Paleobiogeographic patterns of the brachiopod faunas before and during the first radiation of the Great Ordovi-cian Biodiversification Event(GOBE)in South China at 6 different localities from the upper Jiangnan Slope to the vast area of the Upper Yangtze Platform show several interesting features.First,the initial brachiopod diver-sity acme was accompanied by both high origination and extinction rates.Second,no significant changes took place in the taxonomic composition and paleobiogeographic pattern of the brachiopod fauna during the radia-tion at 5 of the 6 localities studied except the near shore locality,where the first brachiopod radiation was much later than at other localities and was marked by a dramatic increase in endemic constituents.Third,orthides were the predominant brachiopod group during the radiation,and regional brachiopod taxa played a significant role in defining the paleobiogeographic pattern of the radiation.Fourth,the first brachiopod radiation was asso-ciated with 3 major pulses of onshore migration from the upper Jiangnan Slope through the central Upper Yang-tze Platform to the near shore settings of the platform,with the middle pulse being the most significant.Finally,paleogeographic dispersal took place in both onshore and offshore directions,although the onshore expansion was more prominent;several key brachiopods,such as Paralenorthis,Nocturnellia,Protoskenidioides,Nere-idella,Euorthisina and Yangtzeella,first appeared on the upper Jiangnan Slope and later formed distinct,and taxonomically diverse,communities on the Upper Yangtze Platform.The paleogeographic dispersal of brachio-pods is considered to be closely related to the tectonic evolution of the Qianzhong Arch.
A continuous Ordovician-Silurian boundary section from the upper Wufeng Formation through the Kuanyinchiao Formation to the lower Lungmachi Formation has been carefully measured and collected at Shuanghe of Changning, southern Sichuan Province. For the first time, the temporal changes of α- and β-diversities of the Hirnantia fauna have been discussed in great detail. The general trend of brachiopod diversity change, increasing upward, is consistent with the regional trend of the Yangtze Platform, which had been controlled by both intrinsic and extrinsic factors. However, the sudden drop of diversity for a short period in the upper Kuanyinchiao Formation might have been controlled by environmental factors rather than normal faunal turnover. Synecological analysis using numerical methods recognizes two brachiopod-dominated associations of the Hirnantia fauna, the Dalmanella-Kinnella Association and the Mirorthis Association, both living in an offshore, deeper water environment corresponding to BA3-upper BA4, particularly lower BA3.
Located northwest of the Jiangshan-Shaoxing Fault Zone,western and northwestern Zhejiang and northeastern Jiangxi provinces experienced major changes in bio-and litho-facies and paleogeography through the Ordovician-Silurian transition (late Katian,Hirnantian,and early Rhuddanian),as manifested by stratigraphic,paleontologic and synecologic records.Three geographic units under consideration are South,Central,and North areas.The western margin of the South area was occupied by the Huaiyu Mountains,whereas the other two parts were covered by the Zhe-Gan Sea during late Katian (Late Ordovician) time.In the early stage of the Changwu Formation deposition (late Katian),the sea was deepening northeastward,but with shallower conditions in the southwest and deeper conditions in the northeast.During mid to late stages of Changwu Formation deposition (latest Katian),the sea became much shallower and the sea bottom was uplifted substantially,which occurred somewhat prior to the global trend.During the Hirnantian (latest Ordovician) and early Rhuddanian (earliest Silurian),the study region became a shallow bay under expansion of the Cathaysian Oldland.There occurred a major drop of sea level and great changes in benthic biota with the occurrence of many new immigrants through the Ordovician-Silurian transition,which are closely related to a unique regional orogeny.The Yangtze and Cathaysian blocks may have amalgamated within the South China Paleoplate during this interval to cause the continuous uplifting and northwestward expansion of the Cathaysian Oldland.
RONG JiaYuZHAN RenBinXU HongGenHUANG BingYU GuoHua
Piperock, a kind of characteristic ichnofabrics in Phanerozoic, was thought to decline gradually from Cambrian to Ordovician. A new compilation on the occurrences of the Cambrian and Ordovician piperocks of China and the world shows that piperocks generally flourished in Cambrian and declined in Ordovician, but a sharp decrease occurred during Middle and Late Cambrian. The case-study on the piperocks from the Lower-Middle Ordovician Hongshiya Formation at Dabaochang of Qiaojia, northern Yunnan Province, Southwest China indicates that the forming and preservation of piperocks were controlled by the depositional environment, the intensities of predation, competition, bioturbation, and the contents of nutrition and oxygen in watermass. A careful study on the development of the Cambrian and Ordovician piperocks suggests that the decrease of nearshore siliciclastic sediments and the low contents of oxygen and nutrition in the watermass may have contributed to the decline of piperocks in Middle and Late Cambrian.
Eospirifer,the oldest known genus of the spiriferide group(Brachiopoda),first appeared on the Zhe-Gan Platform of South China paleoplate during late Katian(Late Ordovician) represented by E.praecursor.It survived the end-Ordovician mass extinction,and reappeared on the Upper Yangtze Platform of South China paleoplate by the end of Ordovician near the upper part of the Normalograptus persculptus Biozone.Starting from the beginning of Silurian,Eospirifer experienced some morphological innovations and expanded its geographical distribution substantially.It reached its species diversity acme and the widest geographic distribution in Wenlock(middle Silurian),with the diversity hotspots in Laurentia,Avalonia,and Baltica.Various shell size frequency curves of E.praecursor under different paleogeographic settings suggest that this pioneer species of Eospirifer,with several macroevolutionary novelties,adopted a range of life strategies to adapt to the changing environments during early spiriferide evolution.There are also some morphological macroevolutionary trends during the evolutionary history of Eospirifer from Late Ordovician to the end of Silurian,such as the ever enlarging shell sizes and the width/length ratios from late Katian to Wenlock,but decreasing apparently of both parameters from Wenlock to Pridoli.
Review of the literature on Ordovician conodont diversification in palaeoplates of North and Western China reveals that four diversity peaks are present in North China,occurring in the middle Tremadocian,early Floian,late Floian,and late Darriwilian,with three of these peaks(excepting that in the late Floian)also being recorded in Tarim.Three diversification intervals are present in North China,during the Tremadocian,late Floian,early and middle Darriwilian;comparable intervals are observed in the early and late Tremadocian,early Floian,and the Middle Ordovician in Tarim.The main conodont diversification episode in both palaeoplates took place in the Darriwilian,at the time of the Great Ordovician Biodiversification Event.A comparison of conodont diversity patterns in different palaeoplates(North China,Tarim,and South China)demonstrates that conodont radiation events mainly occurred within the Tremadocian,Floian,and Darriwilian.Conodont diversifications in these paleoplates also display some differences.In contrasting with Tarim and South China,North China witnessed a rapid conodont diversification during late Floian time.Conodont diversity in North China and Tarim increased continually and reached a peak in the late Darriwilian,concurrent with a prominent decreasing trend in South China.Differences of conodont diversification in these three palaeoplates may be related to their palaeogeography and tectonic history.When conodont diversifications in North China and Tarim are analysed on the background of palaeoenvironments,the main episodes are seen to be partly coincident with second order sea-level changes,particularly in North China.In general,conodont radiation correlates with large scale transgressions.
WU RongChangIan G. PERCIVALSvend STOUGEZHAN RenBin
