【進化】被甲を持つカンブリア紀の蠕虫

The fossil of an armoured worm-like animal from the Cambrian period, approximately 535 million years ago, is presented in Scientific Reports this week. The new species, called Eokinorhynchis rarus has five pairs of large bilaterally placed spines on its trunk and may be related to a group of creatures called kinorhynchs.

Kinorhynchs are a group of approximately 240 living invertebrate species which are found in marine environments. The body of kinorhynchs is divided into three sections: a head, which includes a mouth cone with circlets of teeth; a neck; and a trunk with 11 segments. As such, these creatures could provide clues to the origins of body segmentation, but such efforts have been hampered by the lack of well-preserved kinorhynch fossils.

Shuhai Xiao and colleagues describe several fossils, including E. rarus from the early Cambrian period, discovered in Nanjiang county in China. The authors found that E. rarus has a number of similarities with living kinorhynchs, suggesting a close evolutionary relationship. For example, the trunk of E. rarus and living kinorhynchs have a number of hollow spines and are divided into segments, each consisting of articulated plates. However, unlike modern kinorhynchs, E. rarus is armoured with spines that are larger and more distinct than spines on modern kinorhynchs. As a result of their analysis, the authors suggest that E. rarus may be an ancestor of modern kinorhynchs.