Burrows filled with faecal pellets from the Cambrian (Stage 4) Guanshan biota of South China and their palaeoecological implications

Recent excavations of the Guanshan biota at the Shijiangjun section in the eastern Yunnan Province, South China, have revealed many well-defined faecal pellets and related trace fossils. These can be assigned to three morphotypes, which have affinities with the ichnogenera Tomaculum, Alcyonidiopsis and Tubulichnium. Morphotype 1 is an unbranched burrow filled with randomly distributed faecal pellets. Morphotype 2 is a string-shaped burrow without branching, containing intensely compacted faecal pellets. Morphotype 3 comprises unbranched burrows with a fill of faecal pellets and a central cavity. The ellipsoidal faecal pellets in morphotype 1 and 2 can be assigned to Coprulus oblongus. They are morphologically similar to the excrements produced by modern deposit-feeding polychaetes, as well as the gut content of early Cambrian polychaete worms (Guanshanchaeta felicia) from the Guanshan biota and priapulid worms (Selkirkia) from the Xiaoshiba Lagerstätte. This analogy suggests polychaetes and priapulid worms as potential producers of these burrows and that a diverse range of digestive methods had already been established in the Cambrian Stage 4 ca. 510 Ma ago. Morphotype 3 is interpreted to be related to the storage of faecal pellets and possibly for cultivation of bacteria, which would represent an early case of this behaviour. The discovery of this diverse suite of burrows with faecal pellets indicates that the origin of diverse deposit-feeding strategies in worms can be traced back to at least the Cambrian Stage 4.