New and overlooked occurrences of the rarely reported protochelonibiine "turtle" barnacles from the Oligocene and Miocene of Europe

We report on two clustering chelonibiid shells from Rupelian deposits of southwestern Germany. One of these specimens displays a tripartite rostral complex and disparietal radii that indicate the Oligocene species Protochelonibiamelleni, which was known so far from isolated compartments only. A literature review reveals two additional, overlooked records of the rarely reported genus Protochelonibia, coming, respectively, from the Burdigalian of France and the Langhian of Austria. Both these historical finds likely represent the Miocene species Protochelonibiasubmersa. All together, these occurrences support the notion that the protochelonibiines had acquired a broad distribution as early as in Rupelian times, when P.melleni occurred along the proto-Gulf of Mexico and in the Western Paratethys. Both P.melleni and P. submersa grew in form of peltate shells that evoke a superficial adhesion to some kind of quickly moving hosts. The outer wall of the abraded German colony of P. melleni is comprised of pillar-like blocks of shelly material. In other coronuloids, similar yet more prominent septa abut outward to form T-shaped flanges and intervening longitudinal canals that facilitate the grasping of various kinds of penetrable substrates. The diminutive external longitudinal parietal septa of P. melleni are more likely to represent an early stage in the evolution of the coronuloid shell architecture than vestigial structures. New additions to the pre-Pliocene fossil record of Coronuloidea and a comprehensive phylogenetic analysis of the turtle and whale barnacles will hopefully clarify this and other crucial aspects of the origin and early evolution of these remarkable forms.