Paleoecology of Early Ladinian low-diversity radiolarian fauna from Mt. Svilaja (External Dinarides, Croatia)

• low radiolarian diversity in stratified oxygen-deficient semi-enclosed basins. • presence of some entactinaria and Spumellaria, and monocyrtid nassellaria. • absence of deep-water multi-segmented nassellaria. • absence of eptingiidae and oertlispongidae. • example of a triassic coastal ``dead zone'' due to low oxygen in bottom waters. Dark-grey cherty limestone from Mt. Svilaja contains moderately well-preserved radiolarians of unusually low diversity. Nineteen genera were encountered, namely, only one fourth of genera known from the time equivalent Buchenstein Formation. Based on conodonts, the studied interval is assigned to the Lower Ladinian Budurovignathus hungaricus Zone. Ammonoids and allochthonous fossil elements (calcareous algae, corals, brachiopods, bivalves, benthic foraminifera, terrestrial-plant remains) from the same interval were previously reported. Facies and organic-matter analyses support the interpretation of depositional setting in a semi-enclosed basin with oxygen-deficient bottom waters. The radiolarian assemblage consists of spherical Entactinaria (Pentactinocarpidae, Heptacladidae, Hindeosphaeridae) and Spumellaria ( Archaeocenosphaera, Paurinella, Triassospongosphaera, Spongopallium ), and monocyrtid Nassellaria (mostly Hozmadia ). Among Pentactinocarpidae, Lobactinocapsa ellipsoconcha Dumitrica is abundant and characterized by considerable variability of the cortical shell regarding its shape (ovoid to spherical), wall thickness (single-layered to spongy), and number of external spines. Eptingiidae, Oertlispongidae, Relindellidae, and all multicyrtid Nassellaria, common in the Buchenstein Formation as well as in radiolarian cherts associated with ophiolites, are missing. Similar, although less drastically reduced radiolarian fauna is known from the coeval San Giorgio Dolomite, which was also deposited in an oxygen-deficient intra-platform basin. The San Giorgio fauna lacks multicyrtid Nassellaria but still contains abundant Eptingiidae, Oertlispongidae, and Relindellidae. The likely factor reducing the diversity in the intra-platform basins was the vertical extent of the oxygen-deficient lower water column. Only surface-dwelling radiolarians were successful in stratified basins with expanded deep-water hypoxia.