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A fossil record of cell division

Nov 2006
Kevin Robinson

Using thin section petrology, scanning electron microscopy, transmission electron microscopy and x-ray CT, researchers have uncovered fossilized evidence of cell division. The cells that they found have some similarities to present-day metazoans, which include sea sponges.

The researchers, led by Whitey Hagadorn of Amherst College in Massachusetts, studied 162 fossils of Parapandorina raphospissa embryos, a sea animal from the Ediacaran Period, the last geological period of the Neoproterozoic Era, which occurred 635 million to 551 million years ago. The fossils were found in the Doushantuo Formation in south China. Intact embryo fossils are difficult to identify because of a variety of factors, such as botryoidal void-filling cementation, that destroy or modify cellular structures. The embryos studied range from a few cells up to nearly 1000.

Researchers used a polygon modeling technique to render a 31-celled fossil embryo (500 to 700 mm in diameter). The rendering illustrates that not all cells in the embryos divided at the same time, or equally, which points to sophisticated developmental mechanisms among the earliest animals. Courtesy of Whitey Hagadorn.

In higher-order animals such as mammals, cell division progresses geometrically, meaning that two cells become four, then eight, 16 and so on. Earlier studies of fossilized embryos from the same area assumed that the embryos would follow the same pattern. In this case, many embryos did demonstrate the pattern; however, more than a quarter did not. According to Hagadorn, this pattern is common today among marine invertebrates.

The researchers’ work uncovered information that convinced Hagadorn and his colleagues that the embryos are a metazoan stem group — a precursor from which present-day metazoans evolved. They found that the embryos maintained the same size as they increased in cell numbers, showed evidence of flexible cell walls, had an embryo envelope, and exhibited features such as flexure, tearing and folding that suggest that they were composed of soft tissue. The fossils, however, showed no evidence of epithelial development, which is a hallmark of present-day metazoans.

All four analysis methods were extremely useful, Hagadorn said. “No one technique provided sufficient data to address the questions [we had],” he explained. “That said, the microfocus computed tomography was the real clincher … It helped us screen our sample population for diagenetically and taphonomically altered embryos — which we eliminated from our analyses — and to identify promising embryos with which to conduct further analyses.”

Using electron microscopy, CT scanning and petrology to study fossils deposited in south China about 600 million years ago, researchers found evidence in this four-celled embryo of asymmetrical cell division — such as cells that were about to divide. The information helped convince them that the embryos are from a stem group that evolved into today’s metazoans. The embryo is 500 to 700 µm in diameter. Courtesy of Whitey Hagadorn.

Their research paper also discussed the presence of subcellular shapes similar to one another in several fossilized cells. The scientists believe that these structures are likely to be organelles, possibly fossilized nuclei or spindle bundles.

Hagadorn said that he and his team plan to conduct similar studies on other fossils from the same deposit. “We’d like to examine some of the less common embryo taxa … and determine what, if any, their affinity is with the 2-4-8-16-celled Parapandorina raphospissa embryos. In an ideal world, it would be great if someone found a younger Ediacaran embryo-bearing deposit, or some larval or juvenile forms in rocks of this age.

Science, Oct. 13, 2006, pp. 291-294.


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