進化学：エビに近い大きさのキメラ節足動物がもたらす進化の新知見

The discovery of a fossil arthropod with a body structure that combines features from different groups of arthropods, including five eyes on stalks, is described in Nature this week. The species, Kylinxia zhangi — named after a chimeric creature in Chinese mythology called ‘Kylin’ and the Chinese word for shrimp — furthers our understanding of evolutionary transformations and the relationships between early Cambrian arthropods.

Resolving the early evolution of arthropods is a challenge in evolutionary biology. Well-preserved fossils from the Cambrian (around 541–485 million years ago) have offered some insights, but the relationships between major Cambrian arthropod groups remain unclear.

Diying Huang and colleagues describe six specimens of this previously undescribed fossil arthropod species, which was discovered in the Chengjiang biota in Yunnan, China (which dates to the early Cambrian). The shrimp-sized fossils combine features from different arthropods: five stalked compound eyes on the head, like the enigmatic Opabinia regalis discovered in the Burgess Shale; two large frontal appendages for seizing prey, as seen in predators like Anomalocaris; and a body structure that is similar to those of another group of extinct predatory arthropods, the Megacheira. The chimeric morphology of K. zhangi suggests that it could be a transitioning species that bridges different arthropod groups.

Phylogenetic analyses also suggest that there is similarity between the front appendages of K. zhangi, appendages in front of the mouth of Chelicerata (a subdivision of arthropods that includes scorpions) and the antennae of Mandibulata (a group that includes insects such as bees). The authors conclude that K. zhangi helps to provide insights into the origin of these evolutionary innovations.