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Digital reconstruction of calcified early metazoans, terminal Proterozoic Nama Group, Namibia

¹ Wesley A. Watters and John P. Grotzinger. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139. watters{at}mit.edugrotz{at}mit.edu

A method is presented for the digital reconstruction of weakly calcified fossils within the Nama Group, Namibia. These recently described fossils (Grotzinger et al. 2000) are preserved as calcitic void-fill in a calcite matrix, and individual specimens cannot be freed by conventional techniques. The technique presented here has several integrated steps: (1) the analysis of cross-sections of fossil specimens, (2) the construction of a three-dimensional "tomographic" model that is assembled from the cross-sections, (3) the development of an idealized mathematical model based upon geometric parameters measured from the tomographic model, and (4) the visualization of randomly oriented cross-sections through the mathematical model, which can be compared with fossil cross-sections in outcrop.

In this procedure, rocks containing the fossils are ground and digitally photographed at thickness intervals of 25 µm. 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 tetrahedrons which connect the contours in adjacent layers. We found that most of the fossils represent a single morphology with some well-defined characters that vary slightly among individual specimens. This fossil morphology was described by Grotzinger et al. (2000) 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.

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