Colour patterns in nautilids: a key to phylogeny

ABSTRACT Recent molecular studies of the extant nautilid genera Nautilus and Allonautilus have revealed new information about the relationships among species. However, identifying relevant morphological descriptors for phylogenetic reconstruction continues to be a challenge for the species in these two genera. This study investigates the colour patterns of modern nautilids and defines new characters that can be used in phylogenetic analysis. The relevance of these characters (i.e. the likelihood that they reflect a phylogenetic signal) was assessed by mapping them onto the most recent molecular phylogeny. Ancestral character reconstructions and the retention index were employed to evaluate the strength of the phylogenetic signal and explore the evolution of the characters. Our results reveal that variation in the pattern of colour stripes, including their width, sharpness, number and distribution, as well as the presence of longitudinal bands, branching, intercalation and discontinuities in the stripes, are useful in discriminating clades. Thus, this study provides the basis for a more comprehensive phylogeny of both recent and, possibly, fossil nautilids based on morphological characters.