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

ABSTRACT

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 Kouphichnium isp., different Undichna ichnospecies representing U. bina, U. britannica, U. ?quina, U. unisulca and U. isp., and to an indeterminate trackway. Kouphichnium 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 Kouphichnium trackmaker if compared to known Triassic body fossils. The ichnofauna is described and the ichnotaxonomy of Undichna 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.

KEYWORDS:

• Xiphosura
• kouphichnium
• Undichna
• Wayaobu Formation
• Ordos Basin

Acknowledgments

We thank Ms. Sui-qin Zhao for providing logistical support for the fieldwork, and thank Chunyong Chou, Yusheng Liu and Yu Zhang for their participation in Yulin field study, Kamitoge for his reconstruction image. The manuscript largely benefited from the constructive review and comments of Spencer G. Lucas. This research was funded by the National Natural Science Foundation of China (No. 41888101, 41790455).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/08912963.2022.2162398

Additional information

Funding

This work was supported by the Data Center of Management Science, National Natural Science Foundation of China - Peking University [41790455,41888101].