The articulate brachiopod hinge mechanism: morphological and functional variation

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.

Historical contingency and the purported uniqueness of evolutionary innovations

“…The nervous system is unique to Eumetazoa, but proteins involved in signaling and other functions have precedents in proteins required for food recognition, capture, and defense in unicellular organisms (100). The cyrtomatodont hinge of rhynchonellate brachiopods, in which articulation between the two valves involves resorption and remodeling of the growing hinge teeth, perhaps had a single origin (28), although the possibility that it evolved convergently in some small extinct clades cannot be ruled out.…”

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“…The distribution is positively skewed . The right graph, with the distribution for CarboniferousPermian deltidiodonts, shows that the mean and the minimum increased in the Paleozoic (Carlson 1992). Note that the Carboniferous-Permian distribution has 32 taxa, whereas the Ordovician has 61, raising the possibility that the increase in the minimum is an artifact of rarefaction.…”

“…Distributions for four subclades drawn from the right tail ofthe Ordovician distribution. Data are from Carlson (1992). Sec the caption for figure 8.…”

“…Further investigation with the model is necessary to develop more confidence in the subclade test . In the meantime, the DEMONSTRATIONS Brachiopod Geometry: A Driven System Carlson (1989Carlson ( , 1992) studied hinge and diductor-muscle geometry in Paleozoic brachiopods and found trends in a number of morphological variables. The left graph in figure 9A shows the distribution of values for Ordovician deltidiodont brachiopods in the angle formed by the hinge axis, the cardinal process, and the diductor muscle (angle HBPm in Carlson 1992, fig.…”

“…However, that is unlikely in this case: assuming that the Ordovician distribution, with ten species in the lowest angle class, is representative of the underlying distribution, the probability that a sample of 32 taxa drawn from that distribution would have no representatives in the smallest class is 0.0032. (Carlson [1992 , fig. 8] shows a more complete time series , and the minimum for all post-Ordovician time periods is greater than the Ordovician minimum.)…”