Multiple Non-Destructive Approaches to Analysis of the Early Silurian Chain Coral Halysites from South China

TL;DR

This study uses non-destructive techniques to analyze the structure and composition of Silurian corals, revealing new insights into their growth and reef ecosystems.

Contribution

The study introduces non-destructive methods to analyze Halysites, enabling high-resolution 3D mineralization analysis for the first time.

Findings

Halysites exhibits lateral and vertical growth patterns with flaggy corallites at the initial branching stage.

Septa spines in Halysites were identified, contradicting previous assumptions of their rarity or absence.

Halysites reefs were relatively sparse, with a volume ratio of ~30%, creating space for other reef organisms.

Abstract

Cnidarians are among the most important diploblastic organisms, elucidating many of the early stages of Metazoan evolution. However, Cnidarian fossils from Cambrian deposits have been rarely documented, mainly due to difficulties in identifying early Cnidarian representatives. Halysites, a tabulate coral from Silurian reef systems, serves as a crucial taxon for interpreting Cambrian cnidarians. Traditionally, the biological characteristics of Halysites have been analyzed using methods limited by pretreatment requirements (destructive testing) and the chamber size capacity of relevant analytical instruments. These constraints often lead to irreversible information loss and inadequate data extraction. This means that, to date, there has been no high-resolution three-dimensional mineralization analysis of Halysites. This study aims to introduce novel, non-destructive techniques to analyze the internal structure and chemical composition of Halysites. Furthermore, it seeks to elucidate the relationship between coral organisms and biomineralization in reef settings and to compare Silurian Tabulata with putative Cambrian cnidarians. Techniques such as micro-X-ray fluorescence spectrometry (micro-XRF), micro-X-ray computed tomography (micro-CT), and scanning electron microscopy (SEM) were employed in this research. With the help of high-resolution micro-CT scanning, we identify the growth pattern of Halysites, showing both lateral and vertical development. The lateral multiple-branching growth pattern of Halysites corals is first established herein. The flaggy corallite at the initial stage of branching is also observed. The micro-XRF mapping results reveal the occurrence of septa spines for Halysites, a trait previously thought rare or absent. Additionally, the ratio of coral volume to the surrounding rock was assessed, revealing that Halysites reefs were relatively sparse (volume ratio = ~30%). The cavities between Halysites likely provided more space for other organisms (e.g., rugose corals and bryozoans) when compared to other coral reef types. Additionally, we provide a comparative analysis of post-Cambrian colonial calcareous skeletons, offering insights into the structural features and growth patterns of early skeletal metazoans across the Ediacaran–Cambrian boundary.

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