Internal Structure and Breakage Behavior of Biogenic Carbonate Sand Grains

This Study Investigates the Mechanical Behavior of Biogenic Carbonate Sands from Puerto Rico at Grain-Scale Level. Micro-Computed Tomography Has Also Been Used to Get Insights on the Internal Structure of These Particles Before and after Loading. the Crushing Strength of These Particles Are Smaller Comparing to the Values Reported for Silica Sands. It Has Also Been Shown that These Particles Have Complex Internal Structure Including a Network of Pores Connected with Channels. This Study Also Demonstrates the Effect of Intragrain Structure of Biogenic Carbonate Sands and Shows How Internal Grain Structure Plays a Role on Particle Fracture

Ultrastructure of Ediacaran cloudinids suggests diverse taphonomic histories and affinities with non-biomineralized annelids

Cloudinids have long been considered the earliest biomineralizing metazoans, but their affinities have remained contentious and undetermined. Based on well-preserved ultrastructures of two taxa, we here propose new interpretations regarding both their extent of original biomineralization and their phylogenetic affinity. One of these taxa is a new cloudinid from Mongolia, Zuunia chimidtsereni gen. et sp. nov., which exhibits key characteristics of submicrometric kerogenous lamellae, plastic tubewall deformation, and tube-wall delamination. Multiple carbonaceous lamellae are also discovered in Cloudina from Namibia and Paraguay, which we interpret to have originated from chitinous or collagenous fabrics. We deduce that these cloudinids were predominantly originally organic (chitinous or collagenous), and postmortem decay and taphonomic mineralization resulted in the formation of aragonite and/or calcite. Further, based on our ultrastructural characterization and other morphological similarities, we suggest that the cloudinids should most parsimoniously be assigned to annelids with originally organic tubes

Predators and predation in the Cambrian period : quantitative methods in taphonomy and paleoecology

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] The onset of predation profoundly affected early metazoan communities, likely triggering new biological innovations, such as an evolutionary arms race of prey defenses and predatory attacks, ultimately culminating in the geologically drastic radiation of major phyletic stem groups during the "Cambrian Explosion". Predation is understood primarily from two different means of interpreting the fossil record: either inferred from functional morphologies of organisms and interpreted based on those specializations, or from rare instances where predatorprey interactions, or their traces, are directly preserved. We first taphonomically analyzed the different disarticulated anatomies of the Middle Cambrian predator, Anomalocaris using microchemical analyses. From these analyses the samples appear to be preserved through typical Burgess Shale-type carbonaceous compression, with possible preservational influence of both pyritization and aluminosilicification. In a second study, we examine direct evidence of predatory ichnofossils from recurring associations of the arthropod ichnogenera Rusophycus and Cruziana (most commonly attributed to trilobites) with burrows of vermiform animals, interpreted to represent direct feeding behavior of the arthropod tracemaker on the burrowing worm. Here, we examine the Upper Cambrian Davis Formation, near Leadwood, southeastern Missouri. From our investigation of this material, we observe both size selectivity and possibly orientation of attack of the predatory traces, indicating a likely predator-prey interaction

Biomineralizers from the proterozoic to today : assessing taxonomy and paleoecology through the lens of taphonomy

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] The onset of metazoan predation is presumed to have triggered an evolutionary arms race of prey defenses, including the advent of biomineralization. Escalating biological innovations profoundly affected the dynamics of early metazoan ecosystems and ultimately culminated in the geologically rapid radiation of true metazoans during the "Cambrian Explosion". The first direct evidence of predation was recorded in the first biomineralizing metazoan, specifically as drill holes in the skeletal tube of Cloudina. The terminal Ediacaran Period (~551-541) hosts a diverse array of tubular organisms, and holds vast potential for contributing to our understanding of both biotic turnover and the advent of stem lineages into the Cambrian. The first two chapters herein assess the taxonomy of newly reported tubiculous organisms from a locality in the Great Basin, Nevada, USA, using advanced microscopic techniques. The results of this study erect two new taxa and better resolve the phylogeny of these enigmatic organisms. In the last chapter, I report upon modern biological processes, such as the influence of predation, in molluscan communities. Specifically, this study considers the influence of environmental factors and how they operate within spatial and chemical gradients in order to assess the stratigraphic scale of the incipient fossil record at Pigeon Creek, San Salvador, Bahamas. This study reinforces the idea that small scale local environmental gradients can strongly influence biotic communities and that spatial and environmental variation must be considered in order to properly interpret temporal trends in the fossil record

Quantitative µ-CT Analysis of Scale Topology Formed during Oxidation of High SiMo Cast Iron

High SiMo cast iron components in automotive exhaust systems are exposed to high-temperature oxidation over time. Quantitative analysis of formed oxide scale is therefore important for the assessment of a component durability. The brittle nature of multilayered scale and thermal stress limits capture of a true topology using traditional 2D destructive cut and polish methods. In this study, nondestructive high spatial-resolution 3D µCT analysis was performed on 2.90-mm-diameter oxidized specimens which permitted direct observation with 3.5 µm pixel resolution. The specimens were oxidized in three sequential time steps for a total 100 h at 700 °C and 800 °C in air and combustion gas atmospheres. A MATLAB-coded algorithm was used to quantify the topology, thickness variation in internal and external scale layers, and scale/metal interface unevenness. Scale topology was linked to oxidation temperature and gas atmosphere. A water vapor environment increases scale/metal interface unevenness and scale layer thickness irregularity which were related to an accelerated oxidation rate

Micro-CT Quantitative Evaluation of Graphite Nodules in SGI

The quality of spheroidal graphite cast iron (SGI) castings is directly related to the morphology of graphite nodules. Therefore, accurate and comprehensive quantitative evaluation of microstructure is critical for industrial practices. The most influential morphological characteristics of the graphite phase in SGI can be divided into three categories: (1) shape and size of individual graphite nodules, (2) total volume of graphite phase and nodule number, and (3) spatial distribution. During the last decades, SGI structure analysis has progressed from visual observation of polished cross section and quantitative 2D techniques (metallography and automated SEM/EDX) to observation of the real 3D structure using micro-computed tomography (µCT) scanning. In this article, 2D and 3D morphological characteristics of graphite nodules were determined by µCT scanning and compared. 3D µCT provides accurate characterization of the real shape factor and diameter of graphite nodules. It was shown that large and small graphite nodules in inoculated SGI have the different modes of space distribution which could be linked to casting solidification

3D Characterization of Structure and Micro-Porosity in Two Cast Irons with Spheroidal Graphite

Formation of low density graphite phase during solidification increases the volume of semi-solid cast iron with spheroidal graphite (SGI) and resulted pressure could help eliminate shrinkage micro-porosity in casting. Therefore, micro-porosity in casting can be linked to 3D graphite nodule structure. Two experimental SGI\u27s with different levels of shrinkage micro-porosity in castings were intentionally produced by variation in inoculation parameters which influenced graphite nodule structure. 3D-morphological characteristics of graphite nodules and micro-porosity level were determined by µCT scanning. The obtained statistical data was discussed with respect to graphite nodule solidification mode and casting soundness. The capabilities of µCT scanning in predicting micro-shrinkage in SGI casting were compared to the other traditionally used techniques