Geochemical taphonomy of the Standing Rock Hadrosaur Site: Exploring links between rare earth elements and cellular and soft tissue preservation

The Standing Rock Hadrosaur Site (SRHS) in South Dakota preserves a mass-death assemblage of the hadrosaurine dinosaur Edmontosaurus annectens in the Upper Cretaceous Hell Creek Formation. A previous study of SRHS bones found many of them to yield microstructures morphologically consistent with vertebrate osteocytes, blood vessels, and proteinaceous matrix upon demineralization with weak acid. To examine the geochemical history of these specimens in detail, we herein document the intra-bone distribution of rare earth elements (REE) within nine of these previously-examined Edmontosaurus limb bones from SRHS. Laser ablation-inductively coupled plasma mass spectrometry was used to determine the trace element composition of the specimens, which revealed the bones to possess unusually low REE concentrations for bones of Mesozoic age, consistent and steep declines in light REE concentrations with increasing cortical depth, and uniformly low concentrations of middle REE and elements with moderate and low diffusivities through the middle cortex. These attributes imply: (1) at least partial preservation of early-diagenetic trace element signatures; (2) that trace element uptake occurred primarily during a single, early-diagenetic phase, and; (3) that the duration of trace element diffusion was brief in these bones. As we previously reported, low permeability of the fine-grain host matrix and early-diagenetic cementation of portions of the host matrix by precipitation of siderite concretions impeded pore fluid replenishment; this partial hindrance of fluid flow was also likely responsible for the minimal trace element uptake observed in the nine SRHS specimens further studied herein. From a paleontological perspective, minimal trace element uptake reflects minimal chemical alteration, meaning bones from this site are predicted to be ideal candidates for preservation of original organics according to hypotheses previously advanced by geochemical studies. Because low REE concentrations reflect limited interaction with groundwaters during diagenesis, we propose that late-diagenetic overprinting of early-diagenetic REE profiles was minimal and that hydrolytic degradation of soft tissues was limited in the burial environment at SRHS. Mitigation of biomolecular decay by hydrolysis would favor ultrastructure stabilization and is therefore advanced as a partial explanation for the preservation of cells and soft tissue microstructures in these fossil bones.