Erecting a new family for Spirostyliferina, a truncatelloidean microgastropod, and further insights into truncatelloidean phylogeny

Microgastropods in the subclass Caenogastropoda are diverse and species-rich and often exhibit similar morphologies across families, posing difficulties for species identification. Spirostyliferina Bandel, 2006 is a microgastropod genus that has been placed in the family Litiopidae associated with algae and seagrass across the tropical Indo-Pacific and with only one known member to date (Spirostyliferina lizardensis Bandel, 2006). There are uncertainties surrounding the familial assignment of Spirostyliferina however, due to its unique shell with spiral zigzag ridges, unique pitted protoconch and a lack of molecular data for phylogenetic analysis. This study aims to resolve the position of Spirostyliferina within the Caenogastropoda by employing a multilocus data set for phylogenetic reconstruction, and scanning electron microscopy and micro-computed tomography for morphological analysis. Maximum likelihood and Bayesian phylogenies of 56 species, 30 families and six superfamilies of Caenogastropoda recovered Spirostyliferina within the Truncatelloidea. Spirostyliferina had no affinity with any existing truncatelloidean families, and therefore, the family Spirostyliferinidae fam. nov. is proposed for this genus. The genus Hoenselaaria Moolenbeek, 2009 was synonymized with Spirostyliferina in this study (monotypic Hoenselaaria wareni Moolenbeek, 2009). Spirostyliferinidae is diagnosed by heavy pitted microsculpture on protoconch I and intermittent spiral zigzag ridges on the teleoconch. The unpegged corneous operculum and taenioglossate radula of Spirostyliferina appear similar to other truncatelloideans, confirming the position of Spirostyliferina as a monotypic clade within the Truncatelloidea. In all, this study advances the taxonomic status of a microgastropod with a minute size and rarity that presents significant challenges to its collection and study.