Stromatoporoids and extinctions

Abstract

Stromatoporoids are common shallow marine hypercalcified sponges in two major episodes with distinctive skeletal architectures: 1) Palaeozoic: Early to Middle 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 (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, implying biotic resilience. Palaeozoic stromatoporoids suffered 5 events of decline; Event 1): end-Ordovician Mass Extinction; surviving forms are more typical of the Silurian, 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 affecting conodonts and graptolites, associated with positive carbon isotope excursions, seem not to have affected stromatoporoids, likely because of their shallow marine benthic habit, contrasting pelagic oceanic planktonic and nektonic fauna influenced by oceanographic changes. Expansion to their late Early to Middle Devonian (Eifelian and Givetian) acme, as one of the Phanerozoic’s major global reef systems, was likely linked to global sea-level rise, when epeiric seas expanded, but followed by Event 3): end-Givetian extinction, likely related to sea-level fall; Event 4): Frasnian-Famennian (F-F) extinction; and Event 5): end-Devonian (Hangenberg Event) extinction; 4 and 5 may be related to cooling, anoxia and potentially, magmatism. The apparent stratigraphic gap between end-Devonian and Triassic occurrence is normally interpreted as extinction of Palaeozoic stromatoporoids, but rare Carboniferous examples in England, Russia, USA and Japan prove survival in shallow marine environments. An interpretation that stromatoporoid-grade sponges lost ability to calcify is unlikely, because chaetetid hypercalcified sponges expanded and built reefs in the Carboniferous. Important is those skeletal architectures of hypercalcified sponges, such as stromatoporoids and chaetetids, are regarded as ‘grades of organisation’ of the skeleton, lacking phyletic value; living stromatoporoid- and chaetetid-grade sponges occur in the Demosponge and Calcarea sponge classes based on 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 an unpreserved crisis in non-calcifying Porifera, noting a poor sponge record in end-Devonian strata. Having also survived the end-Permian and end-Triassic extinctions, sponges with ability to produce stromatoporoid-grade skeletons expanded again in the Jurassic, together with sphinctozoan and inozoan grades, then survived the K-Pg extinction although are rare after the Cretaceous. Stromatoporoids seem to be more abundant during calcite seas times, so there may be both an oceanographic chemical control on their development and bias in preservation towards calcite rather than aragonite mineralogy. Overall, the hypercalcifying ability of sponges was not lost throughout their Phanerozoic history; thus, stromatoporoids and other hypercalcified sponges are preserved evidence of resilience of sponges in Earth history, contrasting other celebrated reef-building forms, such as tabulate and rugose corals, and rudist bivalves, that died out.