Ordovician radiolarian biodiversity : insights based on new and revised data from Kazakhstan

Abstract

Abstract Since the 1970s Ordovician carbonate rocks from Kazakhstan have provided well-preserved radiolarian faunas and this country remains one of the most important areas in the world to learn about Lower Palaeozoic radiolarian taxonomy and biostratigraphy. Here we describe a new lower Arenigian radiolarian assemblage and discuss all currently available Ordovician radiolarian data from Kazakhstan. The newly discovered assemblage is curiously oligospecific, despite its good state of preservation. Four morphospecies are recognised, including a new genus and two new species : Inanihella bakanasensis (Nazarov) and I.(?) akzhala n.sp, which characterise the assemblage, together with Triplococcus acanthicus n.gen n.sp. and Proventocitum sp.cf. P. procerulum. This is the oldest radiolarian assemblage from limestone lithologies of Kazakhstan and one of the very few Lower Ordovician assemblages discovered worldwide. We specify in this study the age range of the succeeding Haplentactinia armillata-Proventocitum procerulum assemblage, defined earlier by Nazarov and Popov [1980], which now covers the middle Arenigian – lower Llanvirnian (lower Abereiddian) interval. The succeeding upper Llanvirnian (Llandeilian) Haplentactinia juncta – Inanigutta unica assemblage reported by Nazarov and Popov [1980] reflects a substantial increase in radiolarian diversity, due largely to the introduction of new members of the family Inaniguttidae.