PATTERNS OF MORPHOLOGIC DIVERSIFICATION AMONG THE ROSTROCONCHIA

Morphologic diversification among rostroconch molluscs is explored usi ng multiple phylogenies derived from different methods. In addition to strict parsimony, phylogenetic estimates were derived using four diff erent methods that employ stratigraphic data. The resultant phylogenie s are generally very similar to one another and to the phylogeny propo sed 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 chara cter state change per branch) than among Cambro-Ordovician rostroconhs . However, the data do not suggest that morphologic evolution became m ore constrained among rostroconchs as a whole, but instead suggest a r educed characteristic rate of morphologic change in the clade that hap pened to survive the end-Ordovician extinction. Likelihood ratio tests provide strongest support for the hypothesis that morphologic evoluti on was more constrained within a derived subclade (corresponding to pr evious definitions of the Conocardioidea, minus the Eopteriidae) than it was in a broad paraphylum (corresponding to the Ribeirioidea + Eopt eriidae). 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 biolo gical explanation. Nonphylogenetic analyses of morphologic disparity s uggest a similar history of morphologic evolution, including the appar ent difference in characteristic rates of morphologic evolution betwee n paraclade and subclade.