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Ecological reconstruction of chilinxia. Credit: Huang Diying
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 fossil resembling a five-eyed shrimp, which has provided important insights into the early evolutionary history of arthropods. The study was published in Nature on November 4, 2020.
Anatomical reconstruction of Kylinxia Credit: Huang Diying
The fossil species, Chilinxia, 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 it is a very rare chimeric species. It combines morphological characteristics of different animals, which is analogous to “kylin”, a chimeric creature in traditional Chinese mythology “.
“Due to the very special taphonomic conditions, the Chilinxia the fossils show exquisite anatomical structures. For example, the nervous tissue, the eyes and the digestive system – these are soft parts of the body that we usually can’t see in conventional fossils, “said Prof. ZHAO Fangchen, co-correspondent author of the study.
Chilinxia exhibits the distinctive features of true arthropods, such as a hardened cuticle, segmented trunk, and articulated legs. However, it also integrates the morphological features found in very ancestral forms, including the bizarre five eyes of Opabinia, known as the “strange wonder” of the Cambrian, as well as the iconic raptorial appendages of Anomalocaris, the giant predator at the apex of the Cambrian ocean.
Fossil of Kylinxia, holotype. Credit: ZENG Han
Among the fauna of Chengjiang, Anomalocaris it 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 fossils of Chilinxia. They showed that the first appendages in Anomalocaris and true arthropods were homologous. Phylogenetic analyzes suggested that there was 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 it’s right in between Anomalocaris and true arthropods. Thus, our discovery reached the evolutionary root of true arthropods, “said Prof. ZHU Maoyan, co-author of the study.
“Chilinxia represents a crucial transition fossil predicted by Darwin’s theory of evolution. Bridge 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.
Reference: “An early Cambrian euarthropod with radiodont-like raptorial appendages” by Han Zeng, Fangchen Zhao, Kecheng Niu, Maoyan Zhu and Diying Huang, 4 November 2020, Nature.
DOI: 10.1038 / s41586-020-2883-7
The research was funded by the Chinese Academy of Sciences and the National Natural Science Foundation of China.
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