A diverse trackway-dominated marine ichnoassemblage from the Lower Triassic in the northern Paleotethys: Ichnology and implications for biotic recovery

We document a diverse ichnoassemblage from marine interbeds of the Lower Triassic terrestrial succession in the Houzhougongmiao (HZGM) section of Shaanxi Province, northwestern China. Integrated biostratigraphic data (bivalve, palynology and conchostracan) reveals that the ichnofossil-bearing marine beds are Griesbachian (Induan, Early Triassic) in age. The marine interbeds are interpreted to be the result of the earliest Triassic transgression of the Paleotethys Ocean northward onto the southern margin of the North China Craton. The HZGM ichnoassemblage includes 17 ichnospecies in 16 ichnogenera and is dominated by shallow-tier Asteriacites and Biformites produced by ophiuroids, the scratch marks or trackways Dimorphichnus, Diplichnites, and Monomorphichnus produced by arthropods, and a rare occurrence of the fish swimming trace Undichna. Of these, the hook-shaped imprints Biformites, representing the moving arm impressions of ophiuroids, are reported for the first time from the Lower Triassic. These trace-makers are interpreted to have lived in a low energy, semi-restricted, shallow embayment environment. Although possessing relatively high ichnodiversity, the HZGM ichnoassemblage differs clearly from other coeval diverse ichnocoenoses in the lack of complex burrow forms (i.e. Thalassinoides, Diplocraterion, or Rhizocorallium) and a mixed layer, and in having abundant shallow tiers. In addition, high ichnodiversity may occur at both high and low latitudes during the Griesbachian (or Induan). Accordingly, the resting trace or trackway-dominated ichnoassemblage from the HZGM section may represent the initial recovery of the trace-maker ecosystem after the Permian-Triassic mass extinction. Trace-makers such as arthropods and ophiuroids were perhaps opportunistic organisms that proliferated in marginal marine settings when other biota were still suffering post-extinction biotic depletion and environmental stresses.