Data from: Isotaphonomy in concept and practice: an exploration of vertebrate microfossil bonebeds in the upper Cretaceous (Campanian) Judith River Formation, North-Central Montana

Vertebrate microfossil bonebeds (VMBs)—localized concentrations of small resilient vertebrate hard parts—are commonly studied to recover otherwise rarely found small-bodied taxa, and to document relative taxonomic abundance and species richness in ancient vertebrate communities. Analyses of taphonomic comparability among VMBs have often found significant differences in size and shape distributions, and thus considered them to be non-isotaphonomic. Such outcomes of “strict” statistical tests of isotaphonomy suggest discouraging limits on the potential for broad, comparative paleoecological reconstruction using VMBs. Yet it is not surprising that sensitive statistical tests highlight variations among VMB sites, given the general lack of clarity with regard to the definition of “strict” isotaphonomic comparability and the likelihood of significant variability within natural taphonomic systems. We rigorously sampled and compared six VMB localities representing two distinct paleoenvironments (channel and pond/lake) of the Upper Cretaceous Judith River Formation to evaluate biases related to sampling strategies and depositional context. Few defining distinctions in bioclast size and shape are evident in surface collections, and most site-to-site comparisons of sieved collections are indistinguishable (p ≤ 0.003). These results provide a strong case for isotaphonomy among the majority of Judith River VMBs, and bode well for future studies of paleoecology, particularly in relation to investigations of faunal membership and community structure in Late Cretaceous wetland ecosystems. The isotaphonomic equivalence of pond/lake and channel-hosted VMBs in the Judith River Formation is also consistent with a formative model that contends that channel-hosted VMBs were reworked from pre-existing pond/lake assemblages, and thus share taphonomic history.

脊椎动物微化石骨床（VMBs），即局部分布的小型韧性脊椎动物硬体富集体，常被用于采集此前难以发现的小型脊椎动物类群，并记录古脊椎动物群落内的相对类群丰度与物种丰富度。针对VMBs之间埋藏学可比性的分析常发现其尺寸与形状分布存在显著差异，因此认为这些骨床不满足等埋藏学标准。此类针对等埋藏学的“严格”统计检验结果，似乎限制了利用VMBs开展大范围对比古生态重建的潜力，令人受挫。然而，考虑到“严格”等埋藏学可比性的定义普遍缺乏明确性，且自然埋藏系统内本就存在显著变异的可能性，敏感的统计检验能揭示VMB位点间的差异便不足为奇了。本研究对晚白垩世朱迪河组（Judith River Formation）中代表两种不同古环境（河道与池塘/湖泊）的6处VMB位点进行了严格采样与对比，以评估采样策略与沉积背景带来的偏差。在地表采集样品中，生物碎屑的尺寸与形状几乎不存在明确的区分特征；而对筛洗后样品的多数位点间对比结果均无显著差异（p ≤ 0.003）。上述结果为朱迪河组多数VMBs满足等埋藏学标准提供了有力依据，同时也为未来古生态研究带来了良好前景，尤其是针对晚白垩世湿地生态系统的类群组成与群落结构研究。朱迪河组中池塘/湖泊型与河道型VMBs的等埋藏学等效性，也与一种形成模型相符：该模型认为河道型VMBs是由预先形成的池塘/湖泊生物组合经再搬运形成的，因此二者共享相同的埋藏历史。