Some of the most ancient fossils collected to date were traced back to the Ediacaran period. This is the time interval ranging from around 635 to 541 million years ago, shortly before the time when scientists predict that a wide range of animals started appearing.
A site where geologists have found numerous ancient fossils is the Weng’an Biota, a large collection of fossils in Southern China that is predicted to be around 609 million years old. The Weng’an Biota has been found to host various organisms preserved inside inorganic minerals, such as calcium phosphate. These include, for instance, sponge-like organisms, algae and possible animal embryos.
Researchers at the University of Bristol and the Chinese Academy of Sciences’ Nanjing Institute of Geology and Paleontology carried out a study examining two renowned types of fossils found at this site, known as Megasphaera and Megaclonophycus.
Their findings, published in Biology Letters, suggest that these fossils are unlikely to be preserved embryos of known animals or their extinct relatives, thus offering new clues about the early emergence of animal life.
“Our paper comes from the first project as part of my Ph.D. at the University of Bristol,” Kirsten Flett, first author of the paper, told Phys.org.
“The affinity of the embryo-like fossils that we studied, originating from Weng’an in China, has been a topic of debate for decades. We wanted to help resolve this by studying the development of these fossils to discover whether they follow typical development patterns of animal embryos.”
Studying ancient fossils in extreme detail
As part of their study, Flett and her colleagues collected hundreds of tiny fossils from the Weng’an Biota in the South of China. They focused on two main types of fossils called Megasphaera and Megaclonophycus.
Megasphaera are spherical microfossils consisting of only a small number of cells that are only visible under a microscope or by using synchrotron tomographic microscopy. Megaclonophycus, on the other hand, are spherical fossils that are made up of hundreds to thousands of cells.
Past studies hypothesized that these two types of fossils are preserved remnants of ancient multicellular organisms in an embryo-like stage, as well as being different developmental stages of the same organism. A key objective of the recent study was to further test this hypothesis, by closely examining some of these fossils using advanced imaging techniques and computational tools.
“After collecting and prepping the specimens, we scanned hundreds of embryos at the synchrotron at the Paul Scherrer Institute, Switzerland,” explained Flett.
“Using a technique called computed tomography, I was able to make a 3D model of each specimen and count every cell within 15 specimens to make a training dataset as well as calculate their volume. We then used an approximation method to calculate the total number of cells in each specimen.”
The scans performed by the researchers and the 3D reconstructions they create offered new clues about the ancient fossils’ nature. Specifically, the team found that the analyzed specimens did not follow the typical patterns of development observed in animals.
Most notably, the specimens did not maintain a volume consistent with the repeated splitting of cells that occurs when animal embryos develop (i.e., cell division). Moreover, the cells did not follow typical organization patterns observed in known animals or their extinct predecessors.
“Even though the fossils do show similarities to animal embryos in their lower cell division stages, this does not continue as cells divide into the 100s and 1000s,” said Flett.
“These embryo-like specimens from Weng’an were considered the main evidence for a diversity of animals already existing in the early Ediacaran. However, our results refute this interpretation and instead suggest that animal diversification must have occurred after the deposition of the Weng’an Biota.”
Plans for further research into ancient fossils
The results of this recent study suggest that embryo-like fossils from the Weng’an Biota are not actually remnants of early animal life on Earth. These fossils instead appear to be ancient traces of entirely different types of multicellular organisms.
The insight gathered by Flett and her colleagues could have important implications for the understanding of early life on our planet, as they hint at the possibility that animal life emerged later than previously predicted. The researchers are now planning further studies aimed at analyzing other ancient fossils collected in China.
“I am currently working on a second paper which carries out a comparative analysis between these embryo-like specimens from Weng’an and accepted animal embryos from elsewhere in China to help us distinguish similarities and differences in the preservation of the two,” added Flett.
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Publication details
Kirsten E. Flett et al, Developmental biology of the Ediacaran Megaclonophycus from the Weng’an Biota, Biology Letters (2026). DOI: 10.1098/rsbl.2025.0592.
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