Purported animal embryo fossils have been reported continuously over the last 12 years, mainly by paleontologists working in China. Scientists disagree about whether the fossils are actually animal embryos or even if they are animals.
"The fossils look great. The problem is, if you know anything about embryos, their fossilization just doesn't seem likely," said IU Bloomington Professor of Biology Rudolf Raff, one of the report's authors. "It's like trying to fossilize soap bubbles. Some investigators showed that these fossils are being preserved with calcium phosphate, but they haven't explained how embryos could survive long enough for that to happen. We do that."
The group of scientists, led by Rudolf Raff and IUB Department of Biology Chair Elizabeth Raff, explain what conditions are most likely to facilitate the fossilization of early animal embryos, as well as what factors are not likely to affect fossilization. They used two sea urchin species as models, Heliocidaris erythrogramma and Lytechinus pictus.
"We wanted to find what conditions would allow a dead embryo to be preserved for about a month, enough time for it to be encased in minerals," Rudolf Raff said.
The dead spherical embryos do not last long under normal seawater conditions. If the cell's own degradation processes don't destroy it, nearby bacteria will. The embryos must die in the presence of a so-called "reducing" substance, such as hydrogen sulfide. Such reducing substances slow or stop the internal degradation processes that occur very soon after cells die and also inhibit voracious bacteria.