A burrowing annelid from the early Cambrian.

Soft-bodied fossils of annelids from the Cambrian are relatively rare but provide vital insights into the early evolution and diversification of annelids. Here we describe a new annelid, Xiaoshibachaeta biodiversa gen. et sp. nov., from the early Cambrian (Stage 3) Xiaoshiba biota of Kunming, Yunnan Provence, China. This worm is obliquely oriented in the sediment, and is characteristic of a cephalic cage-like structure formed by the anteriorly directed parapodia and long chaetae of chaetiger 1, strongly suggesting an endobenthic lifestyle. This first report of an annelid worm from the Xiaoshiba biota provides the earliest known plausible evidence of burrowing behaviour in Annelida. Phylogenetic analyses recover X. biodiversa in the polytomy with other crown-group Annelida, indicating that the evolution of cephalic cage in Annelida is most likely convergent.