Life on a lake bottom: diverse horseshoe crab and fish ichnofauna from Yanchang Group (Upper Triassic) of northern Shaanxi Province, China–Implications for palaeoenvironment

An ichnoassemblage comprising trackways and trails of horseshoe crabs and fishes has been discovered in the Upper Triassic Wayaobu Formation of the Ordos Basin, Shaanxi Province, northern China. This is a lacustrine coal-bearing succession with sandstones, mudstones and oil shales. Two trace-bearing beds are present, one with fine sandstone, the other being a mudstone layer. The traces are assigned to <i>Kouphichnium</i> isp., different <i>Undichna</i> ichnospecies representing <i>U. bina, U. britannica, U</i>. ?<i>quina, U. unisulca</i> and <i>U</i>. isp., and to an indeterminate trackway. <i>Kouphichnium</i> isp. shows long trackways dominated by impressions of the pusher legs, often accompanied by a telson trace. While these were probably left by bottom walking horseshoe crabs, others showing elongated curved scratches lacking a telson trace might have been left by swimming individuals. Remarkable is the large size of the <i>Kouphichnium</i> trackmaker if compared to known Triassic body fossils. The ichnofauna is described and the ichnotaxonomy of <i>Undichna</i> and similar forms discussed. In the Late Triassic, a depressional lake basin was formed within the Ordos Basin. Bivalve, crustacean and fish body fossils support a lake environment in a semi-arid to humid climate. Discoveries of horseshoe crab and diverse fish traces indicate a rich shallow lake epibenthic community.