Toarcian–Aalenian (Early–Middle Jurassic) radiolarian fauna from the Los Molles Formation, Neuquén Basin, Argentina: Taxonomy and paleobiogeographic affinities

This paper reports the occurrence of a Toarcian–Aalenian (Early–Middle Jurassic) radiolarian fauna in the Los Molles Formation, Neuquén Basin, Argentina, as well as comments on its paleobiogeographic affinities. The micropaleontologic analysis was carried out in fine-grained rocks from a turbiditic section of the Los Molles Formation. These samples were first chemically processed using only hydrogen peroxide (H2O2), and afterward treated with acetic (CH3COOH) and hydrofluoric (HF) acids. The first chemical procedure permitted the recovery of only few spongy spumellarians, while the second one enabled to recover more diversified radiolarian assemblages. In general, the studied fauna presents low diversity and abundance, with a strong dominance of spumellarians over nassellarians. The fauna is composed by the genera Paronaella, Homoeoparonaella, Praeconocaryomma, Archaeocenosphaera, Orbiculiformella, Praeparvicingula, and some unidentified spumellarians and nassellarians. According to paleobiogeographic models, the studied Toarcian–Aalenian fauna presents a mid to high latitude affinity. It is possible to infer from those data a bipolar distribution of some taxa, such as Praeparvicingula and probably Praeconocaryomma, between the Northern and Southern hemispheres since the Early Jurassic (Toarcian).