In situ otoliths from Triassic Fishes

Otoliths are paired elements housed within the inner ears of fish and are involved in the sense of hearing and balance. They have a calcareous (aragonite, occasionally vaterite) and protein composition, so in the fossil record their occurrence is mostly dependent on the aragonite preservation. Although rare, in situ fossil otoliths have gained major importance because they allow the correlation of each distinct morphology with the corresponding fish taxon. Contrarily to the Cenozoic, the knowledge on the otoliths of Mesozoic fish is still very scarce and the Triassic record of in situ specimen is so far limited to a single taxon, the flying fish Thoracopterus magnificus from the Norian (Late Triassic) Zorzino Limestone (N Italy). We discovered in situ otoliths in several specimens from Middle Triassic and early Carnian sites. In the recently discovered Pelsa/Vazzoler Lagerstätte (Dolomites, N Italy; late Ladinian) at least five actinopterygians have been found with otoliths in situ, namely Habroichthys (Peltopleuriformes), Placopleurus and Marcopoloichthys (Neopterygii), a perleidiform, and a palaeonisciform. Otoliths have also been found in a few Prohalecites porroi specimens (out of the more than 4000 recovered) from the Monte San Giorgio Kalkschiferzone (N Italy; late Ladinian), and at least in Eosemionotus and a halecomorph from the Raibl ‘Rossi Level’ (NE Italy; Ladinian/Carnian boundary). Habroichthys specimens from Slovenia and Xingyi (China) also present otoliths. From the Austrian early Carnian Polzberg Fauna, otoliths are recorded in Nannolepis, Peltopleurus, and Thoracopterus (Peltopleuriformes), in Phaidrosoma (Neopterygii) and in yet undescribed palaeonisciforms. Thus, otoliths are present in both primitive (Palaeonisciformes and ‘Subholostei’) and advanced Actinopterygii (Halecomorpha and basal Teleostei), although only a few specimens preserve them. We suppose that, owing to their mineralogical composition, they were typically dissolved in the acidic anoxic bottom environment that favoured the preservation of articulated fish skeletons. Due to the usually very small size of most fish specimens, micro-CT scanning has proven useful to reconstruct the 3D shape of the otoliths. With this study we shed new light on the very scattered record of Triassic otoliths, allowing future micropaleontological research to better link the loose samples with the most likely producer.