Secular variations of ichnofossils from the terrestrial Late Permian-Middle Triassic succession at the Shichuanhe section in Shaanxi Province, North China

摘要

Ichnological studies are exceptionally important for reconstruction of ecosystems in the aftermath of mass extinctions, but they have only rarely been undertaken from strata deposited in continental settings during the Permian-Triassic transition. This paper systematically documents seventeen ichnogenera and three ichno-types coupled with plant fossils (mainly equisetaceous stems and roots) from the fluvial lacustrine Late Permian to Middle Triassic Shichuanhe section in North China. These ichnofossils constitute several ichnoassemblages, including resting traces (Lockeia), locomotion traces (Diplichnites, Kottphichnium, Protovirgularia), dwelling traces (Arenicolites, Camborygma, Macanopsis, Palaeophycus, Skolithos), grazing traces (Gordia, Helminthoidichnites, Monomorphichnus) and feeding traces (Planolites, Scoyenia, Spongeliomorpha, Taenidium) and some of them were scarcely reported in the coeval sediments of North China. The ichnological criteria, such as ichnodiversity, intensity of bioturbation, and burrow size and depth, exhibit a significant reduction in the latest Permian rocks, accompanied by simplification of trace maker ethology. These signals initially rebound in the early Early Triassic, when the behavioral generalists opportunistically occupied the empty niches and constructed simple but deep Skolithos. A full reconstruction of continental ecosystems should take place in the late Early Triassic to Middle Triassic, with high ichnodiversity, intensive bioturbation and behavior complexity of trace producers. The small, simple but deep burrows occurring in the early Early Triassic would be the ecological responses of the animals to the lethal heat in the terrestrial facies.