What kind of "worm" was the Yunnanworm 518 million years ago? The top ten advances in Chinese paleontology in 2022 are released

When, where, and how did the key evolutionary link from fish to man occur? Why do giraffes have such long necks? What did ancient tigers look like 43,500 years ago? On March 17, the Chinese Paleontological Society released the results of the "Top Ten Advances in Chinese Paleontology in 2022" in Nanjing, Jiangsu Province, and the above puzzles were solved one by one.

The selected achievements were completed by research institutes and universities such as the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, China University of Geosciences (Wuhan), Peking University, Nanjing University, and Chang'an University. The "Top Ten Advances" were selected from 31 nominated achievements, covering multiple geological periods from the Cambrian to the Quaternary.

"Since the first release of the 'Top Ten Advances' in 2017, the Top Ten Advances in Chinese paleontology have become a weather vane for world paleontology." Zhang Yuandong, secretary-general of the Chinese Society of Paleontology, told reporters that in addition to paleontology, the "Top Ten Advances" also involve a large number of related disciplines, such as stratigraphy, paleoecology, paleoanthropology, geochemistry and geotectonics, which are research advances that integrate and cross more than a dozen disciplines.

Yunnan insect ecological restoration map provided by Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences

The most primitive vertebrate to date is the Yunnan worm 518 million years ago

Where do modern vertebrates come from? What is the oldest vertebrate? In the history of life evolution on Earth, the question of the origin of vertebrates has always been the most fascinating scientific hotspot.

Evolutionary biologists usually speculate that the ancestor of vertebrates was a type of deuterostome with a notochord, dorsal neural tube and gill slits based on the characteristics of the living cephalochordate amphioxus. However, this hypothetical vertebrate ancestor has never been supported by fossil evidence.

The answers to these major paleontological questions are hidden in the Chengjiang fauna in Yunnan, my country. On July 8, 2022, a research result completed by the "Early Evolution of the Earth-Life System" research team led by Zhu Maoyan, a researcher at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, and the research group of Jiang Baoyu, a professor at the School of Earth Sciences and Engineering of Nanjing University, was published in the journal Science. The study found that the Yunnanworm produced by the Chengjiang fauna in the Cambrian period 518 million years ago had a cellular cartilage structure unique to vertebrates in its pharyngeal arch, confirming that the Yunnanworm is the most primitive group of vertebrates.

The research team used a variety of modern experimental techniques, including three-dimensional X-ray tomography, Fourier transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy, to re-examine an animal whose classification was previously questionable, from the perspective of its microscopic anatomical structure: the Yunnan worm.

By reanalyzing the gill arch structures of 127 Yunnanworm specimens, the research team discovered for the first time the three-dimensionally preserved disc-like cell structure and protein microfibril structure on the pharyngeal arch of Yunnanworm, proving that Yunnanworm has pharyngeal arches that are unique to vertebrates and composed of cellular cartilage, indicating that Yunnanworm is a primitive vertebrate.

"The research results support that Yunnanzoa is a stem vertebrate, providing key evidence for the origin and early evolution of vertebrates, and will have a far-reaching impact on the exploration of the evolution of key characteristics of the pharyngeal skull of vertebrates." Zhao Fangchen, a researcher at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, said that from another perspective, the study once again demonstrated that the Chengjiang faunal fossils have the potential to preserve fine biological structures at the micro-nano scale.

535 million-year-old fossils suggest that wrinkled cysts were more likely ecdysozoa

▲The cover article of Nature in 2017 proposed that the wrinkled cysts are the most primitive deuterostomes (Image source: Jian Han/Dinghua Yan)

Five years ago, an animal only a millimeter in size with a big mouth appeared on the cover of Nature magazine, sparking heated discussions. A study at the time believed that this microscopic organism called "wrinkled cyst worm" was the most primitive deuterostome and was considered to be a close relative of human ancestors.

▲Three-dimensional artistic restoration of the wrinkled cyst worm drawn by the latest research (Photo source: Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences)

However, in August 2022, a research team composed of domestic and foreign researchers from Chang'an University, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, etc., through in-depth research on microfossil wrinkle cysts from southern Shaanxi about 535 million years ago, confirmed that the wrinkle cysts with sac-shaped body structure (about 535 million years ago) were ecdysoneae, revealing that the sac-shaped body structure appeared in the ancestral type of ecdysoneae, providing more possibilities for the study of the origin of the body structure of ecdysoneae; at the same time, it was confirmed that wrinkle cysts were not the earliest deuterostomes, and the first appearance of the deuterostome subkingdom fossils should still be about 518 million years ago (Chengjiang Fauna). The relevant results were published in the journal Nature.

▲Analysis of the anatomical structure of the fossil of the wrinkled cyst (Image source: Reference [1])

The key feature of the wrinkled cysts that was once interpreted as the earliest deuterostomes is that previous studies believed that the wrinkled cysts had "gill holes". "The wrinkled cysts have no gill holes, and the structure that was once interpreted as gill holes is likely to be a hole-like structure formed by the wear of the closed spiny plates during the fossilization process." Zhang Huaqiao, a researcher at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, told reporters that at the same time, the wrinkled cysts' developed epidermis and epithelium do not have cilia, and the radially symmetrical arrangement of the terminal mouth and ring mouth indicate that it is more closely related to ecdysozoa. Therefore, the research team believes that the wrinkled cysts are more likely to be ecdysozoa rather than deuterostomes.

Ecdysozoa are a branch of protostomes, including cyclospora (branchial worms, roundworms, etc.) and panarthropoda (trilobites, spiders, shrimps, etc.). The earliest known ecdysozoa appeared in the "Lucky Period" of the Cambrian, but they were all cyclospora. The wrinkled cysts added a new type to the earliest known ecdysozoa. At the same time, the "cyst-shaped" body structure of the wrinkled cysts is obviously different from the "worm-shaped" body structure of other ecdysozoa, which shows that "worm-shaped" is not the only body structure type of ecdysozoa, and the latest common ancestor of ecdysozoa may not be a worm.

Top 10 Advances in Chinese Paleontology in 2022

1. Silurian fossil deposits reveal the rise and diversity of early jawed vertebrates

2. Ancient genomes reveal the history of human population formation in Xinjiang over the past 5,000 years

3. Ultrastructure reveals that Yunnanzoon has primitive vertebrate pharyngeal skeleton

4. Pleurocystis is an early molting animal, not the earliest deuterostome

5. Sexual selection promotes the evolution of giraffe-like head and neck specialization

6. Pigment processing and innovative composite tool use by modern humans in East Asia 40,000 years ago

7. Evolution of key ecological behaviors of Mesozoic insects

8. The world’s first ancient tiger genome

9. Changes in atmospheric carbon dioxide levels reveal the mechanism of the end-Permian mass extinction

10. Global warming and ocean hypoxia under the Late Paleozoic icehouse climate (tied progress 10)

10. Triassic-Jurassic volcanism and the impact of terrestrial ecosystem fluctuations (tied progress 10)

References:

[1] Liu, Y., Carlisle, E., Zhang, H. et al. Saccorhytus is an early ecdysozoan and not the earliest deuterostome. Nature (2022). https://doi.org/10.1038/s41586-022-05107-z

[2] New evidence proves saccorhytus is ecdysozoan. Retrieved Aug. 17th, 2022 from https://www.eurekalert.org/news-releases/961928

[3] Progress in the study of the origin of the body structure of ecdysozoa. Retrieved Aug. 18th, 2022 from https://www.cas.cn/syky/202208/t20220818_4844871.shtml