Predatory gastropod boreholes in an Eocene molluscan assemblage from Nigeria

Morphometric data of 1, 346 boreholes belonging to eleven gastropod species and nine bivalve species have been studied. On the basis of hole-wall geometry and concentricity of the inner and outer holes, six different borehole types are identified: (1) concave with concentric openings (A-type); (2) concave with aconcentric openings (B-type); (3) planar tapering with concentric openings (C-type); (4) planar tapering with aconcentric openings (D-typc); (5) vertical with concentric openings (E-typc); and (6) concave-planar with aconcentric openings (F-type). The preferred gastropod and bivalve prey species for each bole type are named. In identifying the borers of a particular hole type, the mode of life of the most preferred prey species is used as a diagnostic parameter. Preferred infaunal prey species are dominantly bored by naticids, preferred epifaunal prey is mainly bored by muricids. The presence of a boss at the base of an inoomplete hole and the presence of countersunk holes are the additional criteria for identifying naticid borings. The hole type analysis suggests that hde types A, B and D belong to muricids and C, E and F to naticids. All the naticid and muricid hole types studied have a preferred site for boring. They are located purposefully where they would intercept internal organs of the prey. It appears that in ornamented bivalve shells these predators would rather drill between ribs than on top of the ribs.