Anatomy of living Nautilus: Reevaluation of primitiveness and comparison with Coleoidea

Abstract. The anatomy of Nautilus pompilius was reinvestigated on the basis of material from the Philippines to evaluate the “primitiveness ” of the living fossil cephalopod. All organ systems were dissected at the gross level, photographed, and described. The results of dissection were compared with the knowledge of other living cephalopods (Coleoidea) and noncephalopod conchiferan molluscs. As a result, 16 characters were identified as synapomorphies of the Recent cephalopods, and there are more possible characters in the buccal musculature and various types of cartilage. Further comparison revealed clear differences in 52 characters between Nautilus and the Coleoidea. These differences include Nautilus autapomorphies and/or coleoidean synapomorphies. By outgroup comparison within the Conchifera, especially with basal gastropods and scaphopods, the polarity and “primitiveness” were evaluated for each character. Sixteen characters were inferred to be obviously plesiomorphic in Nautilus, but others are apomorphic or ambiguous. The large number of arms and ctenidia in Nautilus are possibly derived characters rather than primitiveness. The two-paired renopericardial organs might also be apomorphic, but this hypothesis should be tested in further ontogenetic studies