A burrowing annelid from the early Cambrian

Abstract

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.

Keywords: Annelida; Xiaoshiba biota; burrowing; cephalic cage; early Cambrian.

Figure 1.

Xiaoshibachaeta biodiversa gen. et sp. nov. from the Xiaoshiba Lagerstätte, holoptype YKLP 12466, part. (a) Light microscopic image of the specimen, anterior is up. (b) Fluorescence image of the specimen. (c) Interpretive line drawing of the specimen; numbering denotes the parapodia. (d,e) Three-dimensional information revealed by KEYENCE microscopy, showing the worm penetrating obliquely into sediment. Abbreviations: ch, chaetae; ch1, first chaetae; h, head; pa1, first parapodia; pp, palps; sb, segmental body. Scale bars: 1 mm.

Figure 2.

Detail of holoptype YKLP 12466, part. (a) Detail of anterior part of the specimen. (b) Anterior part of the specimen showing the elongate chaetae with side small spines (arrowed) of first chaetiger. (c) Well-preserved short parapodia with two bundles of capillary chaetae. (d) Detailed morphology of biramous parapodia and capillary chaetae. (e) Close-up of boxed area in (d), showing the detail of one chaeta with fine lateral spines (arrowed). (f–h) SEM-EDS images of the anterior part of the specimen. (f) Backscatter image. (g) Elemental mapping for carbon. (h) Elemental mapping for iron. (a,c) Close-ups of boxed areas in figure 1a. (b,d) Close-ups of boxed areas in figure 1b. Abbreviations: ch1, first chaetae; ch2, second chaetae; h, head; pa, parapodia; pa1, first parapodia; pp, palps; sb, segmental body. Scale bars: (a–d, f–h) 500 µm; (e) 100 µm.

Figure 3.

Phylogeny and reconstruction of Xiaoshibachaeta. (a) Simplified strict consensus tree of maximum parsimony under implied weight (k = 10, 16 most-parsimonious trees (MPTs), 874 steps, consistency index (CI) = 0.295, retention index (RI) = 0.647). Bootstrap support values (>50) are displayed. (b) Majority rule consensus tree of Bayesian inference. Numbers at nodes are posterior probabilities. Scale bar represents the expected changes per site. (c) Reconstruction of the anterior part of Xiaoshibachaeta.