Patterns of morphologic diversification among the Rostroconchia

Abstract

Morphologic diversification among rostroconch molluscs is explored using multiple phylogenies derived from different methods. In addition to strict parsimony, phylogenetic estimates were derived using four different methods that employ stratigraphic data. The resultant phylogenies are generally very similar to one another and to the phylogeny proposed by Pojeta and Runnegar (1976). All estimates (including the Pojeta and Runnegar estimate) imply much lower morphologic separations among post-Ordovician rostroconchs (measured here as the frequency of character state change per branch) than among Cambro-Ordovician rostroconchs. However, the data do not suggest that morphologic evolution became more constrained among rostroconchs as a whole, but instead suggest a reduced characteristic rate of morphologic change in the clade that happened to survive the end-Ordovician extinction. Likelihood ratio tests provide strongest support for the hypothesis that morphologic evolution was more constrained within a derived subclade (corresponding to previous definitions of the Conocardioidea, minus the Eopteriidae) than it was in a broad paraphylum (corresponding to the Ribeirioidea + Eopteriidae). Estimates from each of the phylogenetic methods lead to the same conclusions. Various metrics indicate that the pattern is not due to poor sampling of Cambro-Ordovician species and thus merits a biological explanation. Nonphylogenetic analyses of morphologic disparity suggest a similar history of morphologic evolution, including the apparent difference in characteristic rates of morphologic evolution between paraclade and subclade.