Five-Eyed "Missing Link" In Arthropod Evolution Discovered

Fossil specimen of Kylinxia are remarkably well preserved revealing soft tissue insights. ZENG Han

Since the Cambrian Period, arthropods have roamed the land and sea in the form of extinct and extant insects, arachnids, myriapods, and crustaceans. This invertebrate group of animals have strong exoskeletons instead of bones and segmented bodies with paired jointed appendages. Today, they constitute around 80 percent of all living animals, ranging in size and morphology from the Christmas Island coconut crab to a tiny mite. So, how did these crunchy creatures come to dominate the Earth? New research published in the journal Nature investigated a specimen retrieved from the Chengjiang fauna in southwest China's Yunnan Province to find out.

The origin of arthropods has been shrouded in mystery for decades, but Anomalocaris, an extinct genus of animals thought to be one of the earliest examples of an apex predator, was thought to be a main player in their history. However, huge morphological differences got in the way of understanding their true relation to the phylum, and this evolutionary gap became a crucial "missing link" in the origin of arthropods, one which the researchers hoped to bridge.

Ecological reconstruction of Kylinxia, HUANG Diying

Kylinxia was a shrimp-like fossil with five eyes, whose fossils exhibit an exquisite number of anatomical structures as a result of their specific process of fossilization. These include soft body parts such as nervous tissue, eyes, and digestive structures, which aren’t usually visible in fossils. They’re so named because they appeared to act as a chimeric species, combining the morphological characteristics of a range of different species. In traditional Chinese mythology, 'kylin,' is one such chimeric creature.

To see if Kylinxia played a role in the evolution of true arthropods, the team carried out examinations on their fossils and found that the front appendages of Kylinxia were similar to the small predatory appendages that sit by the mouth of Chelicerata, the group of arthropods that encompasses spiders and scorpions. They were also similar to the antennae in Mandibulata, which contains insects such as bees and ants.

"Missing Link? Lil' ole me? Well I'll be damned" - Kylinxia. Anatomical reconstruction by HUANG Diying

The researchers realized that Kylinxia shared distinctive features of true arthropods, while also integrating characteristics from ancestral forms, including the five eyes of the Cambrian's “weird wonder”', Opabinia, and our early apex predator, Anomalocaris. The researchers suggest that in establishing these ties between Anomalocaris, Kylinxia, and true arthropods, they have discovered the evolutionary root of the true arthropods.

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

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