- A fossil record of cell division
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
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
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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