Otoliths from the Lealt Shale Formation, Great Estuarine Group, Middle Jurassic (Bathonian), Inner Hebrides, Scotland

Abstract

The Lealt Shale Formation is known for its superb preservation of aragonitic molluscs. As fish otoliths are primarily composed of aragonite, and because of the semi-stable nature of this polymorph, records of otoliths become increasingly rare in deep time. Therefore, the Lealt Shale Formation provides a window through which to expand knowledge of Middle Jurassic otoliths. A total of 753 otoliths have been collected, representing 10 species: three in Archaeotolithus, five in Leptolepis, one Archengraulis and a teleost incertae sedis (juveniles only). Four species are new: Archaeotolithus eiggensis, Archaeotolithus invernizziae, Leptolepis flexuosus and Leptolepis skyensis. Two species remain in open nomenclature and the remaining species are known from the Middle Jurassic of southern England and continental Europe: Archaeotolithus doppelsteini, Leptolepis densus, Leptolepis rotundatus and Archengraulis productus. The Lealt Shale Formation was deposited in a low-salinity lagoonal environment with variable freshwater influx and a probably tenuous link to marine waters, resulting in a complex palaeosalinity history. The relevance of the specific environment for the composition of the fish fauna as documented by otoliths is discussed. The evolution of the otolith morphospace during the Jurassic is discussed in the light of the new finds.