Phylogeny of the Bakuellid‐Related Ciliate (Alveolata, Ciliophora) Genera, With Establishment of Three New Families and Note on Morphogenesis of Holostichides heterotypicus

The order Urostylida Jankowski, 1979 remains taxonomically challenging due to incomplete morphological and morphogenetic data for many species. To clarify the relationships within genera such as Holostichides Foissner, 1987 and Paragastrostyla Hemberger, 1985, we investigated the morphology of Holostichides heterotypicus and two Paragastrostyla species ( P. lanceolata and P. terricola ) and redefined relevant genera and species. In addition, we supplemented the morphogenetic traits of H . heterotypicus . We provided the first multiple‐gene sequence data for the genus Paragastrostyla and revealed, through multi‐ribosomal and mitochondrial COI gene trees, that it occupies a distinct phylogenetic position together with the genus Holostichides . Based on molecular phylogenetic analyses and consistent morphological characteristics shared among Paragastrostyla , Holostichides and Eschaneustyla , we propose a new family, Paragastrostylidae fam. nov., to accommodate these three genera. Furthermore, we evaluated the taxonomic status of the Bakuellidae and found that so‐called ‘bakuellids’ can be divided into several independent and robust clades based on ribosomal gene trees. Based on integrated morphological and molecular data, we propose establishing two additional new families, namely Australothridae fam. nov. (including Australothrix and Anteholostichides ) and Metaurostylopsidae fam. nov. (including Metaurostylopsis and Monourostylopsis ). Additionally, the COI gene is recognised as an unreliable marker for distinguishing higher taxa of ciliates.