The valve opening system of the Permian concavo-convex productid brachiopod Waagenoconcha imperfecta is discussed in reference to the mode of the valve articulatory system and the reconstructed musculatory system. The analysis of the articulatory system suggests that it could open to a maximum angle of 6 degrees. The characters that seem to have played the most important roles in that process are: (1) narrow ginglymus, (2) overhanging beak above the dorsal valve, and (3) relatively narrow interior space in the posterior ventral valve, which provides little space for rotating the cardinal process and in result prevents excessive shell opening. For reconstruction of the musculatory system in productide, the relationship between the contracting direction of the adductor muscles and its mode of attachment to the dorsal valve was examined in well preserved specimens of W. imperfecta. The furrows of the attachment scars are cup-shaped, and their axes are concordant to the direction of adductor contraction. This structural relationship indicates that the paired anterior and posterior muscle scars in the median sulcus represent the attachment sites of the adductor and the diductor, respectively. The reconstructed musculatory system is fairly similar to that of the extant terebratulides. The “scars” that were thought in previous studies to be the places for attachment of the massive and powerful diductor muscle in productides seem to be the imprints of the mantle canal on the interior ventral valve. All this suggests that the previous interpretation claiming that productides could open and close valves actively, should be abandoned.
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