Palaeontology, biostratigraphy and palaeoecology of the graptolite from the Hulo Formation (Darriwilian – Sandbian) in northwestern Zhejiang Province, East China

Ma, X.: Palaeontology, biostratigraphy and palaeoecology of the graptolite from the Hulo Formation (Darriwilian – Sandbian) in northwestern Zhejiang Province, East China. A Ph.D dissertation submitted to University of Chinese Academy of Sciences (supervised by Prof. Zhang Y-D). 1-301. 2020 Darriwilian (Middle Ordovician) to early Sandbian (Late Ordovician) is an interval of critical transitions in global tectonics and sedimentary environments, and a key period for the Great Ordovician Biodiversification Event (GOBE) and the rising of the Paleozoic Evolutionary Fauna (PEF). As one of the predominant zooplanktons of PEF, graptolites underwent a rapid phylogenetic evolution and burst of new forms, and reached its taxonomic and morphological diversity acmes in Darriwilian. Among the most significant turnovers is the replacement of dichograptids, a fauna typified by its multiramous morphos and its predominance in the Early Ordovician, by diplograptids, which first appeared in the early Middle Ordovician and predominated in the entire Late Ordovician. Furthermore, a variety of morphological types of graptolites appeared, including the tubaria of scandent, partially scandent, non-scandent, single-stiped and rod-like forms, as well as some extraordinary architectures of thecal morphology and arrangement, such as extremely elongated or strongly deformed thecae, intensely compact proximal end by means of obverse-reverse thecal superposition, or massive reduction of thecal walls. These various kinds of new architectures of tubaria and thecal types represent a significant functional and palaeoecological innovation in the evolutionary history of graptolites. A study on the palaeontology, biostratigraphy and palaeoecology of diverse Darriwilian to early Sandbian graptolite fauna with continuous biostratigraphic occurrences, will enhance our understanding of graptolite macroevolution in response to environmental changes during the GOBE