The articulate brachiopod hinge mechanism: morphological and functional variation

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Abstract
Functional consequences of the variation in geometry and morphology of the articulate brachiopod hinge mechanism are poorly understood, despite the fact that hinge structures have considerable importance in brachiopod taxonomy. Jaanusson (1971) proposed that the ability to resorb shell material during growth, particularly in the hinge structures, can be used to distinguish two groups within the articulates, the deltidiodonts and the cyrtomatodonts. He considered the two groups to be morphologically distinct, “natural” phylogenetic groups, separated by a “functional discontinuity.” In order to test the morphological, functional, and phylogenetic implications of shell resorption, comparisons of the hinge-system geometry and diductor muscle moment are made here between deltidiodont and cyrtomatodont brachiopods. A truss network composed of landmarks relevant to the valve opening mechanism is constructed to characterize hinge-system geometry. The function of the hinge mechanism is analyzed in the context of valve opening, and diductor muscle force, effort lever arm, and moment are compared between deltidiodonts and cyrtomatodonts. The distribution of resorption among brachiopods is investigated with respect to a phylogenetic hypothesis proposed by Williams and Rowell (1965a).Deltidiodont brachiopods are morphologically more variable than cyrtomatodonts, and a greater proportion of the variability is correlated with size. Deltidiodonts and cyrtomatodonts employ different strategies to open the valves; deltidiodont lever arms are relatively longer, whereas cyrtomatodont diductor muscles have relatively larger cross-sectional areas. The greatest muscle moment in deltidiodont hinge systems is realized in the maintenance of a gape angle; in the cyrtomatodont system, it is achieved at the initiation of a gape. Although they are morphologically and functionally distinct, it is doubtful that the two groups are separated by a “functional discontinuity.” Because the phylogenetic relationships among brachiopod orders are not yet resolved, the status of shell resorption as a homologue is still unclear. Resorption is manifest in at least some members of each major group of articulates (except the pentameraceans); it is likely that resorption has evolved independently several times in brachiopod evolution, in part because of the increased morphological flexibility it confers.