The original composition of the pro-ostracum of an early Sinemurian belemnite from Belgium deduced from mode of fossilization and ultrastructure

The pro-ostracum of the early Sinemurian belemnite Nannobelus from the Belgian Province of Luxembourg is preserved as a thin, irregularly mineralized (phosphatized and pyritized), finely laminated structure, which is situated dorsally between the calcified rostrum and phragmocone. It has a median field with a criss-cross pattern of bluntly pointed, curved growth lines and fine longitudinal ridges, as well as two lateral fields characterized by a fine ornament of closely spaced, longitudinal striae, each lateral field showing a narrow anterior belt-like portion, the width of which equals about one-third of that of the median field. Their posterior portion is remarkably asymmetrical, because its free margin (which does not about the median field) curves ventrally and the interspace between striae gradually increases here. The striation of the lateral field is formed by the longitudinally exposed narrow portions of succeeding, overlapping sublayers of the pro-ostracum. Additionally, an internal sublayer with a silicified, honeycomb-like structure is demonstrated in the pro-ostracum. Based on microlamination that is comparable to that of the chitinous gladius in extant squids and on the irregular mineralization (unlike the rest of the shell), the pro-ostracum is considered to have been originally mainly organic, containing an intermediate cartilaginous sublayer with a typical honeycomb-like structure. The cartilaginous sublayer supposedly provided protection of the pro-ostracum against fractures which might have resulted from regular contractions together with the muscular mantle during jet-propulsion. Ultrastructural and chemical data on Nannobelus favour the interpretation of the pro-ostracum as a novelty of the skeleton in coleoids rather than as a dorsal projection of the phragmocone wall.