New bivalves from a Middle Devonian methane seep in Morocco: the oldest record of repetitive shell morphologies among some seep bivalve molluscs

Abstract

A fauna of bivalve molluscs is described from methane seep carbonates of the Middle Devonian (c. 390 Ma) Hollard Mound in the eastern Anti-Atlas, Morocco. We describe a new modiomorphid genus Ataviaconcha gen. nov. with the type species Ataviaconcha wendti sp. nov. This is a very large, semi-infaunal species occurring in large colonies similar to those formed by Recent chemosymbiotic cold-seep and hydrothermal vent bivalves. It is the second modiomorphid bivalve known from Palaeozoic chemosynthesis-based ecosystems, after the roughly coeval Sibaya ivanovi Little, Maslennikov, Morris & Gubanov, 1999, from the Sibay hydrothermal vent deposit in the Ural Mountains, Russia. The second and much less numerous bivalve species described in this paper is the solemyid Dystactella? eisenmanni sp. nov., belonging to a genus known also from Ordovician to Devonian marine environments distinct from cold seeps and hydrothermal vents. As with other fossil and Recent solemyids, it was an infaunal burrower, most likely living in symbiosis with chemoautotrophic bacteria. These new findings show that bivalves are ancient in chemosynthesis-based ecosystems, thriving there for at least 390 Myr, and that the bivalve-dominated faunas predated the first occurrence of dimerelloid brachiopods at seeps. The early evolutionary adaptation of some bivalves to chemosynthesis-based ecosystems is probably related to a symbiosis-based metabolism allowing efficient exploitation of chemosynthetic food resources. Ataviaconcha wendti sp. nov. represents a morphology which recurred several times throughout the following 390 Myr in different bivalve groups that flourished at hydrocarbon seeps. This strongly suggests environmental control on the evolution of adaptations in seep biotas.http://zoobank.org/urn:lsid:zoobank.org:pub:A83D5CB1-67D2-4D05-8EBC-BFCA6E6845D8