Saurichthys and other fossil fishes from the late Smithian (Early Triassic) of Bear Lake County (Idaho, USA), with a discussion of saurichthyid palaeogeography and evolution

A new marine fish assemblage from the late Smithian (Olenekian, Early Triassic) Anasibirites beds of the Thaynes Formation collected near Georgetown (Bear Lake County, south-east Idaho, USA) comprises actinopterygians (Saurichthys cf. elongatus, Actinopterygii indet.), sarcopterygians (Actinistia indet.), and possibly chondrichthyans. We review the global fossil record of the Triassic lower actinopterygian Saurichthys, which is used herein for a case study of trends in morphological adaptations as well as variations in palaeogeographic distribution and diversity dynamics of Early Mesozoic fishes. In the Early Triassic, Saurichthys already occupied a top position in marine food webs, with some species achieving body lengths of up to 1.5 m. Distribution of morphological characters in Saurichthys during the Triassic suggests trends towards a reduction in squamation, stiffening of the fins and axial skeleton, shortening of the postorbital portion of the cranium, and reduction in the number of dermal skull bones. The postcranial adaptations in particular helped to improve the fast-start ability of these ambush predatory fishes. The palaeogeographic range of Saurichthys changed from a virtually global distribution in the Early Triassic (indicating rapid dispersal within marine and freshwater ecosystems after the end-Permian mass extinction) to an occurrence mainly restricted to the north-western Tethys in the Late Triassic, and also towards increasing rarity within continental deposits. Modifications in the palaeogeographic distribution were accompanied by successive loss in global species richness and were possibly related to intra-Triassic extinction events, environmental alterations and/or competition.