Hallucigenia ’s diet illuminates the feeding ecology of Cambrian lobopodians

The armoured lobopodian Hallucigenia sparsa embodies the seemingly uncanny nature of the animals that evolved during the Cambrian Explosion over 500 million years ago. Initially regarded as an evolutionary oddball, the exceptional preserved anatomy of H. sparsa has been substantially revised, leading to a better understanding of its relationships with other lobopodians and phylogenetic affinities with extant panarthropod phyla. However, the ecology and behaviour of H. sparsa remain largely enigmatic owing to the difficulties of interpreting its functional morphology and the perceived lack of modern analogues. Restudy of a composite fossil assemblage from the middle Cambrian Burgess Shale demonstrates swarm-like behaviour of several small H. sparsa individuals scavenging on a dead ctenophore. The lack of grasping, masticatory, or piercing mouthparts in H. sparsa points to suction feeding as a viable strategy to consume the gelatinous carcass. Reassessment of the morphology of H. sparsa reveals functional analogues shared with extant pycnogonids, including an elongate anterior end with a terminal mouth opening and enlarged foregut chambers lined up with sclerotized denticles. These observations suggest that suction feeding is ancestral for stem-group Onychophora and highlights the critical trophic role of small-bodied armoured lobopodians as degraders of soft-bodied carcasses in Cambrian benthic ecosystems.