Abstract: Millions of crustaceans, insects, myriapods (millipedes), chelicerates (spiders, scorpions) and worms, have an inextensible external skeleton and have to grow through successive moults during their lifetime. They belong to the supergroup Ecdysozoa. The moulting process (ecdysis) is strictly controlled by the endocrine system and allows these animals to grow in size by periodically renewing their cuticle.

The lowermost Cambrian fossils from China described here show that this complex process appeared during the early stages of animal evolution, at least 535 million years ago, as evidenced by remarkably well-preserved moults of marine worms.

Microscopic and microtomographic show that these ornamented worms moulted in a manner similar to that of extant priapulid worms, extricating themselves smoothly from their old tubular cuticle or turning their exuviae inside out like the finger of a glove. This is the oldest record of moulting in ecdysozoans. Ecdysis appears as a major innovation in the evolution of animals and of key importance in the appearance and diversification of arthropods during the Cambrian.

Source: Origin of ecdysis: fossil evidence from 535-million-year-old scalidophoran worms. Deng Wang, Jean Vannier, Isabell Schumann, Xing Wang, Xiao-Guang Yang, Tsuyoshi Komiya, Kentaro Uesugi, Jie Sun and Jian Han. Proceedings of the Royal society B, July 10, 2019.