Functional morphology and taxonomy of branch dimorphism in the Paleozoic bryozoan genus Rhabdomeson

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Abstract
Silicified zoaria of the widespread cylindrical genus Rhadomenson were discovered that bear conical branches traditionally assignable to Coeloconus. Other, conical stems gave rise to cylindrical branches. That the dimorphic branches are not encrustations is shown by the absence of overgrowths and the regular alignment of apertures between parent stem and branch. A similar relationship is true of the type species of Coeloconus, which is synonymized with Rhabdomeson [R. rhombicus (C. rhombicus Ulrich 1889), R. granosus (C. granosus Ulrich 1889)]. Both conical and cylindrical branches were broken but survived to resume growth; this is shown by basal healing patterns and growth reversals. Colony fragmentation has been described in certain Cenozoic and modern bryozoans as a mode of increase, and is inferred to have been important in Rhabdomeson. The presence of fragmentation in taxonomically and stratigraphically widely separated bryozoans suggests that this mode of increase may be more important than realized.

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