The end-Triassic biodiversity crisis was one of the most severe mass extinctions in the history of animal life. However, the extent to which the loss of taxonomic diversity was coupled with a reduction in organismal abundance remains to be quantified. Further, the temporal relationship between organismal abundance and local marine redox conditions is lacking in carbonate sections. To address these questions, we measured skeletal grain abundance in shallow-marine limestones by point counting 293 thin sections from four stratigraphic sections across the Triassic/Jurassic boundary in the Lombardy Basin and Apennine Platform of western Tethys. Skeletal abundance decreased abruptly across the Triassic/Jurassic boundary in all stratigraphic sections. The abundance of skeletal organisms remained low throughout the lower-middle Hettangian strata and began to rebound during the late Hettangian and early Sinemurian. A two-way ANOVA indicates that sample age (p < .01, η² = 0.30) explains more of the variation in skeletal abundance than the depositional environment or paleobathymetry (p < .01, η² = 0.15). Measured I/Ca ratios, a proxy for local shallow-marine redox conditions, show this same pattern with the lowest I/Ca ratios occurring in the early Hettangian. The close correspondence between oceanic water column oxygen levels and skeletal abundance indicates a connection between redox conditions and benthic organismal abundance across the Triassic/Jurassic boundary. These findings indicate that the end-Triassic mass extinction reduced not only the biodiversity but also the carrying capacity for skeletal organisms in early Hettangian ecosystems, adding to evidence that mass extinction of species generally leads to mass rarity among survivors.

中文翻译：

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三叠纪末大灭绝期间和之后动物丰度和氧气供应量的减少

三叠纪末的生物多样性危机是动物生命史上最严重的大规模灭绝之一。然而，分类学多样性的丧失与生物丰度的减少相结合的程度仍有待量化。此外，碳酸盐剖面缺乏生物丰度与当地海洋氧化还原条件之间的时间关系。为了解决这些问题，我们通过对横跨三叠纪/侏罗纪边界的伦巴第盆地和西特提斯亚平宁地台的四个地层剖面的 293 个薄片进行点计数，测量了浅海石灰岩中的骨骼颗粒丰度。在所有地层剖面中，骨骼丰度在三叠纪/侏罗纪边界突然下降。骨骼生物的丰度在整个中下赫唐阶地层中仍然很低，并在赫唐阶晚期和西尼穆纪早期开始反弹。双向方差分析表明样本年龄 (p  < .01, η ²  = 0.30) 比沉积环境或古水下测量更多地解释了骨骼丰度的变化 ( p  < .01, η ² = 0.15）。测量的 I/Ca 比率是局部浅海氧化还原条件的代表，显示出同样的模式，最低的 I/Ca 比率出现在赫唐纪早期。海洋水柱氧含量与骨骼丰度之间的密切对应表明三叠纪/侏罗纪边界的氧化还原条件与底栖生物丰度之间存在联系。这些发现表明，三叠纪末的大规模灭绝不仅减少了生物多样性，还减少了早期赫坦格生态系统中骨骼生物的承载能力，进一步证明物种大规模灭绝通常会导致幸存者大量稀有。