Origins of microfossil bonebeds: insights from the Upper Cretaceous Judith River Formation of north-central Montana
Origins of microfossil bonebeds: insights from the Upper Cretaceous Judith River Formation of north-central Montana
复制标题
微化石骨床的起源：蒙大拿州中北部上白垩统朱迪思河地层的见解
DOI：
10.1666/0094-8373-36.1.80
复制
发表时间：
2010
期刊：
影响因子：
1.6
通讯作者：
M. Brady
中科院分区：
文献类型：
--
作者：
R. Rogers;M. Brady
摘要微化石骨床是多个个体的分离和解离的脊椎动物硬质部分的堆积，主要由毫米到厘米大小的元素组成(≥75%的生物碎屑≤最大尺寸为5厘米)。堆积模式通常很难从文献中的报告中破译，尽管通常提出捕食性(散布)和河流/水力来源。我们对蒙大拿州坎帕尼亚朱迪斯河组的27个微化石骨层进行了沉积学和埋藏学研究，以重建其形成历史。在所研究的16个骨层中，寄主于低能量水生环境(池塘/湖泊微体化石骨层)中的细粒相。其中11个骨层埋藏在古河流环境中积累的砂岩中(以河道为宿主的微化石骨层)。我们提出了一个将生物碎屑岩在两个相中聚集联系起来的模型，而不是基于相差异来调用单独的聚集路径。我们认为，脊椎动物的物质最初在池塘/湖泊中堆积到化石级别，后来被重新加工和重新沉积为河道宿主组合。这一解释是基于对湖泊和河流沉积体系的合理预期，并得到了挖掘学数据的支持。此外，这与动物群数据相一致，表明河道宿主组合和池塘/湖泊组合在有无分类群和等级顺序丰度方面是相似的。这一修改后的骨床形成模型对脊椎动物古生态研究具有重要意义，脊椎动物古生态研究依赖于从脊椎动物微化石组合中恢复的动物群数据的分析。朱迪斯河记录中的池塘/湖泊微化石骨床在当地古环境规模的原地保存，没有死后运入或运出生物栖息地的迹象。此外，它们是其来源群落的时间平均样本，这增加了捕捉生态丰富物种和更多稀有或短暂的古群落成员的可能性。这些属性使池塘/湖泊微化石骨床成为寻求重建整体群落成员和结构的古生态研究的极佳目标。相反，朱迪斯河记录中的河道寄主微化石骨层从古环境角度来看是不合适的，因为它们是从先前存在的池塘/湖泊组合中改造而来的，并在更年轻的河道相中重新沉积。然而，尽管有过折返和再沉积的历史，河道承载的微化石骨层被保存在距离原始生油层相对较近的空间。这种挖掘学重建与普遍持有的观点相反，即微化石骨层是经历了长途运输和重大流体动力分选的有偏见的样品。
Abstract Microfossil bonebeds are multi-individual accumulations of disarticulated and dissociated vertebrate hardparts dominated by elements in the millimeter to centimeter size range (≥75% of bioclasts ≤5 cm maximum dimension). Modes of accumulation are often difficult to decipher from reports in the literature, although predatory (scatological) and fluvial/hydraulic origins are typically proposed. We studied the sedimentology and taphonomy of 27 microfossil bonebeds in the Campanian Judith River Formation of Montana in order to reconstruct formative histories. Sixteen of the bonebeds examined are hosted by fine-grained facies that accumulated in low-energy aquatic settings (pond/lake microfossil bonebeds). Eleven of the bonebeds are embedded in sandstones that accumulated in ancient fluvial settings (channel-hosted microfossil bonebeds). In lieu of invoking separate pathways to accumulation based on facies distinctions, we present a model that links the accumulation of bioclasts in the two facies. We propose that vertebrate material initially accumulates to fossiliferous levels in ponds/lakes and is later reworked and redeposited as channel-hosted assemblages. This interpretation is grounded in reasonable expectations of lacustrine and fluvial depositional systems and supported by taphonomic data. Moreover, it is consistent with faunal data that indicate that channel-hosted assemblages and pond/lake assemblages are similar with regard to presence/absence and rank-order abundance of taxa. This revised model of bonebed formation has significant implications for studies of vertebrate paleoecology that hinge on analyses of faunal data recovered from vertebrate microfossil assemblages. Pond/lake microfossil bonebeds in the Judith River record are preserved in situ at the scale of the local paleoenvironment, with no indication of postmortem transport into or out of the life habitat. Moreover, they are time-averaged samples of their source communities, which increases the likelihood of capturing both ecologically abundant species and more rare or transient members of the paleocommunity. These attributes make pond/lake microfossil bonebeds excellent targets for paleoecological studies that seek to reconstruct overall community membership and structure. In contrast, channel-hosted microfossil bonebeds in the Judith River record are out of place from a paleoenvironmental perspective because they are reworked from preexisting pond/lake assemblages and redeposited in younger channel facies. However, despite a history of exhumation and redeposition, channel-hosted microfossil bonebeds are preserved in relatively close spatial proximity to original source beds. This taphonomic reconstruction is counter to the commonly held view that microfossil bonebeds are biased samples that have experienced long-distance transport and significant hydrodynamic sorting.