Ziyang Fauna: A slope facies fossil lagerstätte of Early Ordovician in South China

Abstract

“Great Ordovician Biodiversification Event” (GOBE) is one of the major biological innovation in Phanerozoic. Trilobite is one of the major taxa in Cambrian and Ordovician. In 1998, Adrain divided the global Ordovician trilobites into two groups, i.e. Ibex Fauna and Whiterock Fauna, and subsequently, the former was subdivided into Ibex I and Ibex II faunas in 2004. The Ordovician radiation of the trilobites was associated with temporal and spatial expansion of the Whiterock Fauna and contraction of the Ibex Fauna. Ibex Fauna is also known as the Cambrian Evolutionary Fauna, which originated at Cambrian, had high diversity and differentiation soon, decline rapidly in Early Ordovician, and went extinct in Late Ordovician. The Ibex I Fauna raised and flourished only in the Early Tremadocian, while Ibex II fauna declined after the Early Ordovician radiation. Whiterock Fauna is also called the Paleozoic Evolutionary Fauna, which appears in Early Ordovician and shows rapidly radiation and high diversification in Middle Ordovician, and then survived the Ordovician Mass Extinction, evolved into Silurian Fauna, some of them might develop and extend to Devonian, Carboniferous and Permian. In 2006, Zhiyi Zhou dubbed the Ordovician trilobite radiation in the South China Block, suggested that the time of onset and the environmental pattern of the Ordovician trilobite radiation may have varied geographically, and the major radiation was likely a long-sustained event that extended through about 25 Ma of the late Early-early Late Ordovician interval, coinciding with the worldwide expansion and domination of the Whiterock Fauna. So the taxa of Zhou’s Ibex Fauna were proportionally dominant over those of Whiterock Fauna. Ziyang fauna, an Early Ordovician fossil lagerstatte is discovered and reported from Ziyang County, Shaanxi Province. It is Floian (the late Early Ordovician) in age based on the occurrences of Tetragraptus approximatus Nicholson. The fossiliferous rocks are blackish-grey mud limestone and calcareous mudstone of possible slope facies in the Yangzte platform. The fauna is dominated by trilobites, accompanied with graptolites, brachiopods, arthropods and some trace fossils. With relatively high taxonomic diversity, the trilobites in this fauna mainly consist of Seleneceme , Niobe , Nileus , Ampyx , Degamella , Borogothus , Geragnostus and Dividuagnostus , etc assgigned to seven families, of them, Asaphidae, Nileidae, Agnostoidea, Peragnostoidea and Alsataspidiae belong to the Ibex Fauna, and Cyclopygidae and Raphiophoridae belong to Whiterock Fauna. Asaphidae and Nileidae are main components of Ibex I Fauna, and Agnostoidea, Peragnostoidea and Alsataspidiae are components of Ibex II Fauna respectively. Quantitatively, Alsataspidiae and Asaphidae account for more than 75% of the total fauna, Ibex I Fauna were predominent over Ibex II Fauna. Other associated fossils include graptolites of Tetragraptus approximatus , Tetragraptus bigsbyi , Tetragraptus quadribrachiatus , Tetragraptus sp., and Paratemnograptus sp., and brachiopods of Tritoechia , Lingulella , etc. The distinctive composition of Ziyang Fauna holds promise for providing insights into the evolution between Ibex and Whiterock faunas in South China.