Modern-type reef in ancient time - Palaeoecology of a Middle Devonian coral community from Madène el Mrakib (Anti-Atlas, Morocco)

Abstract

The southernmost Devonian reefs formed along the north-western margin of Gondwana. One of the best-preserved reefal palaeoecosystems of this area is found in the Givetian of Madène el Mrakib (Anti-Atlas, Morocco), exposing a vast, at least ∼1.2 km2 area of reef strata. This contribution documents the ecological succession, taxonomical structure and palaeoecology of this exceptionally diverse reef community. Combined evidence from the sedimentary succession and faunal changes records a trend of progressive shallowing. The ecological succession is characterised by an upsection increase in the abundance and diversity of the reef-building biota, accompanied by a shift in the dominant morphotypes of tabulate corals from platy (pioneering and diversification stages) to branching and massive forms (climax stage). The exceptionally rich, shallow-water reef community occurs in the uppermost part of the succession. The climax community is dominated by extraordinarily large, branching tabulate corals (mostly Thamnopora), with the notable role of massive alveolitid, heliolitid and favositid tabulates, as well as large, solitary rugose corals. A surprising feature of this assemblage is the rather subordinate role of stromatoporoids, generally regarded as a main component of the shallow-water Devonian reefs. Remarkably, the observed community structure is closer to that typical of modern shallow-water reefs, dominated by branching scleractinian corals from the family Acroporidae, than to the archetypal Paleozoic reefs. These distinctive traits of the Madène el Mrakib community make it particularly suitable for comparative studies between Devonian and modern reef ecosystems, showing that these communities shared notable similarities in terms of their ecological successions, zonation patterns, dominance of branching corals and high morphological variability in shallow-water environments, and adaptations of corals to turbulent conditions.