Fossilized Embryo.

From the website:

The tiny fossils are part of South China's Doushantuo Formation, a limestone bed deposited between 635 and 551 million years ago that contains layers composed almost entirely of fossil embryos.

The team behind the research believes the fossils are the developing offspring of extremely primitive sponge-like creatures.

To resolve the delicate internal structures, the scientists used a technique known as microfocus x-ray computed tomography (microCT). The method allowed the team to construct 3D images of the tiny fossils.

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