PALEOECOLOGY OF LATE DEVONIAN HYDROCARBON SEEPS DOMINATED BY BRACHIOPODS (MOROCCAN MESETA)

Competitive animals within marine chemosynthetic communities (e.g. hydrothermal vents and hydrocarbon seeps) are specialized to cultivate symbiotic bacteria that help them exploit their habitats’ abundant chemical energy. Strangely, among the most common fossils within ancient examples of these communities are brachiopods of the extinct clade Dimerelloidea, who dominated Paleozoic and Mesozoic seeps, presumably (1) without the ability to harbor symbionts and (2) without being entirely restricted to chemosynthetic habitats. To better define the ecological tolerances of this troublesome group, we described and analyzed three Moroccan deposits of the oldest dimerelloid Dzieduszyckia using a combination of field, petrographic, and geochemical evidence. The Gara de Mrirt and Bou Tazert deposits are represented by loose or displaced blocks without stratigraphic context, but the Aicha Mych site uniquely preserves an in-place Upper Devonian sequence. Below the brachiopods are black limestone beds with positive δ¹³C_carbonate excursions, interpreted as anoxic Kellwasser facies of the Frasnian-Fammenian extinction event. Above a thin horizon of banded cements, brachiopod-rich beds up to a meter thick interbed with micrite layers containing pelagic and benthic fauna. Upsection, the density and size of brachiopods decreases along with the abundance of cements until the sequence is drowned by shales. Samples from the other two sites resemble specific stratigraphic intervals at Aicha Mych. The close association between Dzieduszyckia and early-diagenetic carbonate cements with δ¹³C_carbonate values as low as -11.5‰ argues that the brachiopods were living at hydrocarbon seeps, though their role within this habitat remains a mystery. The potential relationship between this ecosystem and the underlying Kellwasser beds merits further exploration.