Doushantuo Formation: China's 600-Million-Year-Old Fossil Embryos

The Doushantuo Formation is a sequence of rocks in south China, laid down between about 635 and 551 million years ago, that preserves some of the most detailed fossils of microscopic life ever found. Phosphate-rich layers at Weng’an in Guizhou Province contain tiny balls of cells, each about half a millimetre across, that were first described in 1998 as the oldest known animal embryos. Their true identity has been argued over ever since, making them central to the question of when animals first appeared.

Background: after Snowball Earth

The formation takes its name from Doushantuo, a locality in the Yangtze Gorges area of Hubei Province. Its base lies directly on top of glacial deposits of the Marinoan glaciation, the last of the Snowball Earth episodes, and its lowest layer is a cap carbonate formed as the ice melted.

In 2005 a team led by Daniel Condon dated volcanic ash beds in the formation using uranium-lead methods, giving ages of about 635 million years near the base and about 551 million years near the top. The Doushantuo Formation therefore covers much of the Ediacaran Period, the interval that also saw the rise of the large Ediacaran biota.

The Weng’an biota

The most famous fossils come from phosphate deposits, called phosphorites, mined near Weng’an. Shortly after the organisms died, calcium phosphate replaced their soft tissues. This process, phosphatisation, preserved them in three dimensions, sometimes with cell walls and internal structures intact.

In 1998 two papers announced remarkable finds. Shuhai Xiao, Yun Zhang and Andrew Knoll reported in Nature fossil algae and clusters of cells that looked like animal embryos at successive stages of division: two cells, four cells, eight cells and more. In Science, Chia-Wei Li, Jun-Yuan Chen and Tzu-En Hua described structures they interpreted as tiny sponges.

The embryo-like fossils have names such as Megasphaera and Parapandorina. They are similar in size to the eggs of many living marine invertebrates.

The embryo debate

If the fossils were animal embryos, they would push the record of animals back well before the large Ediacaran fossils. That claim has been tested repeatedly with new imaging techniques, especially synchrotron X-ray tomography, which can image fossils in 3D without destroying them.

In 2007 Jake Bailey and colleagues argued in Nature that some of the objects resembled giant sulphur bacteria related to living Thiomargarita. Most researchers did not accept this for the whole assemblage, but it showed that dividing cell clusters are not unique to animals.

In 2011 a team including Therese Huldtgren, Philip Donoghue and Stefan Bengtson reported structures interpreted as fossilised cell nuclei and patterns of development unlike those of animal embryos. They suggested the fossils were single-celled relatives of animals that divided inside cysts.

In 2014, however, Jun-Yuan Chen and colleagues argued that some specimens showed different cell types, including a separation between reproductive and body cells, which would fit an animal or close relative. Other work has linked the embryo-like fossils to Tianzhushania, a spiny microfossil, suggesting they developed inside resting cysts. Today there is no consensus, and many specialists describe them cautiously as “embryo-like fossils”.

Vernanimalcula and other claims

Bold claims have not always survived. In 2004 a fossil called Vernanimalcula, less than 0.2 millimetres long, was announced as the oldest bilaterian, an animal with a left and right side. Later studies argued that it was a product of mineral layers forming in a microfossil rather than a real animal, and few researchers now accept the original interpretation.

Legacy

The Doushantuo Formation remains one of the best records of life in the aftermath of Snowball Earth. Besides the embryo-like fossils, it contains abundant algae, spiny microfossils called acritarchs and, in the Yangtze Gorges area, carbon films of seaweed-like organisms known as the Miaohe biota.

Its rocks also record chemical changes in the ocean, including evidence for rising oxygen, which help scientists link environmental change to the evolution of complex life that later culminated in the Cambrian Explosion.

Timeline

• c. 635 million years ago: The Marinoan glaciation ends and the lowest Doushantuo cap carbonate forms.
• c. 600 million years ago: The phosphorites of the Weng’an biota are deposited, according to most estimates.
• c. 551 million years ago: Deposition of the Doushantuo Formation ends.
• 1998: Xiao, Zhang and Knoll report animal embryo-like fossils; Li, Chen and Hua report possible sponges.
• 2005: Uranium-lead dating fixes the age range of the formation.
• 2007: A study proposes that some fossils are giant sulphur bacteria.
• 2011: Fossilised nucleus-like structures are interpreted as signs of single-celled relatives of animals.
• 2014: A study reports cell differentiation and argues for an animal-like origin.

Why it matters

The Doushantuo fossils offer a rare look inside individual cells from more than half a billion years ago. Whether or not they are animal embryos, they show the kinds of complex, many-celled life that existed in the Ediacaran, and the debate around them has sharpened how scientists test claims about the earliest animals.