Biogenic brachiopod shell concentrations from the Hongjingshao Formation (Cambrian series 2, Stage 3) in Malong area of eastern Yunnan, South China

Skeletal concentrations are conspicuous taphofacies in the marine record and have been recorded from a wide range of sedimentary facies in Phanerozoic strata, providing baseline data for taphonomy, stratigraphy, sedimentology and palaeoenvironmental reconstructions. Most studied shell concentrations come from post-Cambrian deposits. Here, we document monospecific intrinsic biogenic shell concentrations formed at various stratigraphic levels by the gregarious clustering behavior of the lingulate brachiopod Palaeobolus yunnanensis (Rong, 1974) from the Cambrian Stage 3 Hongjingshao Formation in eastern Yunnan, South China, which represents the oldest-known record of an intrinsic biogenic brachiopod shell concentration in the stratigraphic record. Two types of shell concentrations are represented. Type A concentrations are composed of closely clustered (>28 individuals per square decimeter) in situ complete shells (intrinsic biogenic) buried rapidly near the sediment water interface and forming a distinctive hard pavement in the otherwise completely soft, muddy environment. Type B concentrations are composed of sharply fragmented and broken (but largely unabraded) shells affected by local low energy surface and oscillation currents over longer residence times with destruction and re-deposition exacerbated by surface bulldozing and infaunal bioturbation (extrinsic biogenic). The subsequent occurrence of intrinsic biogenic brachiopod shell concentrations across Laurentia, Siberia, NW Gondwana and South China mostly during Cambrian Stage 4 attests to the initial rise of gregarious aggregation in epifaunal brachiopods during the early Cambrian and the development of new hard substrates in the shallow marine benthos. These relatively rare early Cambrian hard substrates provided new ecospace in soft-bottom benthic communities and were the precursors to the dominant calcareous brachiopod shell concentrations which characterize shallow to deep marine substrates during the Great Ordovician Biodiversification Event.