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The sediments that trapped the specimen were extremely toxic, the researchers said. The water contained no oxygen, but hydrogen sulphide -- described as not only \"stinky\" but deadly -- was dissolved in the water, the researchers said.

An unusual chemical alchemy may have been responsible for the unique way Sue was fossilized, the researchers hypothesized.

About 85% of the animals on Earth today are arthropods -- including shrimps, lobsters, spiders, mites, millipedes and centipedes, the paper stated.

The downside to Sue's unique fossilization is it makes it hard to compare the specimen with other fossils of similar species of the time.

\"So it remains a mystery how she fits into the evolutionary tree of life,\" according to the researchers.

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