The ossicles and fragments of crinoids and their palaeoecology from the Upper Ordovician of Tazhong Platform, Tarim Basin, NW China
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Abstract:
Crinoid fragments and ossicles are very common in Ordovician carbonate rocks of the Tazhong Platform, Tarim Basin, Northwest China. In this paper, which differs from the previous studies that focused on intact crinoid fossils, we present a comprehensive analysis of the types and contents of the crinoid stem fossils including ossicles and fragments, and the taphonomy of the symbiotic animals from the Upper Ordovician core of Well Tazhong 35, as well as the sedimentary environments in order to understand the paleoecology of the crinoids. From the core samples, seven types of ossicles are identified: Cyclocyclicus, Pentagonopentagonalis, Pentagonocyclicus, Ellipsocyclicus, Pentagonoellipticus, round Pentagonopentagonalis, and irregular Pentagonopentagonalis. Four ossicle assemblages are recognized: Pentameri, Elliptici, Cyclici and Varii. Results of the statistic analysis of the shape, size and percentage of the ossicles, combined with the taphonomic characteristics of the symbiotic animals and analysis of the sedimentary environments, indicate that crinoids are absent from the community dominated by cyanobacteria and tetradiids in the restricted lagoon environment. Very few slim crinoid ossicles occurr in the biota of gastropods and ostracods in the fine-grained bank facies covering the lagoon facies. Abundant crinoids and various symbiotic bryozoans, Solenopora, dasycladaceans and other organisms are present in the biota of the open platform facies. Both crinoids and bryozoans are filter feeders, and should have a symbiotic but competition relationship in most cases. However, in the Ordovician Tazhong sea, the difference in their body size determines the two groups belong to different filter feeding groups, which eases the competition between them. In communities dominated by crinoids and bryozoans, crinoid ossicles are the most abundant, and all four ossicle assemblages are present. Solenopora is photosynthetic, it has no competition with the crinoids, and its relationship with the crinoids is mutualistic. The crinoid ossicles in the Solenopora–crinoid community belong to the Cyclici type. They are relatively simple, big, and dense, and their content is the highest. The crinoids are widely symbiotic with the dasycladaceans. Due to the influence of the oncoids formed by filamentous cyanobacteria, the number and size of crinoid ossicles and fragments are smaller than those of the dasycladaceans, but they are relatively richer and more diverse, including all four types. Therefore, we suggest that the most suitable environment is the open platform for the Upper Ordovician crinoids from Well Tazhong 35, and the main controlling factors are hydrodynamic energy and water cleanliness. The crinoids flourish more in cleaner water with higher hydrodynamic energy, while turbulent water with low hydrodynamic energy is disadvantageous to their survival.
