Using fish taphonomy to reconstruct the environment of ancient Shanwang Lake

Abstract

Abstract Shanwang Lake Basin is a palaeolake formed during the Miocene, and is renowned for its quality and abundance of fossil specimens, including one of the most abundant and diverse Tertiary fish fossil deposits in China. Evidence from studies of these fish fossils has revealed much about the climate and environment of the ancient Shanwang Lake during the Miocene. Geological studies have identified two kinds of taphonomical facies in the type section of no. 2 locality of the lake basin. One is in the thinly bedded diatom-bearing mudstone and shale typical of the upper part of the section. The poor preservation quality of fossilized fishes in this facies is interpreted as fish having decayed in a hypolimnion warmer than about 15°C, in lake depths less than 8–12 m and under aerobic conditions habitable by scavengers. The second taphonomical facies in the thinly laminated diatom shale typical of the lower part of the section. That fish fossils here show no examples of scattering, disarticulation or macroscopic scavenger disturbance indicates an environment with low hydraulic energy, anoxic or anaerobic conditions and a temperature lower than 15°C in the hypolimnion. Seasonal variation is recorded in the thinly laminated diatom shale, which is composed of couplets of light-coloured diatom-rich laminae and dark grey or dark brown organic-rich laminae. The light laminae include a large proportion of deciduous leaves, plant seeds and fishes, and are interpreted as autumn and winter deposits, while the dark laminae, which contain few deciduous leaves and a relatively high percentage of insects, are thought to be spring and summer deposits. The length-frequency histogram of 474 specimens of Plesioleucicus miocenicus (Pisces: subfamily, Leuciscinae) shows a series of distinct peaks, which are interpreted as averaged year classes produced by repeated, annual winter deaths of fishes. The main cause of fish death is suggested to be overturn-induced anoxia, which occurred in a monomictic lake with annual winter circulation.