Radiolarians from Cambrian Series 2 Niujiaohe Formation in southern Jiangxi, South China

Abstract

The origin and early evolution of radiolarian fossils have always provoked great interest among paleontologists. Yet, little is known about the earliest morphological and evolutionary characteristics of radiolarians largely due to the lack of documentation of early Cambrian radiolarian fossils. Herein, we conduct a thorough examination of the basinal Niujiaohe Formation in Chongyi County of southern Jiangxi and report two types of radiolarians: a large number of spherical radiolarians and a minor occurrence of ellipsoidal radiolarian fossils with bipolar rodded spines. The lengths of the spines on ellipsoid radiolarian fossils vary, and typically, the main spines are slightly offset from the polar axis, resulting in an asymmetrical appearance in some specimens. Through systematic taxonomic examination, this radiolarian fossil form is tentatively identified as Beothuka sp. of the spherical polycystine group. The genus Beothuka is commonly found in the Early Ordovician and has not yet been reported in the early Cambrian. Combined with previous reports, four species of radiolarians in three genera are identified from the Niujiaohe Formation in South China, including Paraantygopora porosa, Paraantygopora sp., spherical radiolarian, and Beothuka sp. Overall, the radiolarian fossils in the Niujiaohe Formation are characterized by low diversity and small size, in contrast to the relatively more diverse fauna found in the contemporary shallow-water Shuijingtuo Formation. Therefore, we speculate that radiolarians may have initially originated in shallow water areas and gradually expanded to deep water. This study further enriches the fossil record of the Cambrian Series 2 radiolarians. Combined with relevant data from South China, the radiolarian fauna began to exhibit diversity and complex morphology, indicating the presence of various, advanced spherical-skeletons in early radiolarians, and radiolarians from the early Cambrian appeared to display multiple evolutionary trends.