Anatomy, shell morphology, and microstructure of the living fossil Pulvinites exempla (Hedley, 1914) (Mollusca: Bivalvia: Pulvinitidae)

Abstract

Pulvinites exempla is the only living species of the marine bivalve family Pulvinitidae. Previously, the species has been known exclusively from cursory analyses of gross shell morphology. The present study is the first description of the soft anatomy, shell microstructure, and early ontogeny of the shell and ligament in this species. Many aspects of the morphology described herein are unique to P. exempla, considerably expanding the degree of morphological disparity within the superfamily Pterioidea. Many features of the body plan of P. exempla are affected by extensive torsion of the pedobyssal apparatus to accommodate its extreme pleurothetic (strongly inclined to the substratum) habit. The aberrant morphology of P. exempla is compared with that of similarly contorted but distantly related Anomiidae. In addition, P. exempla appears to be the first described pterioidean bivalve whose members brood lecithotrophic-planktotrophic larvae in their gills. Detailed observations on the arrangement of the musculature provide the basis for the re-interpretation of the muscle scars in fossil pulvinitids and establish their homology with other pterioideans. Type specimen designations of P. exempla are amended and taxonomic implications resulting from the analysis of variation in shell morphology within P. exempla are discussed. © 2006 The Linnean Society of London.