Post Permo-Triassic terrestrial vertebrate recovery southwestern United States

Recovery of marine biodiversity following the Permo-Triassic extinction is thought to have been delayed relative to other mass extinctions. Terrestrial vertebrate biodiversity is said to have taken as much as 15 Myr longer to recover than the marine. The present study tests, at the scale of an individual fossil community, whether a disparity in biodiversity existed in the American Southwest, between the Moenkopi Formation, containing an early Middle Triassic (Anisian) terrestrial tetrapod fauna, and the Chinle Formation, containing a successor Late Triassic (Norian) tetrapod fauna. Taking Chinle faunal biodiversity to represent full biotic recovery, comparison of taxonomic and guild diversity of faunas from similar depositional and taphonomic environments in these two formations allowed us to assess the possibility of incipient terrestrial recovery of biodiversity in the Anisian. Comparisons were made between the Holbrook Member fauna of the Moenkopi, a unit best characterized as a low-sinuosity medium- to coarse-grained fluvial deposit, and each of four Chinle stratigraphic units, representing fluvial settings from sandy low-sinuosity to muddy high-sinuosity. Three metrics were applied: generic and familial taxonomic diversity and guild diversity; these were compared by rarefaction. Simpson and Shannon diversity metrics augmented the analysis. Units of extraordinary preservation in the Chinle—the so-called blue layers—were removed from the analysis. In all tests the biodiversity of the Holbrook Member fauna is within the variation seen in Chinle faunas. If the results of our study represent global conditions, they suggest that by at least early Anisian time (6 Myr after the P/T extinction) biodiversity had reached levels comparable to those seen in the Late Triassic. This potentially brings the terrestrial vertebrate recovery in line with the 4–8 Myr it took for recovery in the marine realm.

人们普遍认为，相较于其他集群灭绝事件，二叠纪-三叠纪灭绝事件后海洋生物多样性的恢复进程显著延迟。有研究指出，陆地脊椎动物生物多样性的恢复时长较海洋生物多出多达1500万年。本研究以单个化石群落为研究尺度，检验美国西南部两处地层间是否存在生物多样性差异：其一为产出早中三叠世（安尼期，Anisian）陆地四足类动物群的莫恩科皮组（Moenkopi Formation），其二为产出后续晚三叠世（诺利期，Norian）四足类动物群的钦利组（Chinle Formation）。本研究以钦利组动物群的生物多样性代表完全复苏的生物群落状态，通过对比两个地层中沉积环境与埋藏学背景相似的动物群的分类多样性与功能群多样性，得以评估安尼期陆地生物多样性是否处于初期复苏阶段。研究对比了莫恩科皮组霍尔布鲁克段动物群（该段以低曲度中-粗粒河流沉积为典型特征）与钦利组的四个地层单元——后者涵盖从砂质低曲度到泥质高曲度的各类河流沉积环境。本研究采用三类多样性分析指标：属级、科级分类多样性以及功能群多样性，并通过稀疏化（rarefaction）分析开展对比。此外，本研究还补充使用了辛普森（Simpson）多样性指数与香农（Shannon）多样性指数进行分析。研究剔除了钦利组中保存状态异常优异的所谓“蓝色层”相关数据。所有检验结果均显示，霍尔布鲁克段动物群的生物多样性处于钦利组动物群的自然变异范围之内。若本研究结果可代表全球尺度的普遍情况，则表明至少在安尼期早期（二叠纪-三叠纪灭绝事件发生后的600万年），陆地生物多样性已达到与晚三叠世相当的水平。这一发现或将陆地脊椎动物的复苏时长修正至与海洋生物的400万至800万年复苏时长相近的区间。