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 ichnogenus 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.