Complex benthic ecosystem engineering in the Early Triassic: The trace fossil record as a tool for understanding ecosystem function after mass extinction events

The trace fossil record is an important paleoecological archive which records the diversity and complexity of organisms’ behaviors through time – critical information not necessarily preserved in the body fossil record. More specifically, trace fossils preserve the record of bioturbation through time. Bioturbation is a key ecosystem engineering behavior in modern marine environments due to its effects on substrate rheology, sediment redox chemistry, benthic nutrient cycling, and ecospace availability. Thus, the deep time record of bioturbation is a useful tool for understanding benthic ecosystem function at critical intervals in Earth History. Here, we present the first analysis specifically focused on changes in ecosystem engineering behaviors across a mass extinction boundary. The end-Permian mass extinction (251 Mya) was the largest of the ‘Big 5’ Phanerozoic mass extinction events, during which global warming, ocean anoxia, ocean acidification, and habitat loss resulted in an estimated loss of 81% of all marine species and prolonged instability of biogeochemical cycles. We compiled a dataset of shallow marine trace fossil occurrences from previously published literature and use two ecosystem engineering analysis frameworks to understand how behavioral diversity, complexity, and environmental impact changed in response to the mass extinction event. We report two key findings: first, that high-impact and complex three-dimensional network burrows persisted across the mass extinction boundary, and second, that post-extinction bioturbation was limited to more shallow tiers in the sediment. We conclude that bioturbation may have played a critical role in maintaining local environments and biogeochemical cycling in the wake of the end-Permian mass extinction, particularly where deepest three-dimensional network burrowing persisted. Moreover, because the end-Permian mass extinction is considered to be analogous to our current state of climate change and biotic crisis, these results may have useful implications for how changes in bioturbation behaviors will play a role in the future of benthic ecosystems.