Hanson Ranch Edmontosaurus bonebed taphonomy fits mass death assemblage
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Ben Creisler
Nov 18, 2024, 2:47:37 AMNov 18
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Ben Creisler
The Hanson Ranch (HR) Bonebed is a monodominant assemblage of Edmontosaurus annectens and infrequent remains of other vertebrates within the Lance Formation of northeastern Wyoming. This bonebed, which has yielded more than 13,000 specimens from five Main Quarries over the last 25 years, is interpreted to have formed by deposition of a clastic debris flow, generating a normally-graded, fossil-dense assemblage. To clarify the geochemical history of the HR Bonebed and assess its mode of formation independent of traditional taphonomic means, we conducted trace element analyses of 12 bones from the assemblage using laser ablation-inductively coupled plasma mass spectrometry. A majority of the examined bones were found to exhibit rapidly-declining rare earth element (REE) profiles, indicative of brief exposure to groundwaters, modest overall trace element uptake, and minimal late-diagenetic alteration. However, noteworthy variations were also identified among the specimens. In particular, both positive and negative redox anomalies were identified at the cortical margins and whole-bone level, indicating variable exposure within and among specimens to both reducing and oxidizing conditions. Several of the examined specimens, especially those of taxa other than Edmontosaurus, also exhibit less fractionation among REEs and more homogenous internal compositions. We infer these contrasts to arise from more protracted pore-fluid interactions for some specimens than others, and thus that select bones were attritionally added into the assemblage during transport to the final site of burial. These collective findings independently support prior taphonomic inferences that the bonebed horizon primarily hosts a partially-reworked or short-distance-transported mass death assemblage, and that this depositional history has imparted infrequent attritional signatures into specimens preserved in the bonebed. Our data thereby establish a baseline for comparison for many other fossil sites within the Lance Formation in the region, similar study of which can illuminate the suite of paleoenvironments and diagenetic settings recorded by these highly-fossiliferous strata.
A new paper:
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Paul V. Ullmann, Cooper Caputo, Keith Snyder, Arthur Chadwick & Richard D. Ash (2024)
Trace element taphonomy of the Hanson Ranch Edmontosaurus bonebed supports its origin via transportation of a mass death assemblage
Chemical Geology 122501
doi: https://doi.org/10.1016/j.chemgeo.2024.122501
https://www.sciencedirect.com/science/article/abs/pii/S0009254124005813
Highlights
Trace element analyses of fossil bones from the Hanson Ranch Bonebed are presented.
Modest variation among hadrosaur bones supports primary mass death accumulation.
Bones of other taxa differ, indicating minor attritional input to the bonebed.
Elemental signatures provided independent, corroborating evidence of mass death.
Comparisons to nearby Rose Quarry illuminate the chemistry of the Lance Formation.
Abstract
Paul V. Ullmann, Cooper Caputo, Keith Snyder, Arthur Chadwick & Richard D. Ash (2024)
Trace element taphonomy of the Hanson Ranch Edmontosaurus bonebed supports its origin via transportation of a mass death assemblage
Chemical Geology 122501
doi: https://doi.org/10.1016/j.chemgeo.2024.122501
https://www.sciencedirect.com/science/article/abs/pii/S0009254124005813
Highlights
Trace element analyses of fossil bones from the Hanson Ranch Bonebed are presented.
Modest variation among hadrosaur bones supports primary mass death accumulation.
Bones of other taxa differ, indicating minor attritional input to the bonebed.
Elemental signatures provided independent, corroborating evidence of mass death.
Comparisons to nearby Rose Quarry illuminate the chemistry of the Lance Formation.
Abstract
The Hanson Ranch (HR) Bonebed is a monodominant assemblage of Edmontosaurus annectens and infrequent remains of other vertebrates within the Lance Formation of northeastern Wyoming. This bonebed, which has yielded more than 13,000 specimens from five Main Quarries over the last 25 years, is interpreted to have formed by deposition of a clastic debris flow, generating a normally-graded, fossil-dense assemblage. To clarify the geochemical history of the HR Bonebed and assess its mode of formation independent of traditional taphonomic means, we conducted trace element analyses of 12 bones from the assemblage using laser ablation-inductively coupled plasma mass spectrometry. A majority of the examined bones were found to exhibit rapidly-declining rare earth element (REE) profiles, indicative of brief exposure to groundwaters, modest overall trace element uptake, and minimal late-diagenetic alteration. However, noteworthy variations were also identified among the specimens. In particular, both positive and negative redox anomalies were identified at the cortical margins and whole-bone level, indicating variable exposure within and among specimens to both reducing and oxidizing conditions. Several of the examined specimens, especially those of taxa other than Edmontosaurus, also exhibit less fractionation among REEs and more homogenous internal compositions. We infer these contrasts to arise from more protracted pore-fluid interactions for some specimens than others, and thus that select bones were attritionally added into the assemblage during transport to the final site of burial. These collective findings independently support prior taphonomic inferences that the bonebed horizon primarily hosts a partially-reworked or short-distance-transported mass death assemblage, and that this depositional history has imparted infrequent attritional signatures into specimens preserved in the bonebed. Our data thereby establish a baseline for comparison for many other fossil sites within the Lance Formation in the region, similar study of which can illuminate the suite of paleoenvironments and diagenetic settings recorded by these highly-fossiliferous strata.
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