Some species of spatangoid echinoids possess an intestinal caecum containing sulphide-oxidizing bacterial symbionts. Echinocardium cordatum and Schizaster canaliferus burrow deeply and ingest sulphide-rich, anoxic sediment, supplementing their diet with seafloor detritus. The chemoautotrophic synthesis of nutrients would allow echinoids to burrow deeply beneath the seafloor. Deep-tier Scolicia-group trace fossils therefore may be considered Agrichnia.