Ancestrular morphology in cyclostome bryozoans and the quest for phylogenetically informative skeletal characters

Cyclostomes are an ancient order of bryozoans with more than 500 living species. Traditionally, cyclostome taxonomy has depended almost entirely on characters of the calcareous skeletons of mature colonies. However, the taxonomic distribution of these characters is largely incongruent with well-supported molecular phylogenetic trees, implying the need for major revisions if cyclostome taxonomy is to reflect phylogeny. While far more taxa must be sequenced to ascertain their molecular relationships, there is also a need to discover morphological characters that can be used to support molecular clades, not least to allow these clades to be inferred among fossil cyclostomes for which molecular data will never be available. Here we survey the skeletal morphology of the first-formed part of the skeleton – the protoecium of the founding zooid (ancestrula) – in a variety of Recent and well-preserved fossil cyclostomes, motivated by the recent discovery of a morphologically disparate molecular clade that can be distinguished by protoecial characters. The distribution of pseudopores on the protoecium is shown to vary between species, with eight patterns recognizable, offering a new source of skeletal characters in both living and fossil taxa. However, the patterns seem not to be fully congruent with the cyclostome molecular tree and more data and analyses are required before these patterns can be used to recognize cyclostome clades.