Dear colleagues We are pleased to announce a new paper reporting the occurrence of *Thatchtelithichnus* (trace fossils) on a Pliocene grey whale mandible and suggesting that barnacles are possible tracemakers. Collareat A., Tsai C.-H., Coletti G., and Bosselaers M. 2021 *Thatchtelithichnus* on a Pliocene grey whale mandible and barnacles as possible tracemakers. Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen 302: 53-61. Abstract 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. The paper is open-access and freely available here: https://www.schweizerbart.de/papers/njgpa/detail/302/100308/Thatchtelithichn... Alternatively, please feel free to email Alberto: alberto.collareta@unipi.it or me: craniata@gmail.com or whaletsai@ntu.edu.tw Regards and all the best, Tsai 蔡政修（Cheng-Hsiu Tsai さい まさのぶ) PhD Lab of evolution and diversity of fossil vertebrates（古脊椎動物演化及多樣性實驗室） Department of Life Science & Institute of Ecology and Evolutionary Biology （生命科學系＆生態學與演化生物學研究所） National Taiwan University（台灣大學） No. 1, Sec. 4, Roosevelt Rd., Taipei, 106-17, Taiwan（台灣 106-17 台北市羅斯福路 4 段 1 號） Email: whaletsai@ntu.edu.tw; craniata@gmail.com https://www.facebook.com/NTUWhaleTsai/?modal=admin_todo_tour Tel: 886 - 2 - 3366 - 2448