Terminology and functional morphology of attachment structures in pelmatozoan echinoderms

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Abstract
Fixation on the seafloor is a fundamental, though little studied, aspect of the biology and distributional ecology of pelmatozoan echinoderms. Morphology of pelmatozoan attachment structures (holdfasts) and their component appendages is summarized and a detailed terminology proposed. Three major modes of fixation are differentiated in living and fossil stemmed echinoderms, primary, composite and secondary attachments, in addition to the direct thecal attachment in stemless forms. Specific holdfast morphologies, which characterize certain major taxa (families and orders) of pelmatozoans, may limit the available life-modes in these echinoderms. Significantly, numerous lineages of echinoderms have independently developed secondary attachment modes associated with the evolution of flexible tapered stem ends and/or true cirri. Cirri are rare in early Paleozoic pelmatozoans but appeared in most major groups by the late Paleozoic.