Oases of biodiversity: Early Devonian palaeoecology at Hamar Laghdad, Morocco

Abstract

The mudmound locality of Hamar Laghdad (Tafilalt Platform) contains possibly the greatest palaeobiodiversity, both horizontally in the surrounding sediments and stratigraphically through the Devonian sedimentary succession of the Moroccan eastern Anti-Atlas. From the Ordovician to the Devonian, sediments of the Hamar Laghdad region and their fossil contents tend to differ slightly from those in time-equivalent strata of other parts of the Tafilalt Platform. Our research provides a description of the sedimentological and palaeontological record at Hamar Laghdad covering the Lochkovian to Givetian interval. We use alpha-diversity data based on macrofossils from selected fossiliferous strata and compare the results with the relative abundance of bioclasts in polished slabs. We show that the palaeobiodiversity at Hamar Laghdad was similar to that of the southern Tafilalt with a normal diversity during the Pragian, i.e.prior to mudmound initiation and growth. By contrast, the layers covering the mudmound carbonates (i.e.when their activity had ceased) show a high diversity during the latest Emsian with a trophic nucleus comprising 36 species. This fauna, here exemplified by the 'Red Fauna' (from the Red Cliff), is dominated by benthic suspension feeders (corals, brachiopods and crinoids) and cephalopods. The shift in biodiversity is most likely related to the local occurrence of a favourable environmental setting, characterized by a temporally and spatially varying topography. It is assumed that this setting was influenced by sea-level fluctuations, currents and nutrient availability.