A Horodyskia-Nenoxites-dominated fossil assemblage from the Ediacaran-Cambrian transition (Liuchapo Formation, Hunan Province): Its paleontological implications and stratigraphic potential

Abstract

Because of the scarcity of fossil data, life evolution in deep-water settings through the Ediacaran-Cambrian transitional interval is still poorly understood. This study describes a Horodyskia-Nenoxites-dominated fossil assemblage from the chert of the Liuchapo Formation in Liujiata Village, western Hunan Province, which was deposited in a slope to basinal setting at the terminal Ediacaran and earliest Cambrian. In this assemblage, Horodyskia covers the longest range, possibly from the upper part of the Lower Ediacaran to the base of the Cambrian. Other members, including three types of Nenoxites, ?Helanoichnus, ?Shaanxilithes, and two affinity-undetermined forms, appear in the upper part of the Liuchapo Formation. The whole assemblage exhibits an upward increasing complexity and fossil abundance, which reaches its acme at a few meters below the Cambrian Xiaoyanxi Formation. Regardless of the lithology of the host rock, these fossils are exclusively preserved with clay minerals and devoid of any detectable concentration of carbon. Such a preservation mode is dominant among the known records of these fossils worldwide and possibly related to an unknown biological commonality of these organisms. Microscope observations support a body-fossil origin of Nenoxites and contradict the trace-fossil interpretation. Meanwhile, some large Horodyskia show features implying a close relation to testate amoebae. However, despite some resemblances, the differences between these fossils and any suggested relatives are considerable. Taking all the information together, we tentatively interpret this fossil assemblage as a diverse and complex deep-water community of protist-grade organisms. The worldwide occurrence of some members in this assemblage endows them with a biostratigraphic potential. Nevertheless, to establish such a tool, more quality data of fossil occurrences and referential radiometric ages, and a better understanding of the interplay between these organisms and sedimentary environments are required.