Computed reconstruction of spatial ammonoid-shell orientation captured from digitized grinding and landmark data

Abstract

The internal orientation of fossil mass occurrences can be exploited as useful source of information about their primary depositional conditions. A series of studies, using different kinds of fossils, especially those with elongated shape (e.g., elongated gastropods), deal with their orientation and the subsequent reconstruction of the depositional conditions (e.g., paleocurrents and transport mechanisms). However, disk-shaped fossils like planispiral cephalopods or gastropods were used, up to now, with caution for interpreting paleocurrents. Moreover, most studies just deal with the topmost surface of such mass occurrences, due to the easier accessibility. Within this study, a new method for three-dimensional reconstruction of the internal structure of a fossil mass occurrence and the subsequent calculation of its spatial shell orientation is established. A 234 million-years-old (Carnian, Triassic) monospecific mass occurrence of the ammonoid Kasimlarceltites krystyni from the Taurus Mountains in Turkey, embedded in limestone, is used for this pilot study. Therefore, a 150×45×140mm3 block of the ammonoid bearing limestone bed has been grinded to 70 slices, with a distance of 2mm between each slice. By using a semi-automatic region growing algorithm of the 3D-visualization software Amira, ammonoids of a part of this mass occurrence were segmented and a 3D-model reconstructed. Landmarks, trigonometric and vector-based calculations were used to compute the diameters and the spatial orientation of each ammonoid. The spatial shell orientation was characterized by dip and dip-direction and aperture direction of the longitudinal axis, as well as by dip and azimuth of an imaginary sagittal-plane through each ammonoid. The exact spatial shell orientation was determined for a sample of 675 ammonoids, and their statistical orientation analyzed (i.e., NW/SE). The study combines classical orientation analysis with modern 3D-visualization techniques, and establishes a novel spatial orientation analyzing method, which can be adapted to any kind of abundant solid matter.