Phosphatized embryo-like fossils from the Chang 73 sub-member, middle-upper Triassic Yanchang Formation, Ordos Basin, northwest China: Affinity, preservation and paleoecological implications

Abstract

During the Middle and Late Triassic, Ordos Basin in the Northwestern China deposited the terrestrial freshwater lacustrine hydrocarbon source Chang 7₃ sub-member. The composition of planktonic algae and the ecosystem state of the lake are the key to elucidate the high organic matter content of the Chang 7₃ sub-member. An abundant of phosphatized spherical microfossils in the Chang 7₃ sub-member are discovered. A new genus and species Yanchangparapandorina inornata gen. et sp. nov. is established, which may be animal embryo-like fossil at the cleavage stage from aquatic arthropod based on its shape, envelope, polygonal divisions and size. This is the first report of phosphatized preserved animal embryo-like fossils other than the Ediacaran and the early Cambrian periods. A large amount of pyrite framboids and collophnite have been found associated with these phosphatized microfossils. The diameter size of these pyrite framboids indicates that the bottom water of the lake basin is suboxidized and the abundant presence of collophnite suggests that the pores among sediments are rich in phosphate fluid. It is speculated that suboxidized water-bottom environments and phosphate fluid in pores among sediments are beneficial for the preservation of embryo-like fossils through permineralization of phosphate-rich fluid. Additionally, the extensive preservation of animal embryo-like fossils at the cleavage stage, possibly as resting eggs, is likely related to fluctuations of paleoenvironmental conditions of the terrestrial lacustrine. Therefore, the discovery of Y. inornata gen. et sp. nov. provides a new perspective for the ecosystem state and high productivity of the terrestrial lacustrine during the depositional period of the Chang 7₃ sub-member of the Ordos Basin.