Cambrian Series2Shuijingtuo Formation Radiolarian Fauna From Zigui, Hubei Province

The Cambrian marks an important phase in the history of life on Earth. The system is characterized by the appearance of numerous metazoans bearing mineralized skeletons, and by a rapid diversification of animals commonly referred to as the "Cambrian explosion". Nearly all animal phyla known from the fossil record appeared during the Cambrian Period. The third and also the largest phase occurred in the Cambrian Stage3, which involve all the three supraphylogenetic clades of the Eubilateria, However, most researchers mainly focus on the metazoans in shallow marine ecosystem and overlook the study of the plankton in the ocean.The origin of radiolarians and their early evolution are of great interest for paleontologists and biologists. Due to the rather absence of available radiolarian fauna in Cambrian and late Pre-Cambrian, abundant questions and disputes have been left.Radiolarian is a kind of unicellular plankton with siliceous skeleton in ocean. The micro-organism group evolves rapidly, distributes widespread and possesses a long evolution history. They are a constitution of zooplankton communities from Cambrian period at least. The research of Cambrian radiolarians will contribute us to understand their origin and evolution in the period of Cambrian and also the Early Cambrian bioradiation event.Limestone concretions at the bottom of Shuijingtuo formation of Cambrian Series2in Jiuqunao, Zigui, Hubei province yielded various and abundant radiolarian fossils. After detailed identification to these radiolarian specimens,3genus and3species were recognized, namely Kalimnasphaera sp., Antygopora sp.. Orbes hubeiensis n. gen. n. sp. and some spherical radiolarians. Orbes n. gen. is defined as a new genus and Orbes hubeiensis n. gen. n. sp., as a new species herein. The taxa supplied the taxonomy and biostratigraphy of radiolarians in the Early Cambrian to some extent. Additionally, other conclusions are drawn as follows.1. All reports about Kalimnasphaera are from the Middle and Upper Ordovician in Australia and U.S.A. and all descriptions of Antygopora, from the Early and Middle Ordovician in Norway and Canada The both genus are described from the Early Cambrian for the first time herein.2. The needle-like or spicular construction of radiolarians represented by Order Archaeospicularia has been considered as an ancestor of radiolarians until Braun et al. discovered some radiolarians with lattice shells from the Early Cambrian in South China in2007. The similar radiolarian tests were collected from the Shuijingtuo Formation corresponding to the basal part of Cambrian Series II, in South China. This observation supports the result of Braun et al.(2007). The hypothesis that Archaeospicularia is the representative of the oldest radiolarian group needs to be tested by examining older materials than those reported herein. Meanwhile, Our finds support the idea that spherical radiolarians are a very ancient morphology among radiolarians.3. Three distinct types of shell structure of radiolarians, latticed, spicules and spongy, appeared in this fauna. It shows that the known Early Cambrian radiolarian fauna was complex in structure and diverse in morphological expression. So it is necessary to investigate simpler, original radiolarian faunas in older strata.4. The fauna appeared at the time of the Cambrian Explosion and contained the various and abundant radiolarian specimens. It indicates radiolarians have been a constituent of zooplankton communities in the Early Cambrian and they may go through rapid diversification similar to other metazoans in this period.