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7The geological persistence of biotic assemblages and their reorganization or destruction by mass extinc-8 tions are key features of long-term macroevolutionary and macroecological patterns in the fossil record. 9These events affected biotic history disproportionately and left permanent imprints on global biodiversity. 10Here we hypothesize that the geological persistence and incumbency of paleocommunities and taxa are 11 maintained by patterns of biotic interactions that favour the ecological persistence and stable coexistence 12 of interacting species. Equally complex communities produced by alternative macroevolutionary histories, 13 and hence of different functional structure, may support less stable species coexistence, and are therefore 14 less persistent. However, alternative communities with the same functional structure as a persistent pa-15 leocommunity, but variable clade richnesses, tend to be as or more stable than observed palecommuni-16 ties, thus demonstrating that geological persistence is not the result of constrained patterns, or ecological 17 locking. Numerically modeled food webs for seven tetrapod-dominated paleocommunities spanning the 18 traditionally-recognized Permian-Triassic boundary in the Karoo Basin of South Africa, show that incum-19 bency before the Permian-Triassic mass extinction was maintained by a dynamically stable, community-20 level system of biotic interactions, thereby supporting the hypothesis. The system's structure was lost 21 through successive extinction pulses, and replaced initially by a rich but geologically ephemeral Early Tri-22 assic fauna, which itself was replaced by a novel Middle Triassic community with renewed incumbency. 23The loss of persistence and incumbency, therefore, did not result simply from the extinction of species; in-24 stead the largest declines were accompanied by the addition of new species to the system in the earliest 25 aftermath of the event. We therefore further hypothesize that ecological reorganization and evolutionary 26 innovation in the wake of mass extinctions play key roles in the destruction of highly stable, preexisting 27 systems of biotic interaction. In the case of the Karoo Basin paleocommunities, we estimate that a return to 28 stable interactions, and thus incumbency, was achieved in approximately 4-17 Ma. 29

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The Permo-Triassic mass extinction (PTME) is often implicated in the transition from the Paleozoic evolutionary fauna (PEF) to the modern evolutionary fauna (MEF). However, the exact timing and details of this progression are unknown, especially regarding the vacating and filling of functional ecological space after the PTME. Here, we quantify the functional diversity of middle Permian and Early Triassic marine paleocommunities in the western US to determine functional re-organization in the aftermath of the PTME. Results indicate that while the PTME was selective in nature, many new Triassic taxa either re-filled functional roles of extinct Permian taxa or performed the same functional roles as Permian survivors. Despite this functional overlap, Permian survivors and new Triassic taxa differed significantly in their relative abundances within those overlapping functions. This shift in numerical emphasis, driven by an increase in abundance towards more MEF-style traits, may represent a first step in the transition between the PEF and MEF. We therefore suggest that the extreme impact of the PTME had significant and permanent re-organizational effects on the intrinsic structure of marine ecosystems. Early Triassic ecosystems likely bridged the gap between the Paleozoic and modern evolutionary faunas, as newly originated Triassic taxa shared ecospace with Permian survivors, but shifted functional emphasis.