Article: Early growth stages in rhabdomesoid bryozoans from the Lower Carboniferous of Hook Head, Ireland

A number of specimens of young rhabdomesoid colonies were examined, each carrying a curious conical proximal termination. Sectioning showed that these represent the earliest growth stages, following fixation of the ancestrula. External walls of cones have a two-fold structure comprising outer, primary (recrystallized) and inner, secondary (laminar) layers. The arrangement corresponds with that in the basal walls of encrusting trepostomates. There are reasons for believing that during later development the conical structures were progressively obscured by calcification from an external mantle. Some of the young colonies grew around productid spines or other slender objects, and in such cases an internal 'basal wall' defines an axial tube. In others there is no such complication and zooecial tubes diverge from a simple skeletal axial rod. Differences in the origin and structure of the axial parts of rhabdomesoids and the mesotheca of ptilodictyoid cryptostomates suggest that these groups may not have been more closely related to one another than either was to certain trepostomatous stocks.