The Earth has been beset by many crises during its history, and yet comparing the ecological impacts of these mass extinctions has been difficult. Key questions concern the kinds of species that go extinct and survive, how communities rebuild in the post-extinction recovery phase, and especially how the scaling of events affects these processes. Here, we explore ecological impacts of terrestrial and freshwater ecosystems in three mass extinctions through the mid-Phanerozoic, a span of 121 million years (295--174 Ma). This critical duration encompasses the largest mass extinction of all time, the Permian--Triassic (P--Tr) and is flanked by two smaller crises, the Guadalupian--Lopingian (G--L) and Triassic--Jurassic (T--J) mass extinctions. Palaeocommunity dynamics modelling of 14 terrestrial and freshwater communities through a long sedimentary succession from the lower Permian to the lower Jurassic in northern Xinjiang, northwest China, shows that the P--Tr mass extinction differed from the other two in two ways: (i) ecological recovery from this extinction was prolonged and the three post-extinction communities in the Early Triassic showed low stability and highly variable and unpredictable responses to perturbation primarily following the huge losses of species, guilds and trophic space; and (ii) the G--L and T--J extinctions were each preceded by low-stability communities, but post-extinction recovery was rapid. Our results confirm the uniqueness of the P--Tr mass extinction and shed light on the trophic structure and ecological dynamics of terrestrial and freshwater ecosystems across the three mid-Phanerozoic extinctions, and how complex communities respond to environmental stress and how communities recovered after the crisis. Comparisons with the coeval communities from the Karoo Basin, South Africa show that geographically and compositionally different communities of terrestrial ecosystems were affected in much the same way by the P--Tr extinction.
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Methane seep deposits, comprising large, carbonate-rich mounds formed from hydrocarbon seepage, were widely distributed in the Late Cretaceous Western Interior Seaway (WIS) of North America. Well-preserved, methane-derived authigenic carbonates (MDACs) from these deposits have been shown to retain petrological, paleontological, and geochemical imprints of their ancient depositional setting, all of which are important for understanding the dynamics and evolution of the shallow, epeiric WIS. To better characterize the environmental conditions of WIS seeps, we applied clumped isotope paleothermometry to magnesium calcite MDAC samples from five seep localities in the upper Campanian Pierre Shale, South Dakota, USA. We measured 21 subsamples, including 18 micritic carbonates and demonstrated apparent clumped isotope equilibrium between MDACs and WIS bottom waters. Extreme 13C depletion in most samples (Î13C ranging to â45.44â) indicates they were precipitated with oxidized methane as a major source of dissolved inorganic carbon, which itself implies a close association with ancient methanotrophic metabolism. The average clumped isotope paleotemperature from the micritic carbonates is 23 Â 7 ÂC (1Ï standard deviation), which agrees with bottom water paleotemperatures inferred from Î18O measurements of MDACs and well-preserved mollusk shells at similar localities in the WIS. The calculated average Î18Ow value for these samples is â0.5 Â 1.7â (1Ï SD), which is indistinguishable from previously reported calculation on Campanian seawater Î18Ow from fossil mollusk shells, but elevated above younger fossils collected from other locations in the WIS. Our conclusions are inconsistent with previously hypothesized disequilibrium for WIS MDAC clumped isotope and therefore we propose that fossil MDAC deposits may be used as paleotemperature archives.
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