Turtle barnacles have been turtle riders for more than 30 million years

In contrast to other kinds of biological interactions, symbiosis is a scarcely investigated aspect of the fossil record. This is largely due to taphonomic biases that often frustrate any attempt to make a strong case that two organisms shared an intimate association in life. Among extant marine vertebrates, the sea turtles (Cheloniidae and Dermochelyidae) bear a broad and diverse spectrum of epibiotic symbionts, including specialists such as the turtle barnacles (Chelonibiidae and Platyleapa-didae). Here, we reappraise an early Oligocene (Rupelian) fossil cheloniid skeleton, featuring the remains of cirripedes on the exterior of its entoplastron, from the Rauenberg fossil-lagerstätte, southwestern Germany. The barnacle specimens are assigned to Protochelonibia melleni, an extinct protochelonibiine species and the geologically oldest known member of Chelonibiidae. In the light of taphonomic and palaeoenvironmental considerations, and given that the extant chelonibiids are mostly known as epizoic symbionts of sea turtles, we conclude that this unique fossil association resulted from the epizoic growth of the barnacles on the external surface of the plastron of the turtle during its lifetime. This remarkable fossil association provides evidence that chelonibiids, including the extinct protochelonibiines, have been chelonophilic epizoans for more than 30 Myr. A survey of the trace and body fossil records shows that platylepadids are also likely as old as the Rupelian as is their symbiotic association with cheloniid hosts. This early emergence of the modern-looking, turtle-dwelling barnacle lineages corresponds to a climate-driven phase of major radiation and taxonomic turnover among sea turtles at the Eocene-Oligocene transition.