Smoking guns for cold cases: the find of a Carcharhinus tooth piercing a fossil cetacean rib, with notes on the feeding ecology of some Mediterranean Pliocene requiem sharks

Predation and scavenging by Cenozoic sharks are witnessed by relatively common tooth marks on vertebrate bones and much more infrequent shark teeth or tooth fragments that are found embedded in the skeletal elements of their prey/scavenging items. However rare, finds of the latter type are true “smoking guns” that provide the strongest evidence for the trophic palaeoecology of ancient sharks – one that is remarkably unaffected by the kind of problems that sometimes hinder the unambiguous identification of bite marks while often allowing for a positive taxonomic determination of the biting organism(s). Here, we report on a cetacean rib from the Pliocene of Monterotondo Marittimo (Tuscany, central Italy) that is pierced by a partial requiem shark (Carcharhinus sp.) tooth. Interestingly, in Pliocene times, the Mediterranean Basin was inhabited by a diverse carcharhinid stock, including the mammal-eating species Carcharhinus leucas, Carcharhinus longimanus and Galeo cerdo cuvier, none of which inhabits the present-day Mediterranean Sea on a regular basis. Evidence for requiem sharks (mostly broad-toothed members of the genus Carcharhinus and tiger sharks) foraging upon cetaceans is preserved in the Mediterranean Pliocene fossil record in the form of bite marks, teeth associated with bones and, with the present study, teeth embedded in bones. It is thus tempting to speculate that the abundance of mammal-eating requiem sharks in the Pliocene Mediterranean Sea was at least partly supported by a richer-than-today marine mammal fauna. A different primary productivity regime, a currently unparalleled distribution of nutrients along the water column, and/or higher seawater temperatures may in turn explain the high diversity of the Pliocene Mediterranean marine mammal assemblages.