A microfossil evidence for the composition of fish communities in the Late Triassic of Tethys: examples from Cassian Formation, Italy

During the Triassic, fish were the most abundant group of marine vertebrates – in terms of both species richness and number of niches they occupied – as indicated by their rich skeletal record. However, the Triassic ichthyolith record is relatively poor, unlike that from younger time periods, what skews the published palaeontological data heavily towards skeletal materials. Our bulk collecting targeting fish microfossils from soft siliciclastics in the Cassian Fm. resulted in a collection of several ichthyoliths (i.e. bone fragments, teeth and scales) and otoliths in Picolbach, Settsass Scharte, Stuores and Misurina Skilift localities in Dolomites, Southern Calcareous Alps, Italy. We compare this micro-ichthyofauna with other such faunas and in particular with the Jurassic–Cretaceous otolith-based faunas and their Cenozoic and Recent counterparts. We argue that most of the otoliths from Picolbach belong to a variety of actinopterygians, from stem-neopterygians (most likely peltopleuriforms), to holosteans. Otoliths from Settsass Scharte are reminiscent of teleost otoliths rather than ‘palaeoniscid’ otoliths as it was suggested previously, and may therefore constitute the earliest-known occurrence of such otoliths to date. From the microfossil point of view, the Cassian Fm. fish associations were comparable to the roughly coeval skeletal associations of the late Ladinian–Carnian, and Norian age from Austria and northern Italy (e.g. Raibl, Polzberg and Bergamo Lagerstätten). We argue that Carnian (Late Triassic) Tethyan marine fish fauna was quantitatively dominated by neopterygians other than teleosts, and may represent a very early stage of teleost diversification. It seems also that Triassic Tethyan chondrichthyan fauna was limited to durophagous forms only.