Comparative morphological study of the gnathochilarium of millipedes of the suborder Cambalidea (Juliformia: Spirostreptida) assessed by discrete and morphometric character approaches

Abstract

Abstract Phylogenetic relationships of the suborder Cambalidea have been a well-known focus of discussion over the past 100 years. The most recent approach to the suborder is based on the morphology of the sexual structure of males and of the gnathochilarium. The phylogenetic interpretation of the gnathochilarium has been especially criticized due to the poorly understood evolution of its components since no hypothesis of homology has been put forward. This work provides a comparative study of characters of the gnathochilarium. Twenty-one characters are proposed and optimized for 34 terminals from the orders Julida and Spirostreptida. Two datasets were analysed on the basis of whether the sclerite duplomentum corresponds to a mentum fused to a promentum or only to an elongated mentum (promentum being absent). Concatenated analyses were performed with both discrete and morphometric characters to verify the variation in shape of the mentum. In all topologies obtained, Cambalidea were recovered as paraphyletic in terms of the suborder Spirostreptidea. Through constrained searches, Cambalidae, Cambalopsidae and Iulomorphidae were not recovered, while Pseudonannolenidae were recovered only when Physiostreptinae were excluded. The morphological evidence indicates the convergence of the gnathochilarium in Spirostreptidea and in some members of Cambalidea, with independent reversions of the promentum.