A puzzling occurrence of the bite mark ichnogenus Linichnus from the Lower Miocene Chilcatay Formation of Peru

An upper anterior tooth of Cosmopolitodus hastalis (Elasmobranchii: Lamnidae) from Burdigalian strata of the shallow-marine Chilcatay Formation exposed at Zamaca (East Pisco Basin, Peru) exhibits the remarkable occurrence of a serrated bite mark consistent with Linichnus serratus (Praedichnia: Machichnidae). In marine successions of Cenozoic age, traces belonging to the ich- nogenus Linichnus are typically found occurring on the bones of marine mammals and interpreted as due to predation or scavenging by sharks provided with smooth-edged (Linichnus bromleyi) or denticulated (L. serratus) teeth. Only a few fossil shark teeth exist preserving serrated bite marks, all of which have been interpreted as due to self-biting, which may occur when a shark loses a tooth while feeding and accidentally bites into it. Since C. hastalis is a smooth-toothed species, self-biting cannot explain our unusual find of L. serratus, which in turn may either reflect some kind of trophic interaction between large carnivorous sharks (with a late juvenile or young adult C. hastalis being fed upon by another elasmobranch, possibly a large-sized carcharhinid) or testify to accidental biting during multispecies shark scavenging on the carcass of a third organism that did not get preserved alongside the bitten tooth.