High resolution palaeoecological and taphonomic analysis of Early Cretaceous lake biota, western Liaoning (NE-China)

The faunal content of eighteen bedding planes within the Jianshangou Unit of the Lower Cretaceous Yixian Formation of western Liaoning has been analysed in detail in order to understand the environmental and taphonomic framework of this famous lacustrine fossillagerstätte. The bedding planes were excavated within a 3 m thick section, which is composed of laminated mudstones, graded sandstones and mudstones, and horizontally stratified structureless sandstones and tuffs. All investigated bedding planes occur within the laminated mudstone facies and contain elements of the so-called Jehol Biota, in particular the conchostracan Eosestheria ovata, the ephemerid nymph Hexagenites trisetalis and the crustacean Liaoningogriphus quadripartitus. These three faunal elements occur in high abundance, originally possessed chitinous skeletons and are preserved as compressed, faintly mineralized coats or, in the case of Eosestheria, still possess organic material. The specimens do no represent exuviae, but body fossils. The fossils can be grouped into three low diversity associations, each characterized by one of the three taxa. The plan-view orientation of the taxa is invariably random and size frequency histograms are bimodal or polymodal. This indicates that the fauna is autochthonous and that the histograms reflect several recruitment phases. Apparently, the faunal elements suffered seasonal mass mortality caused by anoxic conditions, which became established in the lake during the summer months. Increased humidity during the winter led to mixing of the lake waters, re-establishment of oxic conditions and to deposition of a thin film of sediment. Such environmental conditions were episodically interrupted by flash floods and volcanic ash falls, which may have been responsible for the preservation of the more spectacular elements of the Jehol biota such as birds and feathered dinosaurs.