Ontogeny and Funtional Morphology of a Lower Cretaceous Carpinid Rudist (Bivalvia, Hippuritoida)

Authors: ROBERT W. SCOTT, MEGHAN WEAVER

Abstract: Caprinuloidea rudists are locally abundant and widespread in Lower Cretaceous (Albian Stage) Edwards Formation in Texas. Landward of the shelf margin on the shallow marine Comanche Shelf rudists built circular and elongate bioherms with coarse-grained flank deposits. Two caprinid morphotypes suggest that some lived as elevators above the substrate and others were recumbent upon mobile grain flats. Elevators have elongated attached valves and weakly coiled free valves and recumbents have arcuate attached valves and strongly coiled free valves. Detailed morphologic studies are not possible on the many molds and casts, but a few specimens are silicified. Their internal structures can be seen by X-ray computed tomographic scanning (CT), which provides three-dimensional representations of internal features. This technique enables the specific identification of caprinid rudists that otherwise could only be identified by sectioning the specimen. The abundant Edwards species is identified as Caprinuloidea perfecta because it has only two rows of polygonal canals on its ventral and anterior margins. X-ray CT images reveal ontogenetic stages of these unusual gregarious bivalves. Allometric to isometric growth characterizes the left-free valve (LV). Although the prodissoconch is unknown, the plots suggest that the initial length was greater than the width, which is like the D-shaped prodissoconch of Cardiacea. The LV has the morphology of loosely coiled gastropods and the right-attached valves are elongated and are unlike most Bivalvia.

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