DISTRIBUTION OF PEDICLE BORING TRACES AND THE LIFE HABIT OF LATE PALEOZOIC LEIORHYNCHID BRACHIOPODS FROM DYSOXIC HABITATS

Pedicle boring traces, Podichnus isp., are concentrated on the anterol ateral commissure of both valves of Leiorhynchoidea carboniferum and L . weeksi from the black ferruginous micrites of the Chainman Formation (Upper Carboniferous; west-central Utah, USA) and black phosphatic (' False Cap') limestone of the Phosphoria Formation (southeastern Idaho, USA), respectively. The absence of Podichnus isp. from the posterior of shells of both species indicates that the beak of the shell was bur ied in the mud, inaccessible to colonization by conspecific larvae. Th e concentration of pedicle boring traces near the anterolateral incurr ent regions of leiorhynchid shells with a well-developed central fold further suggests that settling conspecific larvae behaved rheotaxicall y. Larvae were induced to metamorphose near the commissure of the host shell, where suspended food was drawn to the incurrents of the host. The piggybacked mode of life on the anterior of conspecific hosts prov ided a refuge for juveniles above the dysoxic black sediments. In cont rast, modern brachiopods that live as epibionts on skeletal substrates display either a random or posteriorly concentrated distribution of p edicle boring traces.