The above fossils were collected from a Lower Pleistocene silty marl exposed near the Megara archaeological site east of Augusta, Sicily, Italy. I was on that epic International Bryozoology Association field trip this summer I’ve been blogging about. The shells in this locality are very abundant with hundreds of species represented, from foraminiferans to shark teeth. I thought this little vignette of a predator and its typical prey was worth noting.
On the far right is a naticid gastropod (moon snail). These mollusks are predators who kill and consume their prey by drilling holes into their shells with a specialized radula (a kind of tooth-bearing “tongue”). Their holes are distinctively beveled, with a wider portion on the outside narrowing to a smaller inner opening. The three organisms on the left all show boreholes indicating that they were likely killed and eaten by a naticid.
Or at least that’s the traditional story. A paper came out this year (Gorzelak et al., 2013) comparing predatory drill holes in shells with holes produced by physical abrasion by experimental tumbling. The sizes, shapes and locations of these abrasion-produced holes are shockingly similar to those made by drilling predators. It looks like we must be careful which holes we assign to predation and which were produced by other means.
As I look at the three victims above, two of them (the high-spired turritellid gastropod on the far left and the bivalve second from the right) have nicely beveled holes with almost perfectly circular shapes. The gastropod shell that is second from the left, though, presents problems. First, it has two holes that completely penetrate the shell. Predators occasionally bore a shell twice, but not very often. Second the holes are more irregular in shape and don’t have a noticeable beveling. This could be a feature of the thinner shell of this gastropod not recording the usual naticid boring evidence, or it could be the result of physical abrasion and not predation. It is a difficult call but an important one to those plotting the evolution of this predator/prey system through time.
Gorzelak, P., Salamon, M.A., Trzęsiok, D. and Niedźwiedzki, R. 2013. Drill holes and predation traces versus abrasion-induced artifacts revealed by tumbling experiments. PLoS ONE 8(3): e58528. doi:10.1371/journal.pone.0058528
Kelley, P.H. and Hansen, T.A. 2006. Comparisons of class- and lower taxon-level patterns in naticid gastropod predation, Cretaceous to Pleistocene of the US Coastal Plain. Palaeogeography, Palaeoclimatology, Palaeoecology 236: 302–320.
Kowalewski, M., Dulai, A. and Fürsich, F.T. 1998. A fossil record full of holes: The Phanerozoic history of drilling predation. Geology 26: 1091–1094.
Tyler, C.L. and Schiffbauer, J.D. 2012. The fidelity of microstructural drilling predation traces to gastropod radula morphology: paleoecological applications. Palaios 27: 658–666.