Trace fossil evidence for Osedax Rouse et al., 2004 exploiting shark tooth dentine on a Pliocene seafloor: broadening our understanding of a major taphonomic agent

The zombie worm genus Osedax Rouse et al. 2004 is part of a peculiar group of organisms that thrive at whale fall localities on the ocean floor. In addition to feeding on whale bones, extant Osedax spp. are known to feed on the bones of other marine and terrestrial mammals, birds, and reptiles, and have recently been found to also feed on shark tooth dentine under experimental conditions. The efficacy of Osedax spp. as bioeroders is such that they are thought to have significantly affected both the quantity and the quality of the marine vertebrate fossil record. Here, we report on the occurrence of single-entry borings consistent with those produced by Osedax on fossil shark teeth from the Lower Pliocene offshore deposits of Tuscany, central Italy. Micro-CT investigations reveal that the worms excavated the dentine of the tooth roots, leaving the enameloid-coated crowns seemingly unaltered. The borings in question are assigned to the Osedax-related ichnogenus Osspecus Higgs et al., 2012, which to date had been found on a variety of marine vertebrate fossils, but not on shark teeth. This ichnological record provides the first fossil evidence for zombie worms exploiting shark tooth dentine and serves as a powerful demonstration that such behaviour can and does occur in the wild. Root exploitation by Osedax may contribute to explain the high frequency of rootless teeth in some shark tooth accumulations from modern deep-sea floors and time-averaged horizons inland. Over the past 100 million years, shark teeth have likely served as critical ecological stepping stones between remote marine vertebrate fall localities and may even have provided a suitable substrate for ancient zombie worms during the Cretaceous–Paleogene (K–Pg) extinction event.