Faecal-material recycling in a deep-sea benthic ecosystem: evidence from trace fossils in middle Eocene deep-marine deposits of Georgia

Trace fossils rarely provide direct evidence of trophic interactions because unequivocal behavioural relationships between different tracemakers are seldom preserved. Here we document an unusual association between echinoid locomotion and feeding traces assigned to Scolicia and spiral to meandering structures (Spirophycus and Helminthopsis) interpreted as fossilized faecal strings of enteropneust (acorn) worms from middle Eocene deep-marine volcanogenic-sedimentary deposits of the Likani Suite, Achara–Trialeti Fold-and-Thrust Belt, Georgia. The echinoid traces repeatedly co-occur with Spirophycus and locally overlap with it, suggesting that the echinoids may have followed and exploited the underlying structures as localized sources of organic matter or associated microbial biomass. To quantify the observed geometric correspondence, the geometries of fossil traces and modern enteropneust faecal strings were compared using a morphometric workflow based on discrete Fréchet distance, multidimensional scaling, and hierarchical clustering. The analyses demonstrate substantial geometric overlap between modern and fossil spiral and meandering trajectories, supporting their morphological correspondence and, together with their repeated spatial association, the interpretation that some echinoid traces may have followed the course of enteropneust faecal strings. This association is consistent with either direct consumption of faecal material or exploitation of microbial communities associated with the faecal strings. Preservation of this association may have been favoured by episodic sedimentation and shallow burial in the deep-marine Achara–Trialeti Basin, which may have allowed echinoids to exploit recently buried faecal strings before subsequent burial preserved both structures.,These trace fossils provide rare evidence consistent with faecal-material recycling in a deep-sea benthic ecosystem and suggest that secondary exploitation of enteropneust faecal material by echinoids was developed already in the middle Eocene.