ULTRASTRUCTURE OF BELEMNOTHEUTIS FROM THE OXFORD CLAY (CALLOVIAN), ENGLAND, AS A KEY FOR ELUCIDATING THE ORIGIN OF THE PRO-OSTRACUM

Abstract

The postulate that the pro-ostracum of extinct coleoids is a dorsal extension of the phragmocone wall (conotheca), and is therefore a homologue of the shell wall of ectocochleate precursors (Naef, 1922; Jeletzky, 1966) is analysed in the light of the shell ultrastructure in Belemnotheutis antiquus Pearce. Three shells of B. antiquus from the Oxford Clay (Middle Jurassic, Callovian) of Christian Malford, Wiltshire, England, were examined with the SEM. For comparison, additionally five gladii of the living squids Loligo and Berryteuthis were studied with the SEM. It is shown that in B. antiquus (1) the apical part of the rostrum is composed of a dense, outer prismatic portion and a porous, inner prismatic portion; towards the aperture the rostrum decreases in thickness, losing first the porous inner portion and finally the outer dense portion; (2) the conotheca from the apical to the middle part consists on the dorso-lateral sides of the outer prismatic, nacreous and inner prismatic layers; towards the aperture it loses, first, the outer prismatic layer and then the nacreous layer; (3) the pro-ostracum differs remarkably from the conotheca - it exhibits irregular mineralization while the nacreous and prismatic layers of the conotheca and the prismatic rostrum are standard in their ultratructure; the pro-ostracum is a micro-laminated, possibly mainly organic, structure, made up of vertical, horizontally micro-laminated, columns; (4) fine micro-lamination of the pro-ostracum is similar to the micro-lamination of the chitinous gladius in modern squids. The ultrastructural data on Belemnotheutis favours the concept of the pro-ostracum as a separate, innovative part of the skeleton in coleoids (Doguzhaeva, 2002; Doguzhaeva et al., 2002a, 2003, 2005a, b; 2006 in press) rather than a dorsal projection of the conotheca.