Epibionts on shells in the Šárka Formation: A sparsely occupied niche in the lower to middle Darriwilian (Oretanian, Ordovician) in the Prague Basin (Czech Republic)
Abstract
Epizoic invertebrates attached to shells are a rare component of the fauna of the Šárka Formation (Middle Ordovician; early to middle Darriwilian) of the Prague Basin (Barrandian area) in the Czech Republic. Epibionts are known exclusively from siliceous nodules and consist of echinoderms (edrioasteroids and possible crinoid holdfasts), monoplacophorans, and bryozoans, all attached to a very small number of host taxa.
Among them, hyoliths are the strongly preferred substrate (60% of observed host shells), with edrioasteroids, possible crinoid holdfasts and the monoplacophoran Pygmaeoconus attaching exclusively to these organisms. Epizoan holdfasts were also found on a unique ramose problematic fossil considered to be the first octocoral specimen from the Šárka Formation.
Most of the host are interpreted have been colonized during life, only a few shells were used for attachment post mortem. All epibionts are attached to the outer surface of the shell, with the exception of the monoplacophoran Barrandicella, which is attached to the internal surface of empty conulariid thecae.
Some of the epibionts shed additional light on the mode of life of their hosts, including the bivalve Redonia, which is shown to be semi-infaunal rather than infaunal. More generally, epibionts in the Šárka Formation attest to a very low level of occupancy of this significant niche during the early stages of the Great Ordovician Biodiversification Event.