A Palaeozoic open shelf benthic assemblage in a protected marine environment

A Late Ordovician benthic assemblage containing trilobites (Colpocoryphe, Onnia) and diploporid echinoderms (Aristocystites) taxa that are usually typical of open shelf environments is preserved in protected marine settings in the lower part of the Kermeur Formation (Armorican Massif, NW France). A facies analysis carried out on the Veryac'h section (Crozon Peninsula) allows identification of four sedimentary facies (SF), ranging from the bay/lagoon domain (SF1 and SF2) to the back-barrier (SF3) and barrier (SF4) domains. A benthic fauna comprising of trilobites, brachiopods and diploporids is found in the SF1 and SF2. In SF1, fossils are randomly scattered in sediments (taphofacies 1, TF1) showing two different preservation stages: TF1a with complete articulated skeletons and TF1b with disarticulated shells. These two preservation stages can be related either to the different modes of life of the taxa (i.e., recumbent, burrowing or nektobenthic) or to a composite assemblage with both autochthonous and allochthonous taxa. In this benthic assemblage, some trilobites and echinoderms taxa are mostly preserved either as complete exoskeletons and exuviae (Colpocoryphe grandis) or complete thecae (Aristocystites sp.), indicating an autochthonous origin. These taxa are usually found in low-energy open shelf settings. Their occurrence in a protected environment results from the combination of both allocyclic factors (i.e., change in sea level) and local conditions (i.e., shelf morphology, environmental conditions). Indeed, the lower part of the Kermeur Formation was deposited during a transgressive phase following a significant sea-level fall (Sandbian-Katian boundary). During the transgressive phase, barrier-lagoon systems were formed on the flat surface inherited from the sandbian shelf. Under non-restricted ecological conditions (i.e., normal oxygenation and salinity) in the protected settings, the new ecological niches were suitable for benthic organisms accustomed to open shelf environments. The settlement of these open shelf benthic species in coastal and protected environments may represent an offshore-onshore expansion, contrary to that described in the onshore-offshore diversification model of Phanerozoic shelf communities (Jablonski et al., 1983; Sepkoski, 1991).

Research Highlights► Armorican Katian sediments were laid down during a significant transgressif phase. ► Barrier-lagoon systems were formed on the flat surface inherited from the shelf. ► Benthic taxa (Trilobita, Diploporida, Brachiopoda) occurred in the bay/lagoon facies. ► Some autochthonous species are usually typical of open shelf settings. ► This colonisation represents an offshore-onshore migration.