DIVISION OF LABOR THROUGH LOSS OF FUNCTION IN A CHEILOSTOME BRYOZOAN

Morphological polymorphism is strongly associated with division of labor, as divergent morphologies in groups or multicellular individuals often correspond to specialized functions. Division of labor presumably evolves as phenotypic variation in novel polymorphs is channeled into distinct morphologies, corresponding to particular functions. In the cheilostome bryozoans, the connection between division of labor and polymorphism is unclear because the functionality of certain polymorphs in colonies remains ambiguous. Here we analyze the evolutionary morphology of one such polymorph type, termed avicularia, after it first appears in the fossil record of the middle Cretaceous genus Wilbertopora. In this genus, avicularia first appear as slightly modified feeding individuals, termed autozooids, in colonies. We find that avicularia experience two pulses of high disparity within colonies shortly after their appearance in the fossil record. This high disparity is associated with loss of function —indicated by the loss of associated ovicells showing a reduction of lophophores and a loss of sexual functions and the ability to feed. After this loss of function, avicularia become become progressively divergent from autozooids within colonies, expanding into novel regions of morphospace.