Scientists have moved a step closer to decipher the development biology of the oldest animals on earth after researching on 600-million-years-old embryo fossils found in southwest China's Guizhou Province.

The study by US and Chinese scientists has revealed how the ancient animal embryos developed into mature adult forms, the scientists said in the latest issue of Geology, the journal of the Geological Society of America.

The scientists studied on some 80 pieces of fossilised embryos, which were formed almost 600 million years ago and showed the embryos in the act of cleaving, Yuan Xunlai, a member of the Sino-US embryos research team that carried out the study, said.

Yuan, from Chinese Academy of Sciences, said that each embryo is about the size of a grain of sand, and is composed of two to nearly 1,000 cells.

The embryos cloaked themselves inside an envelope with tiny holes in a pattern similar to stitches on a baseball that they use to transport, store or metabolise molecules, he was quoted as saying by Xinhua news agency.

Scientists used X-ray computed tomography and other scanning equipment to peel off the envelope and discovered that the embryo cells are dividing and unfurling.

"Previously, we discovered that some cells were clustered together, but they showed no signs of dividing," he said.

About a decade ago, palaeontologists from the United States and China discovered thousands of 600-million-year-old embryo microfossils in Doushantuo Formation, a fossil deposit near Weng'an, Guizhou Province.

Later they unearthed fossils of a tubular coral-like animal, which they named Megasphaera ornata for its appearance, and found them to be the grown-ups of the embryos.

The new fossils could provide the missing link between the egg and adult of one of Earth's earliest animals, said Zhou Chuanming, a researcher with the Nanjing Institute of Geology and Palaeontology.

Zhou said the discovery could hint towards how ancient animals developed, and reveal the unending evolvement journey of earth's life forms.

But scientists added they need other intermediary stages to complete the developmental journey of the ancient animals.