2000 GSA Annual Meeting -- Reno, Nevada
2000 GSA Annual Meeting -- Reno, Nevada
Author(s): BENNINGTON, J Bret, 114 Hofstra University, Hempstead, NY 11549, geojbb@hofstra.edu; SELSS, Mark, Plattsburgh State University, Plattsburgh, NY, 12901; VINNIK, Anna, Oceanside High School, Oceanside NY 11572
Keywords: Pycnodonte, shell-coloration, Cretaceous, oyster, predation
A majority of left valves of the Upper Cretaceous oyster Pycnodonte convexa from the Navesink Formation in northern New Jersey preserve original shell coloration in the form of reddish brown radial bands, similar to those reported in other gryphaeidae. The bands vary greatly among individuals in number and thickness. Of particular interest is the observation that the color bands are discontinuous, being laterally offset along multiple shell growth lines. An episode of offset usually affects all bands on the valve, but in some cases only one side of the valve shows offset. Offset occurs equally as left lateral and right lateral relative to the umbo, with both types of offset occurring on the same valve, and in a few cases, along the same growth line. Growth lines associated with the offsets usually show evidence of disruption in the form of abrupt thickening, healed, crenellated breaks, and localized, abrupt changes in the slope of the outer shell surface. Color band offsets do not appear to be a diagenetic feature. There is no evidence of disruption or damage on the valve interior, nor are any of the valves crushed or damaged due to shell dissolution. Apparently, the offsets were generated by repeated disruptions of the valve margin during growth. Our working hypothesis is that the band offsets record unsuccessful predation attempts resulting in damage to the shell margin along the plane of commissure. Injury to the mantle tissue along the plane of commissure or extreme withdrawal of the mantle away from the commissural shelf could have resulted in a repositioning of the regions of the mantle secreting pigmented shell. If so, then band offsets provide a new metric for assessing the intensity of predation on Pycnodonte. However, preliminary analyses show no apparent correlation between number of band shifts on a valve and other variables, such as shell size and thickness, that would be expected to relate to the frequency of predation attempts.
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