Phylactolaemates represent a small clade within bryozoans that exclusively occur in limnic habitats. About 100 species are described in seven to eight families. Since they are sister taxon to the extensive clade of myolaemate bryozoans, understanding their evolution is crucial for all bryozoans. In recent years, advances in imaging and phylogenetics enabled detailed analysis of the morphology of phylactolaemate families and their internal relationship. Also, historical analyses provide profound data of phylactolaemate lifestyle and development and represent the base of modern phylactolaemate research. Previous analysis primarily focused on sole families and rarely included a broader range of phylactolaemates. This study provides the first comprehensive and comparative analysis of the morphology of this group. It combines historical data as well as recent ones and complements new data wherever possible, to fill in gaps in our knowledge. The combined data were used to establish a morphological character matrix which is mapped onto a recent molecular phylogeny. The ancestral state reconstruction allowed to trace the evolution of numerous traits. The results reveal colonies with large interzooidal distances and non-encrusting cystids ancestral. Moreover, it shows that the myoanatomy is largely conserved but several lineages show unique adaptations in detail, especially in the complex lophophoral base. The neuroanatomy is remarkedly conserved across all taxa. Statoblast types and traits concerning the ectocyst, capability of locomotion and some myoanatomical adaptations are likely to have independent evolutionary origins. Overall, the morphological evolution of phylactolaemates reflects both, deep homology and frequent lineage-specific adaptations. Consequently, it is emphasized to integrate ecological and functional perspectives in future analysis.
Bibermair et al. (Mon,) studied this question.