Molluskan Grazing Traces (Ichnogenus Radulichnus Voigt, 1977) on a Pleistocene Bivalve from Southern Brazil, With the Proposal of a New Ichnospecies

The ichnogenus Radulichnus Voigt, 1977 is recorded for the first time from a bivalve, Anomalocardia brasiliana (Gmelin, 1791), in a late Pleistocene molluskan assemblage from the southern Brazilian coastal plain. Grazing traces comprise short (<1mm), parallel furrows, arranged in rows on the inner (concave) shell surface and mostly concentrated in its central area. Radulichnus accommodates scratches on hard substrates produced by the radula of grazing gastropod or polyplacophorans. Our literature survey on fossil and extant traces, as well as studies on the grazing mechanism in living mollusks, document at least two distinct morphotypes that are related to differences in the feeding modes of the producers. We propose to distinguish a second ichnospecies of Radulichnus, in addition to the type, R. inopinatus Voigt, 1977 (produced by gastropods), which is named R. transversus isp. nov., andattributed to polyplacophorans. Grazing traces on the shell of A. brasiliana match the morphotype produced by polyplacophorans mollusks, and are indicative of its complex taphonomic history in comparison with other shells in this assemblage.