A 520-million-year-old five-eyed fossil reveals the origin of arthropods

Arthropods have been among the most successful animals on Earth since the Cambrian period, around 520 million years ago. They are the most familiar and ubiquitous and make up nearly 80% of all animal species today, far more than any other animal.

But how did arthropods evolve and what did their ancestors look like? These have been a great enigma in animal evolution that has baffled generations of scientists for more than a century.

Now, researchers from the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences (NIGPAS) have discovered a five-eyed shrimp-like fossil, which has provided important insights into the early evolutionary history of arthropods. The study was published in Nature on November 4th.

The fossil species, Kylinxia, ​​was collected from the wildlife of Chengjiang in Yunnan Province in southwestern China. The fauna documents the first most complete animal fossils in the Cambrian period.

Prof. Huang Diying, corresponding author for the NIGPAS study, said: “Chilinxia is a very rare chimeric species. It combines morphological characteristics of different animals, which is analogous to ‘chylin’, a chimeric creature in traditional Chinese mythology.” .

“Due to very special taphonomic conditions, the Kylinxia fossils show exquisite anatomical structures. For example, the nerve tissue, the eyes and the digestive system – these are soft body parts that we cannot usually see in conventional fossils,” he said. said the prof. author of the study.

Chilinxia displays the distinctive features of true arthropods, such as a hardened cuticle, segmented trunk, and articulated legs. However, it also integrates morphological features found in very ancestral forms, including the bizarre five eyes of Opabinia, known as the Cambrian “strange wonder”, as well as the iconic raptorial appendages of Anomalocaris, the giant predator at the apex of the ocean. Cambrian.

Among the Chengjiang fauna, Anomalocaris is a superior predator that can reach two meters in body length and has been considered an ancestral form of arthropods. But there are huge morphological differences between Anomalocaris and true arthropods. There is a large evolutionary gap between the two that can hardly be bridged. This gap has become a crucial “missing link” in the origin of arthropods.

The research team conducted detailed anatomical examinations of the Kylinxia fossils. They showed that the first appendages of Anomalocaris and true arthropods were homologous. Phylogenetic analyzes have suggested the existence of affinity between the anterior appendages of Chilinxia, ​​small predatory appendages in front of the mouth of Chelicerata (a group that includes spiders and scorpions) and the antennae of Mandibulata (a subdivision of arthropods including insects such as ants and bees).

“Our results indicate that the evolutionary positioning of chilinxia is right between Anomalocaris and true arthropods. Therefore, our discovery reached the evolutionary root of true arthropods,” said prof. Zhu Maoyan, co-author of the study.

“Chilinxia represents a crucial transitional fossil predicted by Darwin’s evolutionary theory. It bridges the evolutionary gap from Anomalocaris to true arthropods and forms a key” missing link “in the origin of arthropods, providing strong fossil evidence for the evolutionary theory of life.” said Dr. Zeng Han, first author of the study.