Early Triassic Trace Fossils from North China Fluvial-Lacustrine Settings: Implications for Biotic Recovery Following the End-Permian Mass Extinction

The end-Permian mass extinction (EPME) represents the most biotic crisis during the Phanerozoic. Compared with marine records, constraints on the collapse in the EPME and recovery of terrestrial ecosystems in the Early Triassic remains limited. This study focuses on the Lower Triassic Heshanggou Formation in North China, where a suite of trace fossils has been identified, including dwelling traces (Skolithos, Arenicolites, Palaeophycus), locomotion traces (Diplichnites, Kouphichnium), feeding traces (Planolites, Taenidium, Scoyenia), and grazing traces (Helminthoidichnites). Based on analyses of ichnodiversity, ichnodisparity, bioturbation index, burrow size, and ichnotiering, the ichnoassemblage is characterized by moderate diversity but relatively high functional differentiation. Vertical tiering is established, dominated by shallow-tiering structures. In addition, the increase in vertical burrows and the emergence of more complex burrow architectures indicate intensified bioturbation, reflecting ongoing biotic recovery. By integrating a large dataset of terrestrial fossils across the Permian–Triassic transition from the peri-Palaeotethys region, we infer that North China was located along the northeastern margin of the Palaeotethys Ocean during the Early Triassic. The recovery of terrestrial ecosystems in this region was strongly constrained by environmental factors, representing by biotic communities evolve from low-diversity opportunistic assemblages to systems exhibiting vertical ecospace expansion. This recovery pattern was likely controlled by palaeogeographic configuration and climatic zonation associated with Palaeotethys evolution. Overall, Early Triassic terrestrial ecosystem recovery shows pronounced regional variability and strong environmental control, providing key constraints for understanding the reconstruction and increasing complexity of terrestrial ecosystems during the Mesozoic.