How long does a brachiopod shell last on a seafloor? Modern mid-bathyal environments as taphonomic analogues of continental shelves prior to the Mesozoic Marine Revolution

Carbonate skeletal remains are altered and disintegrate at yearly to decadal scales in present-day shallow-marine environments with intense bioerosion and dissolution. Present-day brachiopod death assemblages are invariably characterized by poor preservation on continental shelves, and abundant articulated shells of brachiopods with well-preserved brachidia are thus not expected to be preserved if not rapidly buried. However, such preservation is paradoxically observed in shallow-water Paleozoic and Mesozoic brachiopod assemblages. Here, we show that a bathyal death assemblage time-averaged to several millennia (Adriatic Sea) consists of sediment-filled articulated shells of Gryphus vitreus with complete brachidia. Postmortem age distributions indicate that disintegration half-lives exceed several centuries (~500-1,700 years). The high frequency of articulated but centuries-old shells (>50%) and the fitting of taphonomic models to postmortem ages indicate that disarticulation half-life is unusually long (~200 years). Rapid sediment filling of shells (1) inhibited disarticulation, loop fragmentation and colonization by coelobites and (2) induced precipitation of ferromanganese oxides at redox fronts within shells. Sediment-filled articulated shells, however, still resided at the sediment-water interface as indicated by encrusters and sponges that infested them after death. Sediment-filled shells disintegrated through bioerosion and wear when residence time in the taphonomically active zone exceeded ~2,000 years. We suggest that the articulation paradox is driven by the Mesozoic Marine Revolution (MMR) that escalated predation, bioturbation and organic matter recycling, all intensifying shell disintegration. A scenario with slow disarticulation in bathyal environments can be an analogue of conditions leading to preservation of articulated shells in shallow-water assemblages prior to the MMR.