A 'hermit' shell-dwelling lifestyle in a Cambrian priapulan worm.

The Cambrian 'explosion', about 530 million years ago, marks a rapid diversification of the major animal lineages. A concomitant increase in the complexity of ecosystems is believed to have accelerated this evolutionary radiation, but direct evidence of the ecological modes of Cambrian taxa is nevertheless scarce - even in exceptional Burgess Shale-type deposits. Here, we present new fossil material from the Cambrian (Stage 4) Guanshan biota in southern China that reveals the consistent occurrence of the priapulan worm ?Eximipriapulus within the conical shells of hyoliths. This represents the first direct evidence of a 'hermiting' life strategy - the adoption of a different organism's exoskeleton - in the priapulans and within the Palaeozoic era. It highlights the intense degree of convergent evolution during the Cambrian radiation. Hermiting behaviour has previously been linked with the escalation of predation pressure during the Mesozoic marine revolution; such intensity of predation may also have characterised early Cambrian oceans.