The massive, rounded monoliths of south-central China resemble a vast army of sentinels. Among their guarded treasures none may be so exciting to IU Bloomington biologists Elizabeth and Rudolf Raff as the Doushantuo formation.

This trove of fossils between Vietnam and the Yangtze River has provided stunning new specimens dating to the animal kingdom's radical expansion 500 to 600 million years ago.

Earlier this year, the Raffs, with colleagues in the U.S. , U.K. , China , Sweden , Switzerland and Australia reported in Science the first direct evidence that primitive animals 550 million years ago were capable of asynchronous cell division during embryonic development. Asynchronous cell division allows the formation of unique shapes.

Fossilized embryos predating the Cambrian Explosion by 10 million years provided evidence that early animals had already begun to adopt some of the structures and processes seen in today's embryos.

"We're learning something about how the very earliest multicellular animals formed embryos and how the embryos developed," said Distinguished Professor Rudolf Raff, who coauthored the report with IUB Biology Department Chair Elizabeth Raff and research associate F. Rudolf Turner. "This gives us an enormous and entirely surprising look at half-a-billion-year-old embryos in the act of cleaving. What a window on the past. We've had no prior idea what they might have done."

The researchers also believe they've identified specialized structures inside the cells, such as bubble-like vesicles that the cells might have used to transport, store or metabolize molecules. Slight aberrations during the fossilization of dead embryonic cells even reveal what appear to be dividing nuclei. It was assumed such structures existed in early animals, but until now, no known fossils of the structures existed.

Even in larger embryo fossils estimated to contain 1,000 cells or more, the scientists did not observe a blastocoel, a fluid-filled gap in the middle of the embryo and a common feature among modern animal embryos. Raff said there are two likely explanations for the observation: "Either these embryos are primitive and don't have a clear blastocoel, or a blastocoel existed but didn't survive the preservation process."

In another study of embryos published by the Raffs and colleagues earlier this year, the scientists reported blastocoels were not always preserved under the kinds of preservation conditions that may have been involved in the formation of fossil embryos.

Fossilized embryos are very rare. Intact fossil embryos are even rarer. The Doushantuo Formation has proved a boon to paleontologists and evolutionary developmental biologists interested in the evolution of animal species during and prior to the Cambrian Explosion, a dramatic time period in which animals became bigger, more diverse, ecologically dominant and, in the late Stephen J. Gould's opinion, a lot more wonderful.