Silurian Eurypterida (Arthropoda:Chelicerata:Merostomata) From Southern China:Taxonomy,Palaeobiogeography And Palaeoecology

Eurypterids are a group of aquatic arthropods belonging to the order Eurypterida,class Merostomata,subphylum Chelicerata,and lived from the Middle Ordovician to the end-Permian.Their habitats changed from deep waters in ocean to rivers on land with time going by,therefore they may provide clues about the terrestrialization of early arthropods.Their worldwide distribution and fancy appearance with near two hundred years of research history draw attention both from the masses and scientists.The study of eurypterids from China started very late and has a very low degree with rare and sporadic records,indicating that more efforts should be made to improve this poor condition.Based on previous researches,the Silurian eurypterids from the South China Block are studied in taxonomy,palaeogeography,palaeoecology and evolution in this thesis.Recently over 400 eurypterid specimens had been collected from the Llandovery Houjiatang Formation at Guquan,Nanjing City of Jiangsu Province,the Llandovery Houjiatang Formation at Guishan,Chaohu City of Anhui Province,and in Huangjintang,Wuhan City of Hubei Province,the Llandovery Xiushan Formation at Daping Village,Baojing County of Hunan Province,and in Shuiyuan Village,Xiushan County of Chongqing City,the Llandovery Huixingshao Formation at Chuanhegai,Xiushan County of Chongqing City and the Ludlow-Pridoli Xiaoxi Formation at Kapeng reservoir,Baojing County of Hunan Province.A taxonomic study of these specimens reveals 15 species(including seven undeterminated species)of 12 genera within eight families,of which five species are recognized as new to science including Eurypterus xiangxiensis sp.nov.,Rhinocarcinosoma xiushanensis sp.nov.,Lanarkopterus chinensis sp.nov.,Hughmilleria viriosa sp.nov.and Hughmilleria parca sp.nov..Of these taxa,some are globally common and abundant groups of high biostratigraphical significance like the families Eurypteridae,Adelophthalmidae and Pterygotidae,greatly enriching the record of Chinese eurypterids.Compared with fossil records from other continents,the earliest record of the families Eurypteridae and Hughmileriidae,and the genera Adelophthalmus and Rhinocarcinosoma are indicated not in Laurentia as previously considered but in the South China Plate.This discovery obviously argues with the previous consensus that most eurypterid groups evolved from Laurentia,and this thesis puts forward a new hypothesis that at least some groups initiated in China and started their radiation from South China Plate as the center.This study highly expands the distribution of eurypterids and indicates eurypterids having a much stronger ability of long-distance movement than previously thought.Palaeoecological and palaeoenvironmental data show that Chinese eurypterids lived in deep water during the Ordovician,moved to shallow sea and salty water mixed with fresh water in the Silurian,lived in salty water during the Devonian and in fresh water in the late Paleozoic Era,which is the same condition in other areas out of China and further supports that the terrestrialization of early arthropods probably happened in eurypterids.Comparison of morphological characteristics among eurypterid groups found in China and other regions shows different eurypterid groups occupying different ecological niches with large and middle-sized groups as the top predators in water surface and small groups as common predators on the water bottom.