Criteria for differentiating microbial-caddisfly bioherms from those of marine polychaetes in a lacustrine setting: Paleocene second member, Funing Formation, Subei Basin, East China

Abstract

Paleogene carbonate successions from lacustrine basins across East China yield microbial bioherms containing abundant agglutinated fossil tubes. Historically, these have been interpreted as the dwellings of polychaetes introduced by marine incursions. This is problematic, because these tubes are associated with abundant non-marine fauna and algae. Diverse and exceptionally well-preserved fossil tubes from carbonate microbial bioherms were studied in outcrops and drill cores of the second member of the Paleocene Funing Formation, Subei Basin, East China. Detailed macro-, meso-, and micro-structural investigations of these tube-dominated bioherms reveal that the putative “marine polychaete tubes” better fit the morphologies and construction styles of lacustrine caddisfly (Insecta: Trichoptera) larval cases, eliminating the need for a marine influence on this lacustrine setting.Individual caddisfly cases are commonly 10–25 mm in length and 1–2.5 mm in diameter, and form slightly curved, tapered tubes or coiled spirals. Twelve tubular case construction styles were further identified, based on caddisfly-case wall structures and dominant grain types. Caddisfly cases are commonly single-walled, with rarer nested instar cases comprising double- or triple-walls of different sizes. Most cases are fully agglutinated with neatly arranged, similarly sized and closely connected particles of clastic grains, carbonate particles, plant fragments, and phosphatic fish bones. Sparsely agglutinated cases are also present but much rarer, and conform to the silk larval cases of caddisflies.The caddisfly cases appear as gregarious aggregations within carbonate microbial-bioherms. The microbial-caddisfly bioherms in this study are commonly developed on carbonate hardground or mudstone within the wave-washed, high-energy, shallow-lacustrine littoral zone and can be traced along the western paleolake margin of the Jinhu Depression of the Subei Basin for up to 60 km. The bioherms are composed of repeated microbial-caddisfly couplets, made up of randomly arranged and similarly sized caddisfly cases encrusted by domed, planar, columnar or cauliflower-like microbial layers. Cyclic microbial-caddisfly couplets fit well with caddisfly biology but are not known to occur with marine polychaetes. This is the first report of microbial-caddisfly bioherms in Cenozoic strata of China, and is the oldest Cenozoic record of caddisflies in China.