The first Miocene Osedax traces on a whale bone from the Northern Apennines, Italy

Large cetacean carcasses that sink to the bottom of the deep sea are usually colonised by a wide variety of organisms, forming the so-called whale fall communities. We studied a fragmentary long bone of an unidentified cetacean from the palaeontological collection of the Università di Modena e Reggio Emilia collected near Montegibbio (Modena, Italy). The dating of the associated sediments, by means of calcareous nannofossils, indicates a early-middle Tortonian age for the specimen, allowing us to assign it to the Termina Formation. The bone, which shows signs of bioerosion on the outer surface, was analysed using 3D photogrammetry and micro-CT scanning to characterise the external and internal shape of the fossil traces and to understand which organism produced them. Externally, the bone surface shows rounded to irregular pits, a few millimetres to four centimetres wide, where the compact bone has been removed exposing the underlying cancellous bone. Smaller pits are isolated, while larger ones merge. These surface traces are similar to those made by the bone-eating worm Osedax on modern marine vertebrates, a polychaete of the family Siboglinidae that uses a specialised root tissue to penetrate the bones from which it obtains nutrients via endosymbiotic, heterotrophic bacteria. However, only small cavities with a single hole reflect the shape of an individual animal’s root system and are of diagnostic value. Using digital segmentation of the scan slices, we isolated, measured and reconstructed the 3D morphology of single boreholes. We distinguished three distinct morphotypes and found similarities in size and shape with boreholes produced by modern Osedax species. Our findings, which represent the oldest fossil record of Osedax traces from the Mediterranean area, and the first Miocene record worldwide assessed via micro-CT scan images, are consistent with the presence of a high diversity of cetaceans in the Late Miocene proto-Mediterranean Sea.