Colonial division of labor – Wilbertopora and the evolution of avicularia

Polymorphism, or variation in body type, is a common feature of Cheilostome bryozoans, occurring in several independent evolutionary lineages. Bryozoan polymorphism is often compared to the caste system in ants, where heterochrony allows for different castes in a colony to occupy different morphological spaces. However, ontogeny in bryozoans operates differently than in ants. Individual zooids bud from their mothers, sometimes in several generations at once, and their exoskeletons are constructed separately. In this type of development, there is no ontogenetic progression from juvenile to adult morphology. Therefore, it is not viable to invoke heterochrony as an evolutionary determinant of polymorphism without empirical study. Here we show that the morphological differences between polymorphs and standard feeding zooids in several closely related species of the Cretaceous bryozoan Wilbertopora are perhaps underlain by an entirely different evolutionary mechanism that promoted the differentiation of zooid body type. We propose that the evolution of similarity patterns between zooids in colonies allowed for polymorphism to evolve.