Early Devonian bivalves from Hamar Laghdad, Morocco

This paper discusses some latest Emsian (Early Devonian) bivalves from marls of the Amerboh Group, Hamar Laghdad (Morocco). Species belonging to five identifiable genera are present in the examined material: Panenka Barrande, 1881, Hercynella Kayser, 1878, Jahnia Růžička & Prantl, 1960, Grammysioidea Miller, 1877, and Patrocardia Fischer, 1887. In addition, at least one species could not be ascribed to a genus. All species in the material are described in open nomenclature, partially due to the imperfect preservation, partially due to the outdated systematics to the scarcity of recent revisions. The dominant group in the material are nepiomorph bivalves, which constitute around 60% of the specimens in the studied material. A high proportion of nepiomorphs in the fauna from Amerboh Group is typical for the Silurian–Devonian deeper shelf bivalve faunas from the Rheic Ocean. The different preservation of epifaunal/ semi-infaunal vs. infaunal species, and the lack of any major signs of mechanical abrasion of the specimens indicate a low-energy, slow sedimentation in a deep water setting, coincident with a sea level rise in the area. The absence of nuculoid bivalves, abundant in some other Early Devonian environments of the eastern Anti-Atlas, could have resulted from unfavourable local conditions, such as the substrate cohesiveness or organic matter content.