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Five-eyed fossil reveals secrets of arthropod evolution

Since the Cambrian Period, approximately 520 million years ago, arthropods have proven to be exceptionally successful on Earth, representing nearly 80% of all animal species today. Their ubiquity and familiarity make them a dominant force in the animal kingdom.

The of arthropods and the appearance of their ancestors have long perplexed scientists, presenting a substantial enigma in animal evolution for over a century. Seeking answers, researchers from the Nanjing Institute of Geology and of the Chinese Academy of Sciences (NIGPAS) recently unveiled a groundbreaking discovery—a shrimp-like fossil with five eyes, shedding light on the early evolutionary history of arthropods. The study, published in Nature on Nov. 4, marks a significant stride in unraveling this evolutionary mystery.

The unique fossil species, named Kylinxia, was unearthed from the Chengjiang fauna in Yunnan Province, southwest China. This fauna preserves a comprehensive collection of early animal from the Cambrian period.

Prof. Huang Diying, the corresponding author from NIGPAS, described Kylinxia as a remarkably rare chimeric species, blending morphological features from different akin to the mythical ‘kylin' in traditional Chinese folklore. The fossils of Kylinxia, due to distinct taphonomic conditions, exhibit exceptional preservation of anatomical structures such as nervous tissue, eyes, and the digestive system—typically unseen in conventional fossils, as highlighted by Prof. Zhao Fangchen, co-corresponding author of the study.

Anatomical reconstruction of Kylinxia. Credit: HUANG Diying

Kylinxia displays distinct characteristics of genuine arthropods, including a hardened cuticle, a segmented trunk, and jointed legs. Notably, it incorporates morphological features found in forms, such as the peculiar five eyes reminiscent of Opabinia, known as the Cambrian “weird wonder,” and the iconic raptorial appendages seen in Anomalocaris, the massive apex predator in the Cambrian ocean.

In the Chengjiang fauna, Anomalocaris, a formidable top predator reaching up to two meters in body length, is considered an ancestral form of arthropods. However, significant morphological disparities exist between Anomalocaris and true arthropods, creating a considerable evolutionary gap that has posed a critical “missing link” conundrum in understanding the origin of arthropods.

Ecological reconstruction of Kylinxia, landscape version. Credit: HUANG Diying

The research team meticulously examined Kylinxia fossils, demonstrating homology between the first appendages in Anomalocaris and true arthropods. Phylogenetic analyses suggested an affinity between Kylinxia's front appendages, the small predatory appendages in front of the mouth of Chelicerata (including spiders and scorpions), and the antennae of Mandibulata (a subdivision of arthropods encompassing insects like ants and bees).

Prof. Zhu Maoyan, a co-author, stated, “Our results position Kylinxia squarely between Anomalocaris and true arthropods, filling the evolutionary gap. This finding delves into the roots of true arthropods.” Dr. Zeng Han, the study's first author, emphasized, “Kylinxia serves as a pivotal transitional fossil in line with Darwin's evolutionary theory, bridging the gap from Anomalocaris to true arthropods and providing robust fossil evidence supporting the evolutionary theory of life.”

Source: Chinese Academy of Sciences

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