Meeting Abstract

100.1  Thursday, Jan. 7  Darwin’s Avicularia: How an Early Sense of Modularity Links Vestigiality, Functional Innovation and the Evolution of Polymorphism CARTER, M.C.; LIDGARD, S.*; GORDON, D.P.; GARDNER, J.P.A.; Victoria University of Wellington, New Zealand; Field Museum, Chicago ; National Institute of Water and Atmospheric Research, Wellington, New Zealand ; Victoria University of Wellington, New Zealand slidgard@fieldmuseum.org

Few studies indicate morphological vestigialization, co-option, and functional innovation of polymorphs in the same genetic individual, combined with other evidence of transitional stages of polymorph evolution among closely related living and fossil species. Here we provide that rare example, based on discoveries of vestigial and augmented homologous structures coincident with discrete functional innovations in a zooid polymorph, the bryozoan bird’s-head avicularium originally described by Darwin. Darwin’s studies on colonial animals shaped a remarkable but under-appreciated theoretical synthesis that anticipates modern concepts of biological modularity. This early sense of modularity is that these morphological entities develop and evolve as units that interact with other units and are integrated in an inclusive whole. Darwin later hypothesized that feeding zooids and nonfeeding polymorphic zooids (avicularia) were derived from an ancestral form most similar to the feeding zooid. Our research disclosed an array of muscular and exoskeletal homologies of feeding zooids and avicularia, more numerous and detailed than in any previous studies. Unlike the feeding zooid, the avicularium frequently captures small invertebrates, including predatory epibionts, which we recorded for the first time. Species in a living bryozoan genus and a Cretaceous one also show a skeletal transformation series between feeding zooid and avicularium. Our research thus connects disparate ideas about structural modularity, vestigialization and polymorphism, and confirms Darwin’s hypothesis.