First discovery of trace fossils in the red beds of the cretaceous Chong'an Formation in Wuyishan National Park

The Cretaceous red beds serve as the foundational geological structure shaping the spectacular Danxia landforms, which define the impressive landscape of Wuyishan National Park. However, due to the lithological characteristics of these deposits, the fossil records within the Late Cretaceous Chong'an Formation red beds have traditionally been exceptionally sparse and poorly documented. Previous studies have only provided limited information on sporopollen fossils (palynomorphs) and lacked confirmed fossil records specifically within the Wuyishan National Park. This study offers the first comprehensive and systematic description of a significant ichnofauna found within the Chong'an Formation red beds at Dawang Peak, located in Wuyishan National Park, Fujian Province. Through detailed analysis, four distinct ichnospecies were identified within four ichnogenera: Arenicolites isp., Cochlichnus isp., Skolithos isp., and Planolites beverleyensis. This assemblage of trace fossils is mainly characterized by two primary types densely appearing on the bedding planes: vertically to subvertically oriented tubular burrows attributed to Skolithos and distinctive U-shaped burrows assigned to Arenicolites. Based on its composition, the assemblage can be formally classified as part of the Skolithos-Arenicolites ichnoassemblage. This ichnoassemblage provides crucial diagnostic evidence, indicating that the Chong'an Formation was deposited in a high-energy hydrodynamic environment, specifically as active fluvial sand bars or dynamic distributary channels in a continental river system. Furthermore, this ichnoassemblage exhibits an ecological pattern of "high abundance and low diversity". The trace makers are interpreted as typical opportunistic (r-selected) species, reflecting that benthic organisms in the study area during the early Late Cretaceous epoch likely existed under sustained high environmental stress. Potential stressors driving this adaptation may include periodic droughts and frequent disruptive flooding events. The significance of this research is multifaceted: 1) filling a critical gap in the previously non-existent fossil records for the Chong'an Formation and Wuyishan National Park; 2) providing essential ichnological evidence that enables a refined reconstruction of the specific sedimentary environments associated with these regional red beds; 3) enriching the recognized paleontological natural heritage value of Wuyishan National Park, a UNESCO World Natural Heritage site. Finally, the findings offer novel scientific insights crucial for understanding the complex paleogeographic configuration of the South China continental margin, the patterns of paleoclimatic changes, and the impacts of these large-scale environmental factors on terrestrial and aquatic biological communities during the early Late Cretaceous period.