Evaluation of Degree of Alteration in Fossil Ostracods from Middle Callovian-Lower Oxfordian Beds of Southern Part of Kachchh Basin, Gujarat, India by Characterizing Surface Roughness of the Shell

Abstract

Before proceeding with any type of analysis, the condition of preservation of microfossil shells must be determined. Preservation of the shell affects tasks, such as holotype specimen selection, distribution chart, and specimen selection for geochemical analysis. The typical way to determine the degree of alteration is to use a visual examination under the stereo microscope. The deviation of visual characteristics such as colour and smoothness of the shell surface from unaltered shells depicts the alteration status. However, our eyes are frequently misled by the roughness of the shell created by tiny crystals that form due to recrystallization during the diagenesis process and are perceived to be smooth. In turn, the shell is regarded as well-preserved. To address these limitations, we have analyzed scanning electron microscope (SEM) images of ostracod shells using ImageJ, an image processing software, to determine the roughness parameters based on the grey value of the image. In addition, we compared the values of several parameters with different types of alterations, such as recrystallization and dissolution. This study aims to characterize the surface roughness of eight selected ostracod specimens from Middle Callovian to Upper Oxfordian beds of the southern part of Mainland Kachchh. The selected eight species - Cytherella masuguluensis, Progonocythere sp. A, Progonocythere jaisalmerensis, Bythocypris rajasthanensis, Paracypris contermia, Galliaecytheridea remota, Cytherelloidea sp. A, Cytherelloidea sp. B - belong to six genera and five families.