Comparative morphology and phylogeny of anomalodesmatan bivalves

Anomalodesmatans comprise a large, ancient and ecologically diverse group of marine bivalves, but are nonetheless inconspicuous in most extant shallow water communities. For various reasons, which include their present scarcity and a bewildering array of disparate morphologies, representatives of the group have always proved difficult to interpret, and their systematics lagged behind those of most other major bivalve taxa. Most of this dissertation reports the results of a comparative investigation on the shell morphology and anatomy of extant anomalodesmatans, which formed the basis for a reassessment of hypotheses of primary homology established by previous investigators and identification of novel characters for phylogenetic inference. Due to the chief role played by the hinge ligament in authoritative discussions of anomalodesmatan evolution, this organ was chosen as the focus of a more detailed treatment. Discontinuous ontogeny of fibrous ligament is shown to characterise several members of the group, with the implication that, in contrast to the prevailing model,not all anomalodesmatan adult ligaments may be considered homologous. Likewise, a system of multicellular glands concerned with sediment agglutination was studied with particular emphasis because it is both exclusive to and widespread within Anomalodesmata. Evidence of preserved glandular secretion is recorded for the first time in fossil material and the glands themselves found in extant laternulids and pholadomyids, thus considerably expanding their known taxonomic distribution. Finally, this volume also documents the largest cladistic analysis of extant anomalodesmatans performed to date, including morphological data compiled from both original observations and literature accounts. Among traditionally recognised superfamilies, Pholadomyoidea, Clavagelloidea and Septibranchia were found monophyletic. Taxa commonly referred to Pandoroidea and Thracioidea were recovered as part of two new clades, which are also supported by recent molecular studies. Interpreted in the light of the fossil record, reconstructed phylogenetic relationships favour the iterative evolution of shallow infaunal and epifaunal anomalodesmatans from deep-burrowing ancestors over previously advanced patterns for the history of the clade, namely ventral migration of the ligament and irreversible radiations into a deep infaunal life habit.

Gates Cambridge Trust, Overseas Research Students Award, Emmanuel College, Cambridge Philosophical Society, Malacological Society of London