INSIDE ANCIENT ANIMAL EMBRYOS AND ALGAE – COMPUTED TOMOGRAPHIC ANALYSIS OF NEOPROTEROZOIC MULTICELLULAR EUKARYOTES

Phosphatized animal embryos and algae from the ca. 600 Ma Doushantuo Formation provide critical information about the evolution of early multicellular organisms and exhibit spectacular cellular-level preservation. Previous work on these fossils has been limited to examination of their three-dimensional external morphology using SEM or their two-dimensional internal morphology using thin-section observation. These approaches have limited our ability to identify features which can be used to assess the taxonomic affinity of these embryos, largely because three-dimensional internal information is needed to accurately characterize cell numbers, cell morphologies, and features found within individual cells. Using microfocus X-ray computed tomography (microCT), we have begun to bridge this knowledge gap. We present preliminary results from analysis of 9 morphotypes of animal embryos, 3 algal forms, and 2 problematic fossils from Doushantuo exposures in the Weng’an region of South China.

For each of these samples we created three-dimensional volumes of X-ray attenuation. Based on comparison with SEM and thin-section analyses, this attenuation correlates with variations in the mineralogy and density of cell walls and other internal structures. Thus isocontouring and volume rendering of these attenuation variations allows visualization of the internal biological characteristics of the fossils, including the morphology of individual cells, the three-dimensional arrangement of cells, and the nature of possible organelle-like structures, as well as the visualization of inorganic structures such as fractures, pyrite tunnels, and diagenetic voids. For example, we constructed models of the cells that are not exposed on the surface of 16-celled embryos, and found them to be 15-sided polyhedrons mostly characterized by flat irregular pentagonal faces. Additionally, 4-celled animal embryos contain two similarly-sized ovoidal structures within each cell; such ovoidal structures have been tentatively interpreted, on the basis of thin-section observations, as organelles by previous authors. With further work, this data can be used to test hypotheses about the number of cells in each embryo, the geometry of individual cells, the orientation of cleavage in embryos and algae, and the nature of organelle-like structures.