Study reveals the origin of spiders, bees, and other true arthropods

Arthropods are invertebrates that have a segmented body, an exoskeleton, and paired jointed appendages. Bees, spiders, crustaceans, and other arthropods have been among the most abundant and successful animals on Earth since the Cambrian Period more than 500 million years ago. 

Arthropoda is the largest phylum in the animal kingdom, making up more than 80 percent of all species. Despite the fact that arthropods are so ubiquitous, however, there is still much that is not known about how they evolved and what their ancestors looked like. The answers to these questions have puzzled scientists for more than a century.

In a new study from the Chinese Academy of Sciences, researchers have discovered a shrimp-like fossil with five eyes. The new species, which is now known as Kylinxia, was discovered in Yunnan Province in southwest China. The specimen has provided experts with critical insight into the early evolutionary history of arthropods. 

“Kylinxia is a very rare chimeric species. It combines morphological features from different animals, which is analogous to ‘kylin,’ a chimeric creature in traditional Chinese mythology,” said study co-author Professor Diying Huang.

“Owing to very special taphonomic conditions, the Kylinxia fossils exhibit exquisite anatomical structures. For example, nervous tissue, eyes and digestive system – these are soft body parts we usually cannot see in conventional fossils,” said study co-author Professor Fangchen Zhao.

Kylinxia shares distinctive features with true arthropods, such as a segmented trunk and jointed legs. However, the species also shares the morphological characteristics present in very ancestral forms, such as the bizarre five eyes of Opabinia and the iconic raptorial appendages of Anomalocaris – a giant ancestral form of arthropod.

Anomalocaris reached up to two meters in body length and was a top predator in the Cambrian ocean. Huge differences between Anomalocaris and true arthropods have indicated that there is a large evolutionary gap between the two types of this phylum. According to the researchers, this gap has become a crucial “missing link” in the history of arthropods.

After conducting detailed anatomical studies of the Kylinxia fossils, the experts found that the species had small predatory appendages in front like those in the mouth of spiders, as well as the antennae of Mandibulata – a subdivision of arthropods including bees. This led the team to the conclusion that the first appendages in Anomalocaris and true arthropods had the same structure. 

“Our results indicate that the evolutionary placement of Kylinxia is right between Anomalocaris and the true arthropods. Therefore, our finding reached the evolutionary root of the true arthropods,” said study co-author Professor Maoyan Zhu.

“Kylinxia 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, contributing strong fossil evidence for the evolutionary theory of life,” concluded study first author Dr. Han Zeng.

The study is published in the journal Nature.

—

By Chrissy Sexton, Earth.com Staff Writer