Abstract:
Brachiopods generally have not been considered to be typical or significant faunal components of modern or ancient hydrothermal vent and cold-seep settings. The Early Cretaceous (Neocomian) rhynchonellide brachiopod Peregrinella has long been viewed as a paleontological curiosity because of its distinctive morphology, status as the largest Mesozoic brachiopod, anomalous stratigraphic associations, and widespread, yet discontinuous paleogeographic distribution. Examination of all worldwide Peregrinella occurrences (14) indicates restriction of this brachiopod to ancient cold-seeps. It is probable that Peregrinella grew to large sizes in such great abundances at fossil cold-seep sites because of a richly organic food supply generated by localized fluid seepage and bacterial chemosynthetic activity. Living brachiopods are not known to harbor chemosymbiotic bacteria in their tissues; however, direct chemoautotrophic utilization of reduced fluids by Peregrinella cannot be rejected or demonstrated at present. Peregrinella occurs at widely separated coldseeps of Neocomian age (e.g., California, Mexico, Tibet, Europe), yet its mode of dispersal and larval development is unknown. In modern hydrothermal vents of the deep-sea, organism dispersal occurs along oceanic ridges, where benthic faunas display both planktotrophic and nonplanktotrophic larval-mode types. Peregrinella may represent a Mesozoic relic of a long-lived "lineage" of ventseep associated rhynchonellides from the Paleozoic (e.g., ?Eoperegrinella, Dzieduszyckia), but major gaps in the stratigraphic record between these rhynchonellide occurrences, and the lack of rigorous phylogenetic analysis for these groups preclude a clear resolution of the origin(s) of vent-seep brachiopods at present.