Microbial–serpulid build‐ups in the Norian–Rhaetian of the Western Mediterranean area: ecological response of shelf margin communities to stressed environments

Microbial–serpulid communities are the main reef‐builders of most Norian–Rhaetian carbonate platforms of Italy. They represent minor, yet significant communities in the shallow western Tethys, in contrast with the highly diversified sponge‐ and coral‐dominated Dachstein reefs widespread from Eastern Alps to Australia. The microbial–serpulid build‐ups are systematically associated with narrow intraplatform troughs and developed on the outer margin–upper slope area under marine conditions unsuitable for the development of coralgal reefs. The development of these settings can be related to transtensional tectonics affecting an elongate belt which was roughly parallel to the Piedmont–Ligurian Ocean spreading axis during the Jurassic. Dysoxic and mesosaline waters are the main cause of the success of the Norian microbial–serpulid build‐ups; whereas, from the late Norian to the Rhaetian, eutrophication and climate change control their distribution. These environments acted during Norian–Rhaetian times as refuges, where opportunitistic organisms could survive and build frameworks during a period otherwise dominated by coralgal reefs.