Rong Jia-Yu

A global synthesis of the latest Ordovician Hirnantian brachiopod faunas

A global review of new and existing data on the distribution of uppermost Ordovician (Hirnantian) brachiopods indicates the existence of at least three biogeographically distinct faunas. The typical Hirnantia fauna characterised subtropical and temperate latitudes and comprised a variety of ecological associations; the fauna reached its acme during the bohemicus and uniformis zones. Atypical Hirnantia faunas, developed marginal to Gondwana, are of low diversity and have few species in common with the typical Hirnantia fauna; their spatial distribution probably marked the margin of the polar ice sheets. The extinction of the Hirnantia fauna occurred in response to changes in sea level. Diverse and quite different faunas, including those from the Midcontinent of North America, Kolyma, the Oslo Region and probably Anticosti Island, occupied equatorial latitudes during the latest Ordovician. The Holorhynchus fauna, on evidence to date, predates the Hirnantia fauna.

Facies patterns and geography of the Yangtze region, South China, through the Ordovician and Silurian transition

Abstract Lithofacies and biofacies changes through the Ordovician and Silurian transition on the Yangtze platform are coincident with stepwise mass extinctions, recovery events, global sea-level changes, and regional palaeogeographic configurations. Facies patterns through three intervals, the late-mid Ashgill, Hirnantian, and early Rhuddanian, indicate that black shale occupied most of the Yangtze platform region during the late–mid Ashgill and early Rhuddanian, while the Hirnantian possesses more diverse facies types. The black shale was replaced by carbonate facies during the Hirnantian coincident with a global sea-level low stand. Biofacies changes from the late–mid Ashgill graptolitic to the Hirnantian shelly facies were apparently signalled by gradual and diachronous existence of a mixed facies, the Manosia facies, in the Yangtze region. However, the biofacies change was abrupt from the Hirnantian to the early Rhuddanian when graptolitic facies occupied most areas on the Yangtze platform leaving only a very narrow, limited belt of shelly facies along the southern margin of the platform in northern Guizhou. The early Rhuddanian unified biofacies pattern is a result of a global sea-level rise, which may be synchronized with a recovery bioevent of graptolites after Hirnantian mass extinction.

Intercontinental correlation by sea-level events in the Early Silurian of North America and China (Yangtze platform)

Potential for eustatic sea-level changes on continental platforms during Early Silurian (Llandovery) time was highly propitious due to the double effects of lingering glaciation in Gondwana and the rapid rate of sea-floor spreading which assisted the general dispersion of many era tonic realms through middle Paleozoic oceans. Following the initial mass melting of the Late Ordovician/Early Silurian glaciers in North Africa and Arabia, four smaller cycles of sea-level fluctuation were recorded by recurrent, shelly communities in a predominantly carbonate setting on the North American Platform. Peaks in sea-level rise occurred in late Rhuddanian or early Idwian (A 4 –B 1 positions), early Fronian (C 1 position), early Telychian (C 4 position), and late Telychian (C 6 position) times. Although absolute time control is poor, the periodicity of these events may have been every 2.5 m.y. In the more submerged regions of the North American Platform, such as Iowa and Anticosti Island, diverse faunal associations distinguished by the presence of stricklandiid brachiopods were especially productive at peak times of transgression. Co-occurrence of the calcareous green alga Cyclocrinites with certain of these faunas indicates that maximum water depth was still within the photic zone. At the other extreme, low stands in sea level are identified by diminished faunal diversity, as well as a combination of typical sedimentary structures and evaporite indicators. Located on a well-defined Precambrian craton, the Yangtze Platform is now a large region in central and southwestern China. During successive stages of the Early Silurian, a thick sequence of graptolitic shales, shelly carbonates, and red beds were deposited on the platform. Of the four sea-level peaks defined in North America, three are recognized in the Yangtze Region. There is very good evidence for a latest Llandovery or earliest Wenlock transgression, based on the extensive distribution of the Salopinella - Coronocephalus - Sichuanoceras fauna. Another clear and well-dated sea-level peak involves the occurrence of faunas with characteristic stricklandiid and pentamerid brachiopods at a level equivalent to the lower Telychian (C 4 position). The initial Silurian transgression appears to have reached its peak on the Yangtze Platform primarily during early Idwian time (B 1 position), but this is based less surely on changing patterns in graptolite diversity. At present, there is no compelling evidence for a widespread, early Fronian (C 1 position) transgression in China. Successful matching of the other sea-level events, however, substantially refines intercontinental correlation for Lower Silurian strata in the Yangtze Region of China and the central regions of North America.

Brachiopod survival and recovery from the latest Ordovician mass extinctions in South China

South China contains many complete sections through the upper Ordovician and lower Silurian. Brachiopod data including 130 brachiopod genera, assigned to 13 orders and 27 superfamilies from mid-Ashgill through late Aeronian intervals reveal that brachiopod macroevolution before and after the latest Ordovician mass extinction shows important changes in the diversity, composition and stratigraphical distribution of the phylum. The following six intervals are recognized: (1) a faunal plateau before the latest Ordovician mass extinction (mid-Ashgill, Rawtheyan); (2) a survival–recovery interval following the first phase of the mass extinction (late Ashgill, Normalograptus extraordinarius Zone and lower Glyptograptus? persculptus Zone; Hirnantian); (3) first survival interval following the mass extinction (latest Ashgill, upper Glyptograptus? persculptus Zone; end Hirnantian); (4) a second survival interval after the mass extinction (earliest Llandovery, Parakidograptus acuminatus Zone; early to mid-Rhuddanian); (5) a recovery interval in the Silurian (early to mid-Llandovery; late Rhuddanian to early Aeronian); and (6) a radiation interval in the Silurian (mid-Llandovery; mid- to late Aeronian). Only near-shore, low-diversity, benthic assemblages (mainly BA2), characterized by Ordovician relicts with a few Lazarus taxa and progenitors, are known from the southern marginal area of the Upper Yangtze epicontinental sea during the early to mid-Rhuddanian. They were replaced by newly established Silurian brachiopod communities (mainly BA2–3) in the late Rhuddanian to early Aeronian. These are marked by many newly evolved endemic forms and new immigrants, expressing a clear recovery within the Brachiopoda, but the recovery interval of the major brachiopod groups was heterochronous. In China the typical Silurian brachiopod fauna was mainly composed of indigenous Atrypida, Pentamerida and Spiriferida with stropheodontids derived from elsewhere, such as Baltica and Avalonia, two apparent refugia in the survival interval. The Atrypida was the first major group of Brachiopoda to diversity in the late Rhuddanian. Copyright

Pre-Hirnantian Ashgill climatically warm event in the Mediterranean region

Compilation of the marine, benthic megafossils from approximately the mid-Ashgill of the Mediterranean region, including much of Central and Southern Europe plus North Africa, and elsewhere indicates a warm interval featuring bioclastic limestone and a warm climate marine fauna. These mid-Ashgill faunas immediately precede the latest Ashgillian, Hirnantian, cool interval that featured widespread glaciation, and are underlain by typical, cold water, Mediterranean Realm, older Ordovician rocks and faunas. The cause or causes responsible for the brief warm interval are uncertain, but may have involved a warm water gateway that is geographically still not located. There is a possibility that South Africa was similarly affected by this roughly mid-Ashgillian marine situation. Early Paleozoic bauxite minerals and kaolins in northwestern Sudan and kaolins elsewhere in North Africa may represent the same time interval, which would suggest that there was a non-marine amelioration of the local climate as well as the marine effects.

The classification of athyridid brachiopods

Phylogenetic relationships among higher taxa of articulate brachiopods that developed a spiralium as mineralized support for the lophophore, and the resulting classifications, are not universally agreed upon due to the complex pattern of character evolution within the phylum. Our results, using phylogenetic methodology in conjunction with traditional methods, showed a more complete and easily testable picture of this evolution than either method used alone. A detailed working hypothesis of athyridide phylogeny and the supporting evidence on character distribution is given, and a new classification of the impunctate and punctate athyrids sensu lato is presented with diagnoses to subfamily. Agreement between the stratigraphical first appearance of athyridid subfamilies and their cladistic rank was quite good (except for the koninckinoids), suggesting that both outgroup and traditional paleontological methods indicate a similar direction of character polarity in the evolution of the group. The classification is proposed based on a mix of external and internal characters which persisted during unbroken lineages. New taxa are the families Parazygidae and Triathyrididae; the subfamilies Whitfieldellinae, Triathyridinae, Septathyridinae, Cleiothyridininae, Ochotathyridinae and Plectospirinae, and the genera Brimethyris, Carteridina, Tetraloba and Cooperispira. We studied the great morphological diversity among the athyridids. Their complete stratigraphical ranges help in building phyletic lineages.

Silurian and Devonian biogeography of China

Integration of newly available Chinese data for the biogeography of the Silurian and Devonian makes possible a number of globally sigificant advances. The Early Silurian, the Llandoverian, can now be shown to have been moderately provincial rather than cosmopolitan, as considered earlier. The Uralian-Cordilleran Region defined previously for the Late Silurian can now be shown with Chinese information to have been in existence with a co-occurring North Atlantic Region since the beginnings of the Silurian and to have been somewhat more provincial than the Late Silurian. The view of Late Silurian provincialism with two major regions is further substantiated with the help of Chinese data. For the Early Devonian, it is now possible to define a South China Region that includes the bulk of the Yangtze Platform Region of South China plus adjacent Tonkin in North Vietnam. The boundary of the South China Region is present in central Yunnan and extends in an approximately north-south direction. The South China Region is still prominent during the lower, Eifelian half of the Middle Devonian. Relatively cosmopolitan Devonian conditions affect China by the latter, Givetian half of the Middle Devonian, as is also the case in most other parts of the world except for eastern North America and the Southern Hemisphere Malvinokaffric Realm. The Chinese Upper Devonian is cosmopolitan in a manner similar to that of the rest of the World. It is notable during the Lower Devonian that the nonmarine vertebrate fauna is unusually provincial and very distinct from faunas known in Europe and North America, as well as from those present in northern Asia. It is suggested that previous distinctions between Lower Devonian Uralian and Mongolo-Okhotsk Regions are more likely provincial or even subprovincial. The presence of the atrypacean genus Tuvaella in the Silurian of east-central Asia is shown to represent the restricted occurrence of a single genus, rather than being indicative of a major biogeographic unit. However, it may indicate the presence of a subprovincial unit or of an endemic center.

A silicified Hirnantia fauna (latest Ordovician brachiopods) from Guizhou, southwest China

A silicified brachiopod assemblage living between the first and second episodes of latest Ordovician mass extinction is first described from micritic bioclastic limestone of Kuanyinchiao Bed, northwestern Guizhou, southwest China. It contains six species, Dalmanella testudinaria (Dalman, 1828), Plectothyrella crassicosta (Dalman, 1828), Hindella crassa incipiens (Williams, 1951), Eostropheodonta sp., the characteristic constituents of the Hirnantia fauna, and two new species, Fardenia modica and Dorytreta longicrura. It lacks Hirnantia, Kinnella, Cliftonia, and Paromalomena, the other characteristic elements of the Hirnantia fauna. A new, low diversity community (inner BA3) dominated by brachiopods is proposed as the Dalmanella testudinaria-Dorytreta longicrura Community along with the trilobite Dalmanitina nanchengensis Lu, rugose corals, and gastropods. Composition of this community of the Hirnantia fauna was controlled primarily by temperature, water depth, and substrate. Comparison with other communities of the Hirnantia fauna in south China supports this conclusion. Fluctuations in generic composition of the Hirnantia fauna and the related factors are also discussed.

Continental island from the Upper Silurian (Ludfordian Stage) of Inner Mongolia: Implications for eustasy and paleogeography

An unconformity between the Silurian Xibiehe Formation and Ordovician igneous rocks marks the perimeter of a small paleoisland near Bater Obo in north-central Inner Mongolia, 180 km northwest of the provincial capital of Hohhot. The stratigraphic position of the lower part of the Xibiehe Formation is correlated by means of conodonts with the upper part of the Ancoradella ploeckensis Zone in the basal Ludfordian Stage (corresponds to mid-Ludlovian Epoch, ca. 421 Ma). Elongate in plan (610 m × 200 m), the exhumed diorite core rises 30 m above the lowest elevations of surrounding Silurian strata. Paleoshores along the principal axis of the inlier delineate contrasting facies. Robust stromatoporoids are in growth position within silty limestones, some directly encrusting the unconformity surface of the sheltered southeast margin. A basal conglomerate of diorite cobbles and boulders characterizes the high-energy northwest margin. The depositional constraints and timing of transgressive facies associated with this continental paleoisland have implications for the eustatic and paleogeographic history of the parent Sino-Korean plate. Burial of the island corresponds to the beginning of a global rise in sea level that peaked in late Ludlovian time. Our interpretation of windward and leeward facies requires an approximate 90° clockwise rotation of the parent plate to accommodate the dominant pattern of low-latitude trade winds and storms.

Earliest Silurian faunal survival and recovery after the end Ordovician glaciation: evidence from the brachiopods

Earliest Silurian (basal Llandovery) brachiopod faunas are surveyed and listed from around the globe, and divided between Lower Rhuddanian and Upper Rhuddanian occurrences. 60 genera are known from the Lower Rhuddanian within 20 superfamilies and there are 87 genera in 25 superfamilies in the Upper Rhuddanian. The 29 areas surveyed span the globe, both latitudinally and longitudinally. Only six superfamilies are Lazarus taxa which are known both from the Ordovician and Middle Llandovery (Aeronian) and later rocks but have not been recorded from the Rhuddanian. These are surprising results, since many previous studies have inferred that the Rhuddanian was a time of very sparse faunas. The global warming that followed the latest Ordovician (Hirnantian) ice age did not proceed quickly, with an ice-cap probably present through at least the Llandovery. There is a marked absence of Lower Rhuddanian bioherms even at low palaeolatitudes; however, the ecological recovery rate was far faster than that following the end-Permian mass extinction event. The partitioning of the Rhuddanian shelf faunas into well-defined benthic assemblages progressed slowly over the interval.