Sudden and extreme hyperthermals, low-oxygen, and sediment influx drove community phase shifts following the end-Permian mass extinction

• Thresholds in environmental change can result in phase-shift community.
• Extreme temperature, low oxygen, and high sedimentation impact shallow marine fauna.
• Re-envisioned anachronistic fauna of the Early Triassic as a phase shift community
• Microbialites and mid-sized metazoan fauna are unique and persistent in the Early Triassic.

We present a correlated record of carbon isotope geochemistry and sedimentological analysis for the Lower Triassic Werfen Formation from the Italian Dolomites. Macro- and mid-sized fossil diversity, ecology, and climate sensitivity are included to provide an integrated account of the benthic response to paleoenvironmental change. Novel communities developed in the wake of the mass extinction during pervasive fluctuations in environmental conditions. In the sedimentary sequences of the Werfen Formation of the Italian Dolomites, microbialites, microconchids, foraminifera, and ubiquitous flat-clams, formed a complex community within the first 500,000 years. Later, increased sea-surface temperatures and inundation of the seafloor with siliciclastic sediments favored infaunal bivalves and microgastropods. Persistent trends in the environment can produce directional, often irreversible, community shifts defined here as phase shifts. Phase shifts in ecosystem structure can be driven by environmental shifts across threshold boundaries to produce an “abrupt and dramatic” novel community composition, a phenomenon readily observed during the Early Triassic. Previously described “disaster forms” including flat clams, microconchids, foraminifera, and microbialites are re-envisioned as a phase shift community. We hypothesize that the unique, persistent, and reoccurring microbialite and mid-sized fauna assemblages observed during the initial recovery from the end-Permian mass extinction and re-appearing throughout the Early Triassic typify a phase shift community. In the Smithian, infaunal bivalves and microgastropods represent a second phase shift community developed in response to a persistent, directional rise in sea-surface temperatures and enhanced sediment influx. We compare and contrast phase shifts with other models for ecosystem recovery including trophic, competition, and Earth System Succession.