The paleoecology, habitats, and stratigraphic range of the enigmatic cretaceous brachiopod peregrinella.

Modern and Cenozoic deep-sea hydrothermal-vent and methane-seep communities are dominated by large tubeworms, bivalves and gastropods. In contrast, many Early Cretaceous seep communities were dominated by the largest Mesozoic rhynchonellid brachiopod, the dimerelloid Peregrinella, the paleoecologic and evolutionary traits of which are still poorly understood. We investigated the nature of Peregrinella based on 11 occurrences world wide and a literature survey. All in situ occurrences of Peregrinella were confirmed as methane-seep deposits, supporting the view that Peregrinella lived exclusively at methane seeps. Strontium isotope stratigraphy indicates that Peregrinella originated in the late Berriasian and disappeared after the early Hauterivian, giving it a geologic range of ca. 9.0 (+1.45/-0.85) million years. This range is similar to that of rhynchonellid brachiopod genera in general, and in this respect Peregrinella differs from seep-inhabiting mollusks, which have, on average, longer geologic ranges than marine mollusks in general. Furthermore, we found that (1) Peregrinella grew to larger sizes at passive continental margins than at active margins; (2) it grew to larger sizes at sites with diffusive seepage than at sites with advective fluid flow; (3) despite its commonly huge numerical abundance, its presence had no discernible impact on the diversity of other taxa at seep sites, including infaunal chemosymbiotic bivalves; and (4) neither its appearance nor its extinction coincides with those of other seep-restricted taxa or with global extinction events during the late Mesozoic. A preference of Peregrinella for diffusive seepage is inferred from the larger average sizes of Peregrinella at sites with more microcrystalline carbonate (micrite) and less seep cements. Because other seep-inhabiting brachiopods occur at sites where such cements are very abundant, we speculate that the various vent- and seep-inhabiting dimerelloid brachiopods since Devonian time may have adapted to these environments in more than one way.