of the Crimea (Black Sea)

Abstract

Several exceptionally well-preserved phragmocones of the belemnite Conobelus, with approximately 20 chambers preserved, were collected from the Valanginian of the Crimean Peninsula (Black Sea). The shell structure was studied with SEM. The prismatic wall of the protoconch wedges out near the mural part of the first septum, a short distance after the closing membrane. The prismatic protoconch wall consists of sublayers. In the extremely thin shell wall at the apex, there is a single sublayer. Additional sublayers appear on the inner shell surface at some distance from the apex. The closing membrane, about as thick as the shell wall, is not continuous with wall of the protoconch and conotheca. The primordial rostrum is structurally different from the rostrum, and separated from it by an organic membrane. The first 20 chambers, at least, seem to lack the conotheca proper. The shell wall is here composed of long mural parts of the septa and/or of the prismatic layer of the primordial rostrum. The first septum is prismatic; others are formed by the nacreous layer Type 2 (Mutvei, 1970), which in sections gives an impression of a granular structure; the mural parts of the septa show tabular nacre. At early ontogenetic stages, starting from the sixth septal neck, the transformation from retro- to prochoanitic septal necks occurs. The replacement of retrochoanitic septal necks by prochoanitic (or transitional) necks in Conobelus is similar to that in am- monoids and in some other branches of cephalopods, and, consequently, represents an example of homeomorphy in cephalopod evolution. In addition, the present paper includes an ultrastructural comparison of early shell ontogeny in belemnites and bactritoids.