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INTERNAL CAPRINID MORPHOLOGYRudists were aberrant sessile suspension feeding marine bivalves that lived from the Late Jurassic to the end of the Cretaceous. Together with corals and sponges, rudists were important organisms in shallow-water Cretaceous buildups (Masse and Philip 1981; Burchette 1993; Scott 1981, 1990). Their shell structure and form evolved from a pair of coiled valves having a thin calcitic outer layer and a thick aragonitic inner shell layer to very inequivalved forms, in which the unattached valve varied from cap-shaped to operculiform to elongate. Uncoiling enabled shell accretion along the entire mantle margin and, hence, the growth of conical forms (Skelton 1978). One of the most abundant and diverse Lower Cretaceous rudist families is Caprinidae d'Orbigny (1847). It is divided into two subfamilies, Caprininae d'Orbigny (1847) and Caprinuloidinae Mac Gillavry (1970). These two taxa are differentiated by the cardinal apparatus, ligament, posterior accessory cavity, pallial canals, and the relative protrusion of the posterior myophoral plate, besides other, minor dental differences (Skelton and Masse 1998; Skelton and Smith 2000). The posterior myophore is a salient plate on either the left-free valve (LV-FV) in caprinuloidinids or the right-attached valve (RV-AV) in caprinids that projects across the commissural plane down into a cavity of the opposing valve (Chartrousse 1998, figure 5.1). The anterior myophore in each valve is an inclined ledge that extends ventrally from the hinge plate across the commissure. In Caprininae the myophore plate projects up from the RV-AV and in Caprinuloidinae it projects down from the LV-FV (Chartrousse 1998). However, in 2-D cross sections, as seen in many outcrop specimens, these features cannot be recognized. Thus 3-D views provided by CT images are useful for taxonomic diagnosis (see Figure 1, Figure 2, a 3-D movie). |
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