August 1, 2016, by GEOLOGICAL SOCIETY OF AMERICA, EurekAlert!

Boulder, Colo., USA - The origin and early evolution of animals have been a fascinating topic since Charles Darwin. Definite early animal fossils largely appear from early Cambrian, so the fossil records are often interpreted as documenting a "Cambrian explosion" of animals. The phosphatized embryo-like fossils displaying cellular and subcellular structures from Ediacaran Doushantuo Formation, South China, provide an unparalleled snapshot of life before the Cambrian.

Because the fossils are from the interval in which animal clades were diversifying, according to molecular estimates, they were thought to have great potential to reveal the evolution of animals 600 million years ago. However, the affinities of these fossils are contentious.

In their article for Geology, Zongjun Yin and colleagues report new Doushantuo embryo-like fossils. They used high-resolution synchrotron radiation X-ray microtomography to reconstruct three-dimensional structures of the fossils, and the results demonstrate that these fossils preserve unique features directly comparable to living animal embryos that utilize a special kind of cell division pattern known as discoidal cleavage. Given that discoidal cleavage only occurs in animal embryos, the biological affinities of these fossils are probably animals.

This result substantiates the conclusion derived from molecular estimates that animal lineages had evolved by the mid-Ediacaran after the termination of the Marinoan Glaciation, if not earlier.