Thatchtelithichnus on a pliocene grey whale mandible and barnacles as possible tracemakers

The ichnogenus Thatchtelithichnus Zonneveld, Bartels, Gunnell & McHugh was created for ring-shaped, roughly circular grooves affecting the outer surface of plastral bones of Eocene geoemydid turtles. Such traces were assumed to be attachment scars of aquatic ectoparasites (possibly ticks, leeches or liver flukes). Despite its well-distinctive aspect, Thatchtelithichnus has only been reported subsequently by few works and mostly from the plastron-bottom of freshwater turtles. Here we provide the first record of Thatchtelithichnus from a fossil mammal bone, namely, a partial grey whale mandible from the Belgian Pliocene. Thatchtelithichnus traces from this cetacean fossil commonly penetrate into the outermost portion of the cancellous bone, achieving a maximum depth of about 2 mm. The external margin of these grooves is sharply defined and commonly follows an elliptical, somewhat festooned path. A scrutiny of recent literature in palaeontological and forensic taphonomy as well as new first-hand observations reveal that Thatchtelithichnus-like structures can be produced by the attachment of barnacles on the surface of mammal bones that suffered long-lasting exposure on the seafloor. When encrusting bare bones in marine settings, barnacles can thus produce a variety of traces, including Anellusichnus Santos, Mayoral & Muñiz, Thatchtelithichnus and, possibly, Karethraichnus lakkos Zonneveld, Bartels, Gunnell & McHugh. The modes of trace formation are still largely to be understood, but observations on how barnacles damage paint coatings during growth might help us in envisaging how this kind of process works.