Abstract:

Cassid gastropods are well-known drilling predators of a whole suite of echinoids which are common in both Recent and fossil environments. In many cases, the identification of cassids as possible predators is based on drill hole morphology only. A detailed study of drill hole characteristics is thus imperative for predator recognition and provided here by a thorough descriptive and statistical analysis. Fifty drill holes in the sand dollar Leodia sexiesperforata produced by the predatory gastropod Cassis tuberosa are analyzed for recognizable characters and compared to 174 drill holes in 289 denuded and empty tests from San Salvador, the Bahamas. The drill holes in dead tests are identical to the freshly drilled holes. Drill holes show subcircular to elliptical outlines with irregular margins and can feature notches produced by the predator. The cross-sectional drill hole profile range from concave parabolic to sigmoidal. The collected tests reveal a drilling intensity of 59.5% with only three individuals (1.74%) featuring multiple drill holes. The high drilling intensity and the presence of almost exclusively single drill holes indicate a high drilling success rate for the cassid predator in Caribbean ecosystems. Although the results suggest size selectivity, taphonomic processes and handling effects of the predator might result in a weaker selectivity than that inferred directly from the investigated material. Cassis tuberosa drills predominantly into the oral side of the test (86.3%) particularly in the area below the internal organs (90.9%). Most drill holes occur in the anterior part of the sand dollar's test (66.7%).

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