* Astronomy

Title: Priapulid worms: Pioneer horizontal burrowers at the Precambrian-Cambrian boundary
Authors: Jean Vannier1, Ivan Calandra1, Christian Gaillard and Anna ylinska

The major evolutionary events that characterise the Precambrian-Cambrian transition are accompanied by profound ecological changes in the composition of benthic communities, the nature of the substrate, and the occupation of marine ecospace. The increased animal activity on and within the substrate is attested to by numerous trace fossils, such as the cosmopolitan Treptichnus pedum whose first appearance is used as the global stratotype section and point (GSSP) to mark the base of the Cambrian. In spite of its major importance in biostratigraphy, the maker of Treptichnus  trace fossils, and more generally of treptichnids, has long remained an enigma. Treptichnids were subhorizontal burrow systems produced in the subsurface and had a worldwide distribution throughout the Cambrian. Here we show, by using experimental ichnology, that the treptichnid burrow systems were most probably produced by priapulid worms or by worms that used the same locomotory mechanisms as the Recent priapulids (e.g., Priapulus). Their typical three-dimensional morphology with repeated arcuate probing branches suggests that their function was related to the feeding strategy of the worm such as predation or scavenging upon small epibenthic or endobenthic invertebrates. This interpretation is strongly supported by the preserved gut contents of Cambrian priapulids from the Burgess Shale Lagerstätte that contain effectively a variety of small epibenthic prey. The antiquity of treptichnids would designate priapulids as one of the earliest infaunal colonisers of the substrate that possibly interacted with epibenthic communities, thus playing a leading role in the construction of the early marine food chain, and as important subhorizontal bioturbators in the early stages of the "Cambrian Substrate Revolution."

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