Environmental and diagenetic controls on the morphology and calcification of the Ediacaran metazoan Cloudina

Amy Shore,Rachel Wood

doi:10.1038/s41598-021-90768-5

Abstract

Cloudina is a globally distributed Ediacaran metazoan, with a tubular, funnel-in-funnel form built of thin laminae (ca. 1–10 μm). To what degree local environmental controlled morphology, and whether early diagenesis controlled the degree of calcification of Cloudina, is debated. Here we test these hypotheses by considering assemblages from four, coeval localities from the Upper Omkyk Member, Nama Group, Namibia, from inner ramp to mid-ramp reef across the Zaris Subbasin. We show that sinuosity of the Cloudina tube is variable between sites, as is the relative thickness of the tube wall, suggesting these features were environmentally controlled. Walls are thickest in high-energy reef settings, and thinnest in the low-energy, inner ramp. While local diagenesis controls preservation, all diagenetic expressions are consistent with the presence of weakly calcified, organic-rich laminae, and lamina thicknesses are broadly constant. Finally, internal ‘cements’ within Cloudina are found in all sites, and pre-date skeletal breakage, transport, as well as syn-sedimentary botryoidal cement precipitation. Best preservation shows these to be formed by fine, pseudomorphed aragonitic acicular crystals. Sr concentrations and Mg/Ca show no statistically significant differences between internal Cloudina cements and botryoidal cements, but we infer all internal cements to have precipitated when Cloudina was still in-situ and added considerable mechanical strength, but may have formed post-mortem or in abandoned parts of the skeleton.

Highlights

Cloudina is a globally distributed Ediacaran metazoan, with a tubular, funnel-in-funnel form built of thin laminae
In the Nama Group, Namibia, Cloudina which grew associated with shallow, hydrodynamically-energetic reefs shows the largest tube diameters recorded, but individuals with smaller tube diameters were dominant in low-energy, microbial mat settings[10]
The walls of Cloudina show a granular, micritic microstructure, even in phosphatised s pecimens[13]. This calcification may have proceeded via growth of initial amorphous calcium carbonate (ACC) nanoparticles, comparable to that found in modern echinoderms, molluscs and c nidarians[16]

Summary

Introduction

Cloudina is a globally distributed Ediacaran metazoan, with a tubular, funnel-in-funnel form built of thin laminae (ca. 1–10 μm). To what degree local environmental controlled morphology, and whether early diagenesis controlled the degree of calcification of Cloudina, is debated We test these hypotheses by considering assemblages from four, coeval localities from the Upper Omkyk Member, Nama Group, Namibia, from inner ramp to mid-ramp reef across the Zaris Subbasin. Internal ‘cements’ within Cloudina are found in all sites, and pre-date skeletal breakage, transport, as well as syn-sedimentary botryoidal cement precipitation Best preservation shows these to be formed by fine, pseudomorphed aragonitic acicular crystals. Cloudinomorphs share a similar, funnel-in-funnel organisation without transverse structures, with a straight or sinuous morphology[1,5] They can be organic-only or preserved as calcite, phosphate, limonite/pyrite, or silica. Cloudina may have biomineralised via calcification of pre-existing organic laminae as revealed by Raman spectroscopy, which are 1–10 μm in thickness. This calcification may have proceeded via growth of initial amorphous calcium carbonate (ACC) nanoparticles, comparable to that found in modern echinoderms, molluscs and c nidarians[16]

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Conclusion