Oligocene basking sharks (Lamniformes, Cetorhinidae) of the Carpathian Basin with a reconsideration of the role of gill rakers in species diagnostics

Basking sharks are presented in the modern fauna by a single species (Cetorhinus maximus), although they have been much more diverse in the past. This group of sharks first appeared in the fossil record in the middle Eocene of the Antarctic and the U.S.A., but most of the described extinct taxa are known from the Oligocene and lower Miocene of Europe. Gill rakers are the most abundant among basking shark remains and their morphological details play an important role in species diagnostics. The shape of isolated gill rakers from 16 Oligocene localities of Poland was analyzed using various morphological approaches, including geometric morphometrics. Results indicate that descriptive characters have a wide range of variation and low diagnostic value, and they are associated directly with the position of gill rakers on the gill arch. Morphological indices describe proportions by discrete structures and could be effective only in the identification of the stratigraphically most distant taxa. Geometric morphometrics revealed significant differences between all of the species designated earlier except for †Caucasochasma zherikhini and †Keasius parvus. At the same time, considering the obtained results on morphology along with the geographic distance, Oligocene basking sharks from Poland should be assigned to †K. parvus. Geometric morphometrics of gill rakers supports the taxonomic distinctness of both †K. rhenanus and †K. septemtrionalis. Study results indicate that reliable taxonomic attribution of extinct basking sharks by the shape of gill rakers, in the absence of other skeletal elements (teeth and/or vertebrae), should be sample-based using multivariate approaches.