Palaeoecology of a post‐extinction reef: Famennian (Late Devonian) of the Canning Basin, north‐western Australia

Abstract

Reefs were decimated by the Frasnian/Famennian (Late Devonian) mass extinction event (371 Ma), and are assumed to have survived only as depauperate calcimicrobial communities dominated by disaster taxa. Description of Famennian proximal reef‐slope communities within the Windjana Limestone, Canning Basin, north‐western Australia, shows that, notwithstanding the loss of large metazoans, novel ecologies were established in this setting by a rich biota of survivor and progenitor taxa. Diverse calcimicrobes together with algae, crinoids, bryozoans, brachiopods, and abundant sponges (stromatoporoids, inozoans, sphinctozoans, lithistids and hexactinellids) formed a reef framework of either elevated platy structures up to 4 m in diameter and 0.35 m thick, or mounds up to 15 m in diameter. This framework was dominated by a complex intergrowth of calcimicrobes, where Rothpletzella formed the primary framework, Ortonella, and Girvanella were secondary encrusters, and Shuguria spp. occupied small crypts 2–30 mm in diameter. Contiguous columnar stromatolites up to 50 mm in height and 1 m in width grew upwards from substrate sheltered beneath large sheltered primary cavities: based on minimum growth rates of 50–100 μm/year these are estimated to have been between 500 and 1000 years old. The elevated platy community is inferred to have grown in conditions of episodic siliciclastic sediment input; the reef mounds grew during either episodes, or in localised areas, of low sedimentation. At least 14 morphospecies of spicular sponges are now identified from the Windjana Limestone, where only two were previously documented. These fore‐slope reef communities exposed in Windjana Gorge flourished in high‐energy carbonate environments dominated by coated grain sediments, and where rapid, early lithification was pervasive.Such observations demonstrate that no protracted interval of time was necessarily required for post‐extinction ‘recovery’ in regions where some reef‐building taxa survived and suitable carbonate habitats persisted or returned. Moreover, they show that new ecologies, rather than remnants of the pre‐extinction community, could be established rapidly. The reef‐slope communities of the Windjana Limestone offer little evidence to support the ideas of resurgence or invasion of taxa from deeper waters after the Frasnian/Famennian extinction. Indeed, there is evidence to suggest that similar microbial‐sponge communities were already established in margin and reef slope communities in the latest Frasnian. As such, the most dramatic ecological changes caused by the extinction occurred in back‐reef communities.