The Latest Ordovician Craniiform And Linguliform Brachiopods From China And Myanmar

There were five major mass extinction events in Earth's history,which were generally called "Big Five".The end Ordovician extinction was the first one,which occurred in two pulses.During the first episode,the Gondwana glaciation triggered a rapid climate cooling and a sharp fall of sea level,and resulted in the decreasing of warm-water organisms and the development of the widespread cool-water Hirnantia fauna.In the second episode,the Hirnantia fauna became extinct due to the ending of the glaciation along with climate warming and rise of sea level.The Hirnantia fauna is important for global stratigraphical correlation for its short time interval and global distribution.Craniiform and linguliform brachiopods have not underwent enough study due to their fossils being tiny in size and rarity in number,but they made up about a quarter of the Hirnantia fauna in number of genera.The fossils studied were collected from the Kuanyinqiao Bed(Hirnantian)of South China and the Hwe Mawng Purple Shale Member(Himantian)of Mandalay Division,Myanmar.The fossils were studied by observing under microscope and making latex cast molds.Except a few fossils of the Superfamily Linguloidea that are very rare and poorly preserved,and then not studied herein,there occur nine species belonging to seven genera(Schizotretinia sp.,Trematis sp.,Paracraniops sp.,Pseudopholidops partibilis,Acanthocrania sp.,Petrocrania cribrum,Petrocrania?sp.1,Petrocrania?sp.2,Xenocrania haimei)which are described in this paper.Based on previous reports and the present study,there occur 18 genera of craniiforms and linguliforms recorded in the Hirnantian.They belong to four superfamilies,i.e.,Craniopsoidea(Craniops,Paracraniops,Pseudopholidops),Cranioidea(Acanthocrania,Petrocrania,Philhedra,Xenocrania),Discinoidea(Tehtyrete,Trematis,Kosoidea,Orbiculoidea,Schizotretinia)and Linguloidea(Casquella,Eodinobolus,Lingula,Lingulella,Palaeoglossa,Pseudolingula).Even though craniids are not the most common fossils,they are special in the non-rhynchonelliforms in the Hirnantia fauna due to their highly variable ornaments.Craniide includeds 23 genera that fall into three groups.The first one is characterized by the presence of concentric laminae,with no spines and papillae,including 10 genera;the second by the ornament of radial aligned ribs,along with variable concentric laminae,including 7 genera;the third by the possession of and/or spines randomly or concentrically aligned spines and papillae,containing 6 genera.Xenocrania is a very peculiar genus with highly variable ornamentation.It is hypothesized that the variable morphological characters may have been reflected by phenotypic pleiotropy.In some sense,these morphological and evolutionary novelties probably enhanced survival rates during the crisis.This genus reached its peak in the interval between the two episodes.Geographically,Xenocrania occurred in Sibumasu,South China,Avalonia,Baltica and Perunica and ecologically,mainly lived in BA3 to upper BA4(mid to out-shelf)and rarely BA2(near shore).