IS THE PRESENT THE KEY TO THE PAST? PALEOECOLOGY OF A MIDDLE DEVONIAN REEF COMMUNITY FROM MADENE EL MRAKIB (ANTI-ATLAS, MOROCCO)

The Devonian was a period of the most extensive reef development in the Earth’s history, with the peak of the reef growth reached during the Givetian to early Frasnian. The most poleward Devonian reefs are those from the shelf of NW Gondwana. One of the most prominent localities with reef communities in this area is the Givetian Madene el Mrakib site, located in the Moroccan Anti-Atlas, with at least 670 000 m² of the exposed reef strata. Here, we present preliminary results on the palaeoecology and taxonomical structure of this reef community.

The presence of a well exposed sedimentary succession at Madene el Mrakib allows for studying the environmental changes and accompanied reef fauna succession. The lower part of the section is dominated by platy tabulate (alveolitid and heliolitid) corals and stromatoporoids. The platy morphology of coral colonies is generally recognized as an adaptation to photosymbiosis and is especially common in mesophotic coral ecosystems. Therefore, the development of this pioneering assemblage probably took place within the lower part of the photic zone. A gradual increase of the fauna abundance and diversity is observed towards the top of the succession, a trend accompanied by a shift of the reef builders’ morphotypes to branching and massive ones. This part of the succession hosts also several horizons of reworked fossils, probably recording storm events, attesting to the progressive shallowing. A diverse shallow-water reef community is present in the uppermost part of the succession. Based on almost 1500 assigned fossils, we show that the climax assemblage is dominated by exceptionally large, branching pachyporid corals with a significant role of massive alveolitids, heliolitids and favositids, as well as solitary rugose corals. Stromatoporoids, generally regarded as a main component of the shallow-water Devonian reefs, are of surprisingly subordinate role. Interestingly, the observed community structure is similar to modern shallow water reefs, dominated by branching corals like Acropora, rather than to the typical Paleozoic reefs. This unique characteristic of the Madene el Mrakib community makes it particularly suitable for comparative studies on reproduction and growth patterns of the Devonian and modern branching corals.