New species of the miniature fish Placopleurus (Neopterygii) from the Middle Triassic of Italy and Slovenia reveal the oldest near-homocercal tail

The miniature fish Placopleurus (Neopterygii) is commonly reported from Middle Triassic marine deposits of the Western Tethys but has remained poorly understood due to the incompleteness of the original material. We present a comprehensive redescription of this genus based on new specimens from Middle Triassic Southern Alpine localities. Five new species are established: Placopleurus taibonensis sp. nov. from the upper Ladinian Pelsa/Vazzoler Lagerstätte (Dolomites), Placopleurus verbanensis sp. nov. from the mid-Ladinian of Castelveccana (western Lombardy), Placopleurus seidli sp. nov. and Placopleurus zmelkooworum sp. nov. from Anisian sites in the Kamnik-Savinja Alps (Slovenia), and Placopleurus evelynae sp. nov. from the Anisian Prà della Vacca (Dolomites). The new material provides clearer insights into the skull and tail anatomy of the genus. Placopleurus species-specific characters chiefly involve ornamentation of bones and scales, bone shape (e.g., cleithrum, dermopterotic), presence/absence of particular features (teeth, hook-like element), and overall size. These are combined with meristic characters such as number of deep flank scales, number of horizontal scale rows, and insertion of the pelvic, dorsal, and anal fins. Notably, a juvenile from the Dolomites permits ontogenetic inferences of the taxon. Moreover, some Slovenian specimens exhibit the oldest known near-homocercal tail, resembling that of teleosts. These findings support placing Placopleurus closer to more advanced neopterygians, in the new order Placopleuriformes.