Worm-lobopodian assemblages from the Early Cambrian Chengjiang biota: Insight into the “pre-arthropodan ecology”?

Well-preserved assemblages dominated by scalidophoran worms (Cricocosmia, Mafangscolex) and lobopodians (Paucipodia, Microdictyon, Onychodictyon and undetermined xenusiid) are described from the Early Cambrian Chengjiang biota of China. This fossil material provides the opportunity to reassess key aspects of the functional morphology and lifestyles of the two groups. Both lobopodians exemplified here by Paucipodia and worms relied on a hydrostatic skeleton for support and locomotion and have direct functional analogues in extant onychophorans and priapulids, respectively. Retractor-like muscles attached to terminal claws probably assisted leg elevation in the lobopodians. Whereas lobopodians were essentially walkers and climbers, Cricocosmia and Mafangscolex were most likely subhorizontal burrowers. Their external ornament (phosphatic circular microplates or spiny sclerites) is interpreted as a possible adaptation for tunnelling close to the water-sediment interface. These assemblages seem to represent in-situ faunal associations and suggest that lobopodians and worms might have shared the same habitat and possibly exploited the same detrital food source. However, no evidence indicates that they interacted in predator-prey relationships. Finally, we hypothesize that during a time interval preceding Cambrian Stage 2, scalidophoran worms and lobopodians were the prevalent ecdysozoan component of early animal communities, and introduce the notion of a possible “pre-arthropodan ecology”.