A Cambrian fossil from the Chengjiang fauna sharing characters with gilled lobopodians and radiodontans

Background: Lobopodians are worm-like animals with simple legs. Probably representing a grade of organization, rather than an explicit clade, some lobopodians are thought to have given rise to both Euarthropoda and Onychophora (velvet worms). Another subset has been referred to as gilled lobopodians, and are characterized by flap-like appendages along the trunk and large, raptorial forelimbs. These animals probably include the ancestors of another important Cambrian group, the Radiodonta: large predatory or filter-feeding stem-arthropods such as Anomalocaris.

Results: Parvibellus atavus gen. et sp. nov. from the Early Cambrian Chengjiang fauna of China is a small fossil preserving a distinct cephalic region bearing a pair of lateral projections and a circular, ventral mouth. The trunk bears eleven pairs of flap-like appendages and a short pair of terminal projections. A circular ventral mouth is also seen in Radiodonta and in some gilled lobopodians. Parvibellus atavus, gilled lobopodians and radiodontans also share the character of flap-like appendages along the trunk. However, the new fossil differs from radiodontans and gilled lobopodians by its small size and the absence of enlarged and/or raptorial frontal appendages. It also differs from gilled lobopodians in lacking ventral lobopod limbs, and from radiodontans in lacking stalked eyes.

Conclusions: Parvibellus atavus expresses a unique combination of characters among Cambrian arthropods, and could be part of an early radiation of nektonic stem-Euarthropoda. Lobopodians have emerged as a diverse grade of proto-arthropods ('worms with legs'), walking on the substrate of the early Palaeozoic seas. The new fossil hints at a similarly diverse fauna of nektonic (swimming) stem-group arthropods in the Cambrian, from which gilled lobopodians and radiodontans may have evolved.