Shell microstructure and composition illuminate the classification and evolution of early calcifying foraminifera

Foraminifera is one of the most abundant and diverse groups of marine shelled micro-organisms whose fossil record extends as far back as the early Cambrian. Throughout the early Palaeozoic, foraminifers were mainly represented by simple-shaped agglutinated forms. More complex, calcareous, forms did not evolve until the Middle Devonian (⁓394 Ma) when multichambered foraminifera with complex internal shell structure emerged during the so-called ‘Givetian Revolution’. However, due to poor preservation, the systematics and evolutionary relationships of these early calcifying foraminifera are still far from being resolved. Here we present the exceptionally well-preserved Middle and Upper Devonian specimens derived from numerous localities of Euramerica and Gondwana. Diagenetically unaltered specimens of Semitextularia, Moravammina, Vasicekia and Tolypammina from Poland and Russia exhibited identical lamellar microstructure of their primary calcitic shells. This suggests, that unlike the younger, Carboniferous fusulinids having microgranular shell texture, calcareous foraminifera of the Middle and Upper Devonian likely belonged to a distinct higher-level taxonomic group characterized by uniform lamellar calcitic texture. We propose that this unique group be included within the new class Laminida, whose representatives possibly evolved from morphologically simple Silurian forms. Their further radiation and evolution were likely related to the onset of widespread shallow and warm carbonate platform and reef deposition during the Middle Devonian. Such shallow and well-illuminated habitats could have also promoted foraminiferal endosymbiosis with microalgae or chloroplast sequestration, which might have further promoted the radiation of multichambered calcareous forms.