Devonian rugose coral 'Amplexus' and its relation to submarine fluid seepage

The rugose coral 'Amplexus' occurs frequently in the sedimentary cover of the Devonian submarine volcanic intrusion in the eastern Anti-Atlas, southern Morocco. This study elucidates the palaeoecology of 'Amplexus', which forms very rich and mostly monospecific, spot assemblages within the Lower and Middle Devonian carbonates. Geological and isotopic evidences show that these associations developed at hydrothermal vents and at a cold seep site. The assemblages are always spatially associated with micritic carbonate bodies, occurring within bedded, hemipelagic deposits. The 'Amplexus' corals preferred locations in the close proximity of submarine hydrothermal fluid seepage, but they generally avoided places with the most elevated temperatures. The corals colonised also a hydrocarbon seep, probably only in its terminal phase of development, when the fluid flow was still at least periodically active. This was only possible as a result of the corals following a calice-in-calice growth, developed due to the environmental toxicity, which facilitated selective survival of larvae that settled in the shelter of empty calices. The 'Amplexus' corals appear to have constituted ecological opportunists, thriving in the nutrient-rich, venting- and seepage-affected areas that were hostile for other benthic organisms. It can be suspected that the unusual, extremely simplified morphology of 'Amplexus' made it particularly well adapted to living in environments typified by harsh and unstable conditions.