The origin of primitive archaediscacean foraminifers and the development of their wall structure
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Abstract:
Archaediscacean foraminifers are perhaps one of the most useful biostratigraphic marker microfossils from the early Visean to early Serpukhovian. They are widespread in normal marine, benthic, shallow-shelf carbonate environments. The subfamily Planoarchaediscinae of the superfamily Archaediscacea appeared in the middle early Visean. The wall structure of the planoarchaediscins is the most primitive in this superfamily, which provides an evolutionary link between the Archaediscidae and the ancestral pseudoammodiscids. The wall of the archaediscaceans is predominantly dark and microgranular. The hyaline-radial layer is either limited to the umbilical area, or it forms a thin covering over the sides, or sometimes it covers the periphery of the interior volutions. Regarding the ancestor of the archaediscids, this topic is still open to debate. This study focuses on the specimens of Lapparentidiscus and Viseidiscus with continuous stratigraphic appearances from the Shidengzi Formation at the Malanbian section in South China. The result suggests that Lapparentidiscus is a unique pseudoamrnodiscid that gives rise to the archaediscids. The evolution trend of primitive archaedis cacean foraminifers is: Lapparentidiscus talasicus–Viseidiscus eospirillinoides–Viseidiscus monstratus. The hyalin-radial layer of the archaediscids appears as a replacement instead of a filling. It replaces the dark, microgranular layer gradually, and extends from the umbilical area to the sides.
