The spiral cords of the Muricidae (Gastropoda, Neogastropoda): importance of ontogenetic and topological correspondences for delineating structural homologies

The Muricidae includes about 2500 Cenozoic and Recent species. One characteristic of this radiation is its large sculptural diversification. Therefore, the shell features were used for the supraspecific classification, until anatomical studies suggested that they represent a morphological pitfall, because of their propensity to be homoplastic. However, an overlooked problem in this reconsideration is the accuracy of the shell descriptions, which generally lack ontogenetic data and precise homologies. This paper is focused on the definition of structural homologies through the spiral cords. An analytical approach emphasizes the necessity of three preliminary tests before plausibly defining them. Three main sequences of appearance of cords differing in their mode of insertion on the shell are recognized using the ontogenetic correspondence. This powerful test demonstrates that cords may evolve independently and ontogenetic changes represent a pitfall when attempting to homologize them by using the adult morphology. The topological correspondence allows identifying cords within the two major sequences and is completed by a conjunction test in order to compare all members of the family. The related characters are divided into a cord group and a groove group. Their analysis shows that differences in the expression of cord, cord spines or nodules may exist in spite of a same topological position. It also confirms that labral spines of a same type are not always homologous. Finally, the proposed method suggests that much progress is expected in order to fully understand the morphogenesis of sculptural changes characterizing this gastropod radiation.