Reconstructing the evolution of umbonal sculptures in the Unionida

Abstract

‘Umbonal sculptures’ of freshwater mussels (Unionida), which ornament the early ontogenetic shell, have long been used for species identification. Specificity of these sculptures to higher taxonomic levels and their value for phylogenetic reconstruction are still under considerable scientific debate. In particular, the distribution of beak sculpture morphotypes across the unionoid phylogeny and, consequently, evolution of this character remain poorly understood. Based on an examination of 187 taxa, covering five of the six extant unionoid families, this study presents a new model of character evolution of umbonal sculptures in the order. Ten morphotypes were recognized and conceptualized into the cladistic characters sculpture presence and category. Optimization of sculpture presence on two recent hypotheses of palaeoheterodont phylogenetic relationships using the program Mesquite indicates a sculptured common ancestor of the extant Unionida, with multiple losses of the umbonal ornament occurring subsequently within the clade. Reconstruction of changes in sculpture category is ambiguous and demonstrates the need for further research into the evolutionary relationships of freshwater mussels in general and of their early ontogenetic sculptures in particular. Ambiguity is reduced in analyses applying a model with unequal costs of transformation between character states, which was derived from observations on intermediate forms and polymorphisms. These analyses suggest ‘V-shaped’ or ‘nodulous’ sculpture as the plesiomorphic sculptural category for Unionida. The relatively low levels of homoplasy inferred for V-shaped, pseudo-radial and double-looped sculptures suggest that these types may comprise useful guides to relationships within Unionida. The high degree of homoplasy of W-shaped, pseudo-concentric, wrinkled and single-looped sculptures, on the other hand, renders these sculpture types less fit for such purposes.