Digital reconstructions of fossil morphologies, Nama Group, Nambia

Abstract

Previously undescribed fossils of weakly calcified metazoans were recently discovered in the terminal Proterozoic Nama Group of central and southern Namibia (Grotzinger et al., 1995), in sediments that contain the terminal Proterozoic index fossil Cloudina. The new fossils are closely associated with thrombolites and stromatolites that form laterally continuous biostromes, isolated patch reefs, and isolated pinnacle reefs. Because these fossils are preserved as calcitic void-fill in a calcite matrix, individual specimens cannot be freed by conventional techniques. Rocks containing the fossils are ground and digitally photographed at thickness intervals of 25 pm. A battery of image processing techniques is used to obtain the contour outlines of the fossils in serial cross sections. A Delaunay triangulation method is then used to reconstruct the morphology from tetrahedral components which connect the contours in adjacent layers. It is found that most of the fossils resemble a single morphology with some well-defined characters that vary slightly among individual specimens. This fossil morphology is described in this thesis as Namacalathus hermanastes. A mathematical description of the morphology is used to obtain a database of randomly-oriented synthetic cross sections. This database reproduces the vast majority of cross sections observed in outcrop. In addition, the most common orientation, the mean size, and other population statistics are measured for Namacalathus fossils within an individual rock sample.