NAUTILOIDS WITH MULTIPLE PAIRED MUSCLE SCARS FROM LOWER–MIDDLE ORDOVICIAN OF BALTOSCANDIA

Abstract: The number, outline and arrangement of muscle scars on the internal mould of the body chamber of five species of Early and Middle Ordovician nautiloids from Baltoscandia are described. The ellesmeroceratids Oelandoc-eras haelluddenense Foerste and Pictetoceras oliviae King have 9–10 pairs of muscle scars. The oncoceratid Phtanoncoceras ellipticum (Lossen), and Botellusoceras torpense gen. et sp. nov., order and family indeterminate, have eight and nine pairs of muscle scars, respectively. The tarphycerid Estonioceras imperfectum (Quenstedt) has four pairs of muscle scars. Key words: cephalopods, muscle system, nautiloid phylo-geny. All shelled cephalopods must have had some way of anchoring their body to the shell wall. Recent Nautilus is attached by two strong, head-retractor muscles and two sheet-like tendons (aponeuroses). These muscles form two characteristic imprints on the inner surface and a continu-ous band around the posterior end of the shell of the body chamber (see e.g. Wells 1988; Mutvei et al. 1993; Isaji et al. 2002). Imprints of such muscles are preserved in numer-ous nautiloids (Dzik and Korn 1992). The shape and num-ber of the complex of muscle scars differs strongly between taxa in both lower and higher taxonomic ranks. Accord-ingly, muscle scars bear at least some potential for infor-mation about the soft anatomy of extinct forms and can be used for systematic grouping of the taxa. This paper contributes new information on the value of muscle scar shape for understanding nautiloid relationships