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

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. In the Jiangnan Region, Darriwilian to early Sandbian strata are continuous and well-exposed, which are dominated by graptolitic shale facies, and yield abundant planktonic and nektonic fossils. This is ideal for the study of the graptolite evolution and palaeoecology. This dissertation deals with the systematic palaeontology, biostratigraphy and paleoecology of Darriwilian to early Sandbian graptolites (Hulo Palaeontology, biostratigraphy and palaeoecology of the graptolites from the Hulo Formation (Darriwilian–Sandbian) in northwestern Zhejiang Province, East China Formation) from the region with a special reference to the driving mechanism of graptolite macroevolution. During 2013-2018, over 3000 graptolite specimens were collected from four sections of the Hulo Formation in the northwestern Zhejiang Province, i.e. the Jiumulong and Lajichang sections in Anji County, and the Shengcun and Songtian sections in Tonglu County. Based on the collection, 52 species of 25 genera belonging to 11 families and 1 subfamily are identified, and all of them are systematically described, compared and discussed in the present dissertation. A continuous graptolite biozonation comprising some six biozones is proposed for the Hulo Formation in the two areas. Four graptolite biozones are identified for the Hulo Formation in the Anji area, which are, in ascending order, the Acrograptus ellesae Biozone, Nicholsonograptus fasciculatus Biozone, Pterograptus elegans Biozone and Jiangxigraptus vagus Biozone, suggesting a Darriwilian age. For the topmost Ningkuo Formation and the Hulo Formation in the Tonglu area, 5 graptolite biozones and 1 subbiozone are identified, which are, in ascending order the Undulograptus sinicus Subbiozone (of the Undulograptus austrodentatus Biozone), Acrograptus ellesae Biozone, Nicholsonograptus fasciculatus Biozone, “Hustedograptus teretiusculus Biozone” and “Nemagraptus gracilis Biozone”, suggesting an age spanning Darriwilian to the earliest Sandbian. These biozones are well correlated with those equivalent graptolite biozones in some other blocks or regions of the world. By means of functional morphology and analogue analysis, the dissertation attempts to understand the origination and functioning of some extraordinary morphological features in Darriwilian, e.g. the extremely elongated thecae, intensely compact proximal end of tubarium and the massive reduction of thecal walls. The results show the appearances and development of these novel complexes of graptolite morphology in the Darriwilian are resulted from the enhancement of feeding efficiency and the protection and defense strategies, driven by the pressure derived from consistently limited food sources and bursting planktonic organisms, and the increasing predation pressure from giant predators such as the nautiloids during the GOBE.A dissertation submitted to the University of Chinese Academy of Sciences in partial fulfillment of the requirement for the degree of Doctor of Philosophy in Palaeontology and Stratigraph