Drilling Predation on Scaphopods from the Upper Cretaceous of Manitoba: Prey and Predator

During the past several decades, abundant information on predation upon fossils like bivalves, gastropods, and brachiopods has been accumulated, but very little is known for the worldwide distributed marine infaunal carnivorous scaphopods. One drilled fossil Dentalium specimen was mentioned in an abstract, several museum collections were documented in a paper studying the predation on sub-Holocene scaphopods, and several drilled Oligocene scaphopods were listed in a newsletter. Over 330 specimens of Dentalium gracile were collected from the Upper Cretaceous Millwood Member (Campanian) of Pierre Shale in a site near Russell, SW Manitoba. The study shows that the dramatic increase of predation in Late Cretaceous as recognized by previous studies is also recorded in D. gracile. About 35% of the specimens were drilled, higher than previously noted for the same time period (5-13%), but around or lower than that of the sub-Holocene (34%-61%). Morphologically, the drill holes are bevelled with a countersunk profile. The inner margin (Id) of the boreholes appears to be oval [Id1/Id2 =0.81, standard deviation (DS)=0.0903, N=48], whereas the outer margin (Od) is very close to circular (Od1/Od2 =0.96, SD=0.0701, N=48). The inner margin of Sub-Holocene boreholes (Id1/Id2=0.69, SD=0.10, N=40) tends to be more oval than Cretaceous one, whereas the outer margin of Cretaceous boreholes appears to be more circular than the sub-Holocene one (Od1/Od2=0.86, SD =0.07, N=40). Among the Cretaceous specimens of Manitoba, the boreholes seem to be more common on the lateral side. The dorsal preference as among the sub-Holocene predation is not observed. The drill hole dimensions do not correlate with the size of the prey, this may indicate that the prey size does not correlate with the predator size. Over 300 specimens of the naticid gastropod Euspira ?obliquata were collected from the same site, they were believed to be the predators of D. gracile.