MORPHOLOGY AND SYSTEMATICS OF THE MODERN NAUTILIDS (Invited Presentation)

Nautilids are an ectocochleate cephalopod group that has a long geological record. Their modern descendants Allonautilus and Nautilus were studied frequently by paleontologists in the 1980s to reveal the biology and ecology of extinct ectocochleate cephalopods such as ammonoids. Although, thanks to such studies, we have an overview of modern nautilids, a number of questions regarding their reproductive strategy, morphology, behavior and lifestyle still remain unanswered. It is assumed that detailed morphological analysis of the conchs allows us to answer some of those question. Among the various morphological characters, septal morphology is of great interest for paleontologists because of the potential importance for taxonomy, systematics, and phylogeny in both fossil and modern cephalopods. Here, we study the septal morphology in several species of modern nautilids (A. perforatus, A. scrobiculatus, N. belauensis, N. macromphalus, N. pompilius, N. pompilius suluensis, and N. repertus) to test the hypothesis that septa serve as a diagnostic character. To achieve this, we applied X-ray computed tomography to conchs. On the basis of the CT-scans, we produced image stacks of conchs, which were then used to reconstruct the septal surface (3D models) in each species through ontogeny. To quantitatively analyze the morphology of septa, we employed geometric morphometry using semi-landmarks placed along the surface. These landmarks were normalized by centroid size. We carried out principal components analysis to reduce the dimensions of the data. Results reveal that all species share a similar pattern of morphological changes through ontogeny. We also investigated how septal spacing (septal rotational angle) differs among the above-mentioned species. Lastly, conch geometry was investigated through ontogeny. We will discuss the ontogenetic change of septal morphology, septal spacing, and conch geometry, and the taxonomic/systematic implications of these different characters.