STUDYING THE MACROEVOLUTION OF POLYMORPHISM IN THE BRYOZOAN STEGINOPORELLA

Morphological differentiation is very common in colonial organisms, but the pathways by which differentiation evolves are not always clear. Colonial Cheilostome bryozoans have different body types (termed polymorphs) and are often calcified, providing an opportunity to directly observe the evolution of morphological differentiation in the fossil record. The genus Steginoporella is a presumptive model for the evolution of polymorphism because many of its living species have two similar polymorph types, termed A-zooids and B-zooids. Here we present the first analysis of A- and B-zooid macroevolution in Steginoporella. We quantify the divergence, disparity, and allometry of zooid types among species and across time. Contrary to expectations, we find no evidence that B-zooids are diverging systematically from A-zooids. The modern set of species are particularly misleading because the most divergent species with B-zooids do not survive past the Miocene. Moreover, A-zooids and B-zooids appear to possess highly conserved allometric parameters, indicating that the two zooid types are developmentally coupled. Steginoporella may have been considered a model system for the study of the origin of polymorphism, but its A- and B-zooids appear to be losing, and not gaining, morphological differentiation as the genus evolves. Therefore, to understand polymorphism, it will be important to consider examples of novel origins, such as in the Cretaceous genus Wilbertopora.