The Balanoglossites ichnofabrics in the aftermath of the Permian-Triassic mass extinction

Abstract

Trace fossils are widely distributed in pre-extinction Permian successions, and are also relatively common in the Lower Triassic strata. They have been used for evaluating the recovery process of marine infaunal ecosystem following the Permian-Triassic (P-Tr) mass extinction. However, one complicated ichnogenus Balanoglossites has not been clearly recognized and described in the Early Triassic previously, and its importance in revealing biotic recovery in the aftermath of the P-Tr mass extinction has not been evaluated. Here we documented Balanoglossites ichnofabrics from the Lower Triassic Yongningzhen Formation of the Yongningzhen section from Guizhou Province, South China. Balanoglossites is characterized by three-dimensional complex burrow system with Y-shaped and J-shaped galleries and multiple openings, and the herein described form is assigned to B. triadicus. Microstructures of Balanoglossites under SEM and EDS revealed oxidized pyrite framboids occurred merely near the burrow wall, which may be related to activities of the trace maker and symbiotic microorganism such as sulphate-reducing bacteria. Raman spectrum mapping showed that organic matter was enriched along the burrow boundaries of Balanoglossites, but was lacking in the burrow fill. This pattern may result from the impregnation of the burrow walls with organic mucus and bioirrigation by the trace maker. Polychaetes are considered as the most possible trace makers of Balanoglossites, and they were opportunistic pioneers that proliferated in the Early Triassic. The low ichnodiversity of Balanoglossites-bearing strata contrasts with coeval ichnoassemblages, which probably results from intense bioturbation by deeper-tier burrows that prevents the preservation of surficial to shallow-tier trace fossils. Although Balanoglossites is a complex burrow, its occurrence in the Early Triassic is not an indicator of the full recovery of infaunal ecosystem.