Structural control and Ostwald ripening as essential mechanisms during the fossilization process of sea urchin (Balanocidaris? Lambert) spines from Lower Cretaceous of Southern Spain

ABSTRACT

A comparative study has been made between some cidaroid fossil spines from Lower Cretaceous of southern Spain and other contemporary ones. The modern spines are a composite material formed by calcite nanocrystals oriented with their optical axes parallel to the elongated direction, which behave as a single crystal. Transmission Petrographic Microscopy (TPM) observations of the fossil spines prove that it is a calcite single crystal, whose optical axis coincides with the direction of elongation (as in the current spines). This paper presents a crystallographic approach to propose a secondary crystallisation mechanism, specifically the Ostwald ripening process, through a structural analysis that compares the spines and the conditions observed during the fossilisation process.

KEYWORDS:

• Calcite
• Lower Cretaceous
• echinoid spines
• Ostwald ripening
• single crystal
• taphonomy

Acknowledgments

To UCM Research Group nº 910386: “Crystallographic and geological techniques. Non- conventional applications”, for the financial support provided. To Geological Techniques Unit (UCM). To Dra. Estibaliz López-Samaniego and Adrián Aguilar Rodríguez for mediating the transfer of the modern spines that have been provided by the “Asociación Vertidos Cero” through the MARNOBA project.

Disclosure statement

No potential conflict of interest was reported by the author(s).