Relationships between the extant Anomalodesmata: a cladistic test

Although ancient anomalodesmatans were apparently abundant shallow and deep burrowers. in Recent seas the subclass comprises some of the most specialized and rarest of all bivalves. The morphological adaptations associated with diverse life habits has persistently frustrated attempts to achieve a widely accepted scheme for the relationships between extant families. A cladistic analysis, using 43 informative anatomical and shell characters for each of the extant anomalodesmatan families has produced a single, reasonably robust tree which is in broad. albeit imperfect, agreement with the known fossil record of the subclass. This total evidence tree places the Pandoridae, Lyonsiidae. Cleidothaeridae and Myochamidae, and also the Thraciidae, Periplomatidae and Laternulidae in monophyletic groups. Carnivory appears diphyletic. with the Parilimyidae separated from the 'septibranch' families (Cuspidariidae, Verticordiidae, Lyonsiellidae and Poromyidae) which Form a monophyletic group. The enigmatic tube-dwelling Clavagellidae appear as a sister group to the 'septibranchs'. Re-analysis of the data matrix using only those 18 characters which could be scored from shell characters alone, produced a tree which contradicted the total evidence tree rather than producing a poorly resolved version. The degree of convergence shown by shell characters make it. at least at present, difficult to include the extinct anomalodesmatan families in a cladistic analysis.