Priapulid worms: Pioneer horizontal burrowers at the Precambrian-Cambrian boundary

Abstract

International audienceThe major evolutionary events that characterize 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 Lagerstatte that contain effectively a variety of small epibenthic prey. The antiquity of treptichnids would designate priapulids as one of the earliest infaunal colonizers of the substrate that possibly interacted with epibenthic communities, thus playing a leading role (1) in the construction of the early marine food chain, and (2) as important subhorizontal bioturbators in the early stages of the "Cambrian Substrate Revolution.

Ediacaran matground ecology persisted into the earliest Cambrian

“…1i) and T. pollardi. These full-relief branching structures were emplaced slightly below the mat surface, and therefore are regarded as produced by undermat miners (Treptichnus-Saerichnites ichnoguild), most likely worm-like organisms, such as priapulids 19 .…”

“…This paper is based on an integrated detailed sedimentological and ichnological study of 19 Figure 2 | The ichnological record of animal-matground interactions across the Ediacaran-Fortunian transition. Some types of interactions are restricted to the Ediacaran (mat digestion), whereas others cross the Ediacaran-Fortunian essentially unaffected (mat grazers), display significant compositional changes (mat scratching), or represent Fortunian innovations (undermat mining).…”

Ichnological evidence for meiofaunal bilaterians from the terminal Ediacaran and earliest Cambrian of Brazil

“…Diverse ichnofossil assemblages in the earliest Cambrian place an important constraint on the tempo of bilaterian origins, as they indicate that some groups, including total group panarthropods and priapulid-like scalidophorans 2,9 , were globally distributed and abundant by this point. The major bilaterian divergences (that is, the protostome-deuterostome and ecdysozoan-lophotrochozoan divergences) must therefore pre-date the Ediacaran-Cambrian boundary.…”

Trilobites, their traces and associated sedimentary structures as indicators of palaeoenvironment of Cambrian of the Ociesęki Range (Holy Cross Mountains, Poland)

“…Dur ing this time, in en vi ron ments deeper than the Skolithos ichnofacies, the only sed i ment-reworkers were worm-like or gan isms (Sepkoski, 1978(Sepkoski, , 1979Droser, 1991; Droser et al, 1994), prob a bly mostly priapuloids (Vannier et al, 2010). Deeper-reach ing bur row ing only took place in mid dle shoreface and shal lower en vi ron ments dur ing the Early Pa leozoic (Droser, 1991;Droser et al, 1994).…”