Brooke Lamonte Long-Fox , Laurie C Anderson

Lucinidae, an ancient clade of chemosymbiotic bivalves dating back to the Late Jurassic, have undergone changing taxonomic classifications. Older morphology-based classifications conflict with recent molecular phylogenies. Current taxonomies rely on molecular data, limiting phylogenetic placement to extant taxa with available molecular data. To better understand lucinid evolutionary history, a phylogenetic hypothesis including fossil taxa and morphological characters is needed. Here, morphological and molecular character data are examined using species-level phylogenetic analyses of 52 Neogene and Quaternary lucinid taxa from the Western Atlantic. A morphological matrix of 58 shell characters was developed to describe interior and exterior shell features, including ornamentation, hinge and dentition, muscle scars, pallial line, and inhalant channel, a feature inferred to be associated with chemosymbiosis in lucinids. Published molecular data included two nuclear ribosomal genes (18S and 28S rRNA) and the mitochondrial cytochrome b gene for 18 extant species. We examine congruence and resolution in cladograms produced using 1) parsimony and Bayesian inference methods, 2) morphological characters and combined morphological-molecular characters, and 3) pruned morphology-only, combined morphological-molecular cladograms, a reanalyzed molecular-only tree, and a pruned previously published tree for the family. Bayesian cladograms based on morphological and combined morphological-molecular data were better resolved than those from parsimony methods. While morphological trees had poor resolution at deeper nodes and were uninformative for subfamily-level designations, they successfully placed species into genera and aligned with molecular phylogenies at the tips. Combining molecular data with morphological characters improved resolution at deeper nodes and increased congruence with published phylogenies. Thus, integrating both data types provided clearer species-level placement than morphology alone. Recent phylogenetic studies often overlook morphological characters in place of molecular data, however, this study indicates that the combined use of morphological and molecular characters allows the generic-level placement of fossil taxa and living taxa that do not have molecular data.