Stromatoporoids and extinctions: A review

Stromatoporoids are common shallow marine hypercalcified sponges in two major episodes with distinctive skeletal architectures: 1) Palaeozoic: Ordovician to Late Devonian; and 2) Mesozoic: Late Triassic to Cretaceous and rare Cenozoic, but not confirmed in Permian and earlier Triassic strata. Stromatoporoids appeared in Early to Middle Ordovician strata, important in buildups from late Middle Ordovician metazoan expansions (part of the Great Ordovician Biodiversification Event). Throughout the Palaeozoic, some stromatoporoid taxa occur across several palaeocontinents, and, if they are the same biological taxa, presumably migrated as larvae across oceans. Palaeozoic stromatoporoids suffered 5 events of decline; Event 1): end-Ordovician Mass Extinction; surviving forms are typical Silurian taxa, marking change of abundance from labechiid to clathrodictyid forms. Event 2): late Silurian to Early Devonian contraction: stromatoporoids became scarce with low generic diversity, presumably related to global sea-level fall. Intra-Silurian extinction events principally affected conodonts and graptolites, associated with positive carbon isotope excursions, but not stromatoporoids, likely because of their shallow marine benthic habit, contrasting pelagic oceanic planktonic and nektonic fauna influenced by oceanographic changes. Stromatoporoid expansion to their late Early to Middle Devonian (Eifelian and Givetian) acme, forming a major Phanerozoic global reef system, was likely linked to global sea-level rise, when epeiric seas expanded, but followed by Event 3): end-Givetian extinction, possibly related to cooling; Event 4): Frasnian-Famennian (FF) extinction; and Event 5): end-Devonian (Hangenberg Event) extinction; 4 and 5 may be related to sea-level fall, cooling, anoxia and potentially, magmatism. The apparent stratigraphic gap between end-Devonian and Triassic stromatoporoids was not extinction of Palaeozoic stromatoporoids, because rare Carboniferous examples in England, Russia, USA and Japan prove survival in shallow marine environments. Prior interpretation that stromatoporoid-grade sponges lost ability to calcify is unlikely, because chaetetid hypercalcified sponges expanded and built Carboniferous reefs. Important is that skeletal architectures of stromatoporoid and chaetetid hypercalcified sponges are regarded as ‘grades of organisation’ of the skeleton, lacking phyletic value; living stromatoporoid- and chaetetid-grade sponges occur in the classes Demospongiae and Calcarea based on their spicules. This implies that extinction of sponge taxa that just happened to have been stromatoporoid-grade hypercalcifiers may explain stromatoporoid loss in the end-Devonian, and may point to unpreserved crises in non-calcifying Porifera, noting poor sponge records in end-Devonian strata. Having also survived the end-Permian and end-Triassic extinctions, stromatoporoid-grade hypercalcification expanded again in the Jurassic, together with sphinctozoan and inozoan grades, and then survived the K-Pg extinction although stromatoporoid-grade sponges are rare after the Cretaceous, perhaps due to the large progressive sea-level fall of the Cenozoic and consequent loss of habitat. Stromatoporoids appear to be more abundant during calcite seas times, so there may be both an oceanographic chemical control on their development and a preservation bias towards calcite rather than aragonite mineralogy. Overall, the ability of sponges to hypercalcify was not lost throughout their Phanerozoic history; thus, stromatoporoids and other hypercalcified sponges are preserved evidence of the resilience of sponges to environmental change, in contrast other famous reef-building forms, such as tabulate and rugose corals, and rudist bivalves, which became extinct.