Brachiopod-Dominated Shell Concentrations from Cambrian Series 2 in 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. Nevertheless, brachiopods also produced a wide variety of monospecific or paucispecific brachiopod shell concentrations from Cambrian Series 2 in eastern Yunnan, South China. Monospecific intrinsic biogenic shell concentrations occurred at various stratigraphic levels, formed by the gregarious clustering behavior of the lingulate brachiopod Palaeobolus yunnanensis from the Cambrian Stage 3 Hongjingshao Formation. This represents the oldest-known intrinsic biogenic brachiopod shell concentration in the stratigraphic record. Four types of biogenic brachiopod shell concentrations occur in the overlying Wulongqing Formation, including Linnarssonia, Eoobolus, Neobolus and Westonia. Those brachiopods formed a geographically widespread phenomenon of many monospecific or paucispecific shell-bed concentrations during Cambrian Age 4. Previous studies recognized taphonomic feedbacks in the early Palaeozoic, whereby post-mortem hard parts can promote the processes associated with the Cambrian substrate revolution by hardening the mixed soft substrate and thus providing a firm or hard substrate for firm-sediment dwellers to colonize. Whilst most pavement-type substrates are formed by the postmortem remains of skeletal debris, there are also instances where living skeletonized organisms provide a hard substrate in benthic environments. A unique, enduring kleptoparasitic life association between tube- dwelling organisms encrusted on densely clustered shells of Neobolus wulongqingensis has been recently documented from the Cambrian (Stage 4) Guanshan Konservat-Lagerstätte in the Wulongqing Formation. Associations of sessile epibenthic brachiopods encrusted by kleptoparasitic tube-dwelling worms, together with infaunal paleoscolecidans, and swimming Tuzoia were discovered in the Wulongqing Formation. This community represents a unique, brachiopod-supported, well-developed vertically stratified benthic community, composed of multiple phyla filling various ecological niches, and exhibits a complex trophic network and community structure with increased ecospace utilization during Cambrian Age 4.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.