WORMS ON FILM: UNDERSTANDING THE INFAUNAL COLONISATION OF ANOXIC, MATGROUND-DOMINATED EDIACARAN–CAMBRIAN SEDIMENTARY ENVIRONMENTS

Throughout the Phanerozoic, burrowing organisms have played a key role in controlling the physio-chemical conditions of marine benthic environments. Prior to the evolution of large burrowing organisms during the early Cambrian, seafloor sediments were predominantly anoxic, hosting abundant microbial matgrounds composed of a consortia of EPS-secreting microorganisms. The physical and chemical conditions in these environments were hostile to oxygen-dependent macro-organisms. Despite this, the fossil record shows a rapid increase in the diversity and complexity of burrows during the Cambrian, indicating that the earliest bioturbators were tolerant of low oxygen porewater conditions or were capable of modifying the environment within and around the burrow in their favor.

While the trace fossil record is crucial to a better understanding of this interval in Earth history, it cannot provide a complete insight into the adaptations and life history strategies that enabled early animals to colonize anoxic marine sediments. To overcome this, our study combines a geological investigation of early Phanerozoic metazoan-microbial interactions with experimental setups to determine how modern vermiform organisms behave in Precambrian-like low-oxygen sediments, including how these organisms modify their burrow morphology in response to such conditions.

Our investigation into the Early-Ordovician metazoan-microbial interactions preserved in Bell Island, Newfoundland, documents settings in which the destruction of microbial matgrounds can be directly linked to the presence of dense, homogeneous assemblages of simple, near-surface, trace fossils, which are succeeded by an ichnocoenosis composed of diverse deeper-tier trace makers. Similar trends are seen in experimental setups involving anoxic sediments, wherein opportunistic organisms with simple burrow morphologies thrive in anoxic sediments, while other animals must alter their burrowing behavior to survive.

These results support field observations on a global scale which indicate that the colonization of latest Ediacaran sediments was initiated by simpler, possibly opportunistic animals that were capable of engineering sedimentary environments in a manner conducive to the evolution of large, more oxygen-dependent burrowing metazoans.