An Unexpected Deadly Meeting: Deep-Water (Hexanchid) Shark Bite Marks on a Sirenian Skeleton from Pliocene Shoreface Deposits of Tuscany (Italy)

Shark bite marks incising fragmentary ribs from a single specimen of the extinct sirenian Metaxytherium subapenninum are described from lower Pliocene shallow- water, marginal- marine (shoreface) deposits exposed at Arcille (Tuscany, Italy). Two of the bitten ribs display bite marks whose morphology matches that of juvenile teeth of the extant bluntnose sixgill shark Hexanchus griseus, which is known from the same site thanks to several dental remains. Our new finds comprise the second published occurrence of hexanchid bite marks on fossil marine mammal bones; furthermore, they represent the first piece of evidence documenting a trophic interaction between H. griseus and sirenians across both the fossil and the Recent records. In light of the environmental preferences of
extant sirenians (which thrive in shallow- marine, nearshore seagrass meadows), and considering that the bitten skeleton takes its place in shoreface sands, our finds are seemingly at odds with the current understanding of hexanchids as deep- water sharks. Juveniles of H. griseus, however, are known for their diel vertical migrations, which result in frequenting relatively shallow waters at nighttime.
Frequentation of the marginal- marine, shoreface Arcille palaeo- biotope by bluntnose sixgill sharks might have been facilitated by a steep morphology of the seafloor as well as by the presence of a
bathymetrically high thermosphere- psychrosphere interface in the early Pliocene Mediterranean Sea.