Discovery and significance of ancient cod fossils in hydrothermal fluid deposition areas: a case study of Chang 7-3 from the Triassic Yanchang Formation in the Ordos Basin

Large depression-type lake basins developed during the Late Triassic in the Ordos Basin, and hydrothermal activity at the bottom of these lakes was frequent. Here, we report an example from the Chang 7 hydrothermal deposition area of the Triassic Yanchang Formation in the Ordos Basin, China: Yanchanglepis Tongchuanlepis gen. et sp. nov. Its general morphological characteristics include a long spindle-shaped body, a long dorsal fin base, a rear-facing gap between the ventral fin and anal fins, thick fins, small decorative spine scales, inclined hanging bones, high gillcap bones, small scales and a number of horizontal scales. Its unique composition, morphology and distribution characteristics have become the key to exploring the habitat of organisms living in extreme environments. This discovery provides a fossil record of cod evolution in Late Triassic continental sedimentary basins. The study is expected to provide insights into the life forms of various extreme organisms in hydrothermal environments and further our understanding of nutrient cycling and ecosystem evolution in deep lakes.