Showing papers in "Geological Society, London, Memoirs in 1990"

Revised World maps and introduction

Abstract: We review the highlights of the 1988 symposium on Palaeozoic Biogeography and Palaeogeography, and present a revised set of 20 Palaeozoic base maps that incorporate much of the new data presented at the symposium. The maps include 5 major innovations: (1) A preliminary attempt has been made to describe the motion of the Cathaysian terranes during the Palaeozoic; (2) a more detailed description of the events surrounding the Iapetus Ocean is presented; (3) an alternative apparent polar wandering path for Gondwana has been constructed using the changing distributions of palaeoclimatically restricted lithofacies; (4) new palaeomagnetic data have been incorporated that places Laurentia and Baltica at more southerly latitudes, and adjacent to Gondwana, during the Early Devonian; Siberia is also placed further south in the light of biogeographic data presented at the symposium; (5) Kazakhstan is treated as a westward extension of Siberia, rather than as a separate palaeocontinent. The relationships between climatic changes, sea level changes, evolutionary radiations and intercontinental migrations are discussed

Phytogeographic patterns and continental configurations during the Permian Period

Abstract: The climatic influences on vegetation in the present world can serve as an excellent model for the Permian, there being approximately the same level of floral differentiation and the same relatively cold polar regions. Accordingly, a system of present day climatically defined biomes, developed by Walter, is adapted herein for palaeophytogeographic purposes. There are ten biomes altogether which range from the tropical 9everwet9 (biome 1) to the polar 9glacial9 (biome 10). Biome 1 is represented by the tropical rainforests of the Cathaysian province sensu stricto which was populated by the arborescent lycopods and sphenophytes, and the Gigantopterids which are interpreted as lianas (vines). This is flanked by the lower diversity 9summerwet9 biome 2, the Atlantic Province, a zone which today is characterized by savanna vegetation, and in the Permian was represented by the pteridosperm Callipteris and the primitive conifers. Biome 3 was effectively 9abiotic9 and is expressed geologically by evaporite belts. The 9winterwet9 or Mediterranean climate of today (biome 4) was not well developed in the Permian, or perhaps just not well preserved, but I attribute some low diversity floras of Kazakhstan to this biome. Biome 5, the 9warm temperate9 biome is well developed in both hemispheres, and, like biome 1, is characterized by high diversity floras and abundant swamp deposits. Not surprisingly, the biome 5 floras have been mistaken for biome 1 with which they are transitional in the modern world and with which they share similar climatic conditions. In the Permian Angaran Realm, the Pechora Province represents this biome, while the Austroafroamerican Province is the Gondwanan equivalent. Often, biome 5 floras have been simply termed 9mixed9. Proceeding poleward across the 9hard frost line9, the cool temperate floras of biome 6 were populated by diverse herbaceous sphenophytes and deciduous trees (the cordaitids of Angara and the similarly diverse glossopterids of Gondwana). Areas in the temperate zone that were remote from moisture sources are assigned to biome 7 and are only known in the southern hemisphere by some aeolian deposits in Argentina. The 9cold temperate9 biome 8 is a low diversity equivalent of biome 6, and has been given the name Gangamopteris Flora in the southern hemisphere. The 9tundra9 environment, biome 9, and the truly glacial deposits, biome 10, are known only from the southern hemisphere.

Palaeoclimatic constraints for palaeozoic Palaeolatitudes of Laurentia and Euramerica

Abstract: Palaeozoic lithic palaeoclimatic data were plotted at the series or stage level for Laurentia (North America) and Euramerica. Their distribution was used to infer the extent of humid and arid climatic belts across the continent for specified time intervals. Climatic data are potentially useful for constraining palaeolatitudes for times when zonal atmospheric circulation remains the dominant pattern. Arid lithic indicators (evaporites, carbonate oolite) and humid indicators (coal, bauxite) were constrained to provide the best fit of these data into a simple zonal climatic pattern delineating subtropical arid belts and equatorial and/or temperate humid belts, respectively. An attempt was made to minimize occurrences of arid indicators in interpreted equatorial regions to maximize a zonal fit. Climatic belts can be arranged in a temporally consistent zonal climatic scheme for much of the Palaeozoic, but monsoonal and orographic effects apparently disrupted the general zonal pattern during portions of the Late Palaeozoic. The interpreted climatic patterns suggest progressive southward movement of Laurentia during the Cambrian through Early Devonian followed by general northward drift of Euramerica during the Middle Devonian through Permian. Lithic palaeoclimatic data provide an independent means to test palaeolatitudes interpreted from palaeomagnetic data. There is general agreement between palaeoclimatic and palaeomagnetic interpretations, but some discrepancies, particularly for the Devonian and Early Carboniferous, warrant further attention.

Biogeography of Ordovician and Silurian faunas

Abstract: The new reconstructions generated for this volume fit well with the majority of Ordovician and Silurian faunal data and are a great improvement on previous attempts The distribution of selected trilobites, brachiopods and graptolites are plotted on the new maps and confirm the importance of palaeolatitude in controlling the faunal distributions, particularly of the old cratons, which are shown for the Early Ordovician and Late Silurian Two contrasting patterns of cratonic faunas are (a) disjunct or (b) gradational across a large palaeocontinent, eg Gondwana Marginal and deeper-water biofacies show different patterns, which help to define the edges of palaeocontinents, but which are not so constrained in their palaeolatitudinal distributions In contrast pelagic trilobites do not help to define palaeocontinents, but were sensitive to palaeo temperature and palaeolatitude Specific case histories are considered, in particular the closing of Iapetus and the contemporary widening of the Rheic Ocean and the positioning of Avalonia, and the ancestry of the various associated trilobite and brachiopod genera during the later Ordovician The Ashgill deeper-water Foliomena fauna is also plotted

Early-Middle Palaeozoic biogeography of Asian terranes derived from Gondwana

Abstract: Contiguity of the Shan-Thai Terrane and NW Australia is suggested for Cambro-Ordovician times by the close faunal affinities seen in Late Cambrian trilobites, Ordovician molluscs, stromatoporoids, brachiopods and conodonts. Taxa such as Spanodonta and Georgina are found only on these two blocks whilst others have a Shan-Thai North China, Australian distribution. This, with a re-evaluation of early Palaeozoic palaeomagnetism, places Shan-Thai against NW Australia, N China against N Australia, S China against the western Himalayan-Iran region with Indo-China and Tarim lying between S China and Shan-Thai. A palaeomagnetically required anticlockwise rotation of this greater Gondwana from the Early Cambrian to the Middle Ordovician satisfactorily accounts for the changing biogeographic patterns, in particular the differences between North and South China during the Ordovician. Recent studies on micro vertebrates and conodonts suggest that Shan-Thai was still very close to Australia in the Middle Devonian, as seen by similar turiniform thelodont species from western Yunnan, northern Thailand, and South Australia, as well as the polygnathid P. labiosus lineage, species of which have recently been found in Thailand. Continental fish faunas were highly endemic in South China from Silurian through to Early and Middle Devonian, indicating prolonged isolation of this terrane and separation of South China from Gondwana probably in the Silurian. In the Late Devonian, shared biotic assemblages between North China and South China (endemic Zhongning/Wuting plant flora; endemic antiarch placoderm and polybranchiaspid agnathan fishes); and between these Chinese terranes and Australia (sinolepid antiarchs, earlier appearances of certain antiarchs) indicate close continental proximity of these three major regions. New palaeomagnetic data for Ningxia, together with biogeographic data suggests that this region may have constituted a separate 9Hexizoulang Terrane9.

Gondwana's movement over the South Pole during the Palaeozoic: evidence from lithological indicators of climate

Abstract: A statistical technique is described that uses the geographical distribution of lithological indicators of climate (carbonates, evaporites, coals and tillites) to estimate the past position of the geographic pole. This technique was used to estimate the movement of the South Pole across the supercontinent of Gondwana during the Palaeozoic. Our results indicate that during the Cambrian and Early Ordovician the South Pole was located adjacent to northwestern Africa. The pole moved into the Amazon Basin during the Late Ordovician and into south-central Argentina during the Silurian. Throughout the Devonian and Early Carboniferous the pole moved slowly from a location in southern Argentina to a position near the south coast of Africa. From the Late Carboniferous and into the Permian the South Pole swung eastward across central Antarctica. The Early Palaeozoic and Late Palaeozoic portions of the palaeoclimatically determined APW path are in good agreement with available palaeomagnetic data. The Middle Palaeozoic portion of the palaeoclimatically determined APW path agrees better with the palaeomagnetic data that places the South Pole in southern Argentina, than with the palaeomagnetic results that place the Devonian pole in central Africa.

Constraints on the locations of Asian microcontinents in Palaeo-Tethys during the Late Palaeozoic

Abstract: Useful constraints for Late Palaeozoic reconstructions of the Palaeo-Tethys and the development of Asia come from tectonic, palaeomagnetic, biogeographic and palaeoclimatic data. Tectonic constraints include the timing of collisions of the microcontinents which traditionally have been regarded as proceeding from north to south, and ranging in time from Late Palaeozoic to the Tertiary. Such a view gives a good account of the palaeobiogeographic connections, but the Mongolo-Okhotsk suture of northern Mongolia continued to close by counterclockwise rotation as late as the Jurassic. The biogeographic provinces are well developed in the Late Palaeozoic due to the relatively steep equator-to-pole gradients, thus as continents rifted from the southern margin of Palaeo-Tethys, they lost their south temperate Gondwanan affinities and acquired sub-tropical to tropical floras and faunas. Eventually, the north temperate Angaran floras and faunas inhabiting the northern margin of Palaeo-Tethys invaded some of the Cathaysian microcontinents by the end of the Palaeozoic. Unfortunately, the tropical Cathaysian floras could have occurred over a considerable latitudinal range (25° N to 25° S) and thus do not provide precise constraints. These floras, however, do contain seed-dispersed plants which implies that most of these separate microcontinents must have been geographically connected while apparently tectonically distinct. The climatically sensitive sediments include tillites and glacio-marine deposits associated with some of the terranes, and with Gondwana, but they are clearly temperate in origin, not polar, and are overlain by carbonates in Gondwana and the terranes. We interpret this as due to a climatic amelioration, rather than to a latitudinal plate motion. Palaeomagnetic data are now available for a number of south Asian microcontinents and enough determinations have now been made in North and South China to show consistency with the tectonic, biogeographic and climatic information. In detail, a southern belt of terranes, from the Helmand block in Iran and Afghanistan, through the Western Qiangtang and Lhasa blocks of Tibet and to the Sibumasu block of Thailand and Malaya, all rifted off the margins of Gondwana in the Permian. Few palaeomagnetic data are available to support this, but the tillites, floras and faunas are shared with the midlatitude portions of Gondwana from Iran, India and Australia. In the Late Permian the Cathaysian flora is known from at least the Helmand and Western Qiangtang blocks, suggesting that they reached lower latitudes, and were in physical contact with other Cathaysian floras, although the nature of this connection is not understood. The major Cathaysian microcontinents of Yangtze, Indochina, Eastern Qiangtang, Sino-Korea and Tarim were tropical throughout the Carboniferous and Permian. Again, a degree of geographic interconnection is implied by their common floras, and this is shared with low latitude portions of Gondwana, including Arabia and North Africa, but the location and nature of the 9land bridges9 are unknown. Collision of Tarim with Asia in the Early Permian and of Sino-Korea with the Mongolian arcs in the Late Permian is indicated from tectonic and biogeographic data, but, as stated above, the Mongolo-Okhotsk suture did not close until the Late Jurassic, thus rotation of the combined Mongolia and Sino-Korean block continued until that time. The other Cathaysian microcentinents collided with Asia about the Late Triassic giving rise to the Indosinian Orogeny.

Relations between conodont provincialism and the changing palaeogeography during the Early Palaeozoic

Abstract: World-wide distribution patterns of conodont species during the Cambrian, Ordovician, and Silurian are assessed quantitatively using a Coefficient of Similarity (CS) formula. The validity of the idea is examined that there is a correlation between CS values and the geographic distance between the sample points, and that changes in CS values may, at least in part, reflect plate motions. To minimize effects of local palaeoecological control and the varying degree of exploration of the faunas, well known faunas are used from comparable depositional environments on the several continental plates. Conodont provincialism can be traced back to Upper Cambrian time, when there was an initial differentiation into low-latitude, warm-water (Midcontinent Faunal Region) and high-latitude, colder-water (Atlantic Faunal Region) faunas. The Lower Ordovician is characterized by great taxonomic diversification as well as striking provincial differentiation. Based on regional distribution patterns, it is possible to recognize a North American Interior Province, a Mediterranean Province, a North Chinese Province, a Siberian Province, a Baltic Province, and an Australian Province during Arenig time. A similar lateral differentiation prevailed during Llanvirn to lower Caradoc time. It is difficult to recognize a close correlation between CS values and inferred distances of plate separations in the Cambrian and Lower and Middle Ordovician. Unlike some megafossil groups that markedly decreased in provincialism during the Upper Ordovician, the conodont faunas continued to exhibit pronounced lateral differentiation despite the fact that the Baltic, Siberian, and North American plates were then relatively closely together in the equatorial zone. Near the Ordovician-Silurian boundary (probably in the uppermost Ordovician) there was a global, quite conspicuous turn-over in the conodont faunas. During that time, the taxa characteristics of the Atlantic Faunal Region disappeared and apparently, the ancestors of virtually all Silurian taxa are to be found in the Midcontinent Faunal Region. The Silurian conodont faunas have a cosmopolitan character although there are minor local differences that may be attributed to local environmental control. During Silurian time, the plates that have produced the best conodont faunas (North America, Baltica, Siberia) were all in the equatorial zone, and a general uniformity in environmental conditions may be one of several reasons behind the cosmopolitan character of the Silurian conodont faunas. Many Lower Palaeozoic conodont species were capable of crossing water bodies of oceanic dimensions, and the distribution of many taxa seems to have been more closely controlled by water temperature and other ecological parameters than by potential migration barriers related to size of water bodies and emerged continental blocks. Nevertheless, because of their pronounced latitudinal differentiation during the Ordovician, the conodont faunas may be used as tools to decipher some of the positions of continental plates.

Reconstruction of the Laurasian and Gondwanan segments of Permian Pangaea

Abstract: A reappraisal of the geological and geometrical constraints on the fits of the continents around the Atlantic, Indian and circum-Antarctic oceans is used as a base upon which to re-examine Early and Late Permian palaeomagnetic data. The palaeomagnetic poles from each of the three main Late Palaeozoic palaeo-continents: Eurasia, North America, and Gondwana define statistically well determined means for the Early and Late Permian. When these poles are reconstructed, the means, and their associated A 95 cones of confidence do not superimpose. The lack of superposition of Late Palaeozoic palaeomagnetic poles on Wegener-style Pangaea reconstructions based on Mesozoic and younger seafloor spreading geometries has been noted for some time. Most solutions to this problem have involved alternative 9tight fits9 of the Pangaean continents for the Late Palaeozoic. Solutions invoking the non-dipole behaviour of the magnetic field have also been proposed. We examine the geometric consequences of the 9tight fit9 Mesozoic-based Pangaean reconstructions that allow better fit of the palaeomagnetic data and conclude that the magnitudes of overlap (650-950 km) required by these modifications are not compatible with the geological and tectonic evidence from these regions. We prefer to use a 9looser9 Pangaea fit that does not necessarily result in the superposition of palaeomagnetic poles. However, when the individual poles from Europe, North America, and Gondwana are combined into a global mean pole, it is observed that both the 9tight9 and the 9loose9 fit yield virtually indistinguishable mean pole positions. Considering the strong geological and geometrical arguments against the palaeomagnetically derived fits and the lack of a clear statistical difference between the two we use our revised Pangaean reconstruction to determine palaeo-latitudinal framework of the Eurasian, North American, and Gondwanan segments of Pangaea for the Early and Late Permian. The validity of these palaeo-latitudinal reconstructions is tested with climatically sensitive floristic and lithological data by Ziegler, and found to be satisfactory.

Devonian vertebrate distribution patterns and cladistic analysis of palaegeographic hypotheses

Abstract: Early Devonian vertebrate faunal provinces are clearly defined for four regions: Euramerica, Siberia, China, and East Gondwana. The presence of osteostracans in southwest Siberia (Tuva, Minusa Basins) suggests either proximity of the Siberian block to Euramerica, or palaeogeographic separation of the Tuva region. The Knoydart fauna of Nova Scotia demonstrates that the Avalon Terrane was connected to Euramerica by Gedinnian time. Widespread antarctilamnid sharks in Gondwana suggest a distinctive Gondwana vertebrate fauna, isolated by marine barriers from Euramerica in the Early - Middle Devonian. Late Devonian patterns indicate faunal communication between Gondwana and Euramerica by Frasnian time, and between China and East Gondwana in the late Famennian. The Late Devonian base maps require anomalously wide latitudinal distributions for some taxa. Displacement of Turkey along the northern margin of Gondwana provides an intermediate occurrence of phyllolepid placoderms between disjunct distributions in Euramerica and East Gondwana, but the fossil data do not necessarily corroborate geological evidence for displacement. Biogeographic data generally must be interpreted in the context of palaeogeographic hypotheses, and lack of integration with geological and geophysical data sets has been a major problem. Hierarchical analysis using cladistic techniques has the potential for integrating biological, geological, and geophysical data, as illustrated in a cladistic analysis of the Williams and Hatcher model of Appalachian terranes. As an adjunct to map representation, an area cladogram enables a historical sequence of palaeogeographic events to be represented on a single diagram, together with crucial supporting evidence; it presents an analysis rather than synthesis of empirical data, and the hypothesis is more exposed to falsification.

Palaeozoic palaeogeography from palaeomagnetism of the Atlantic-bordering continents

Abstract: Revised palaeogeographic reconstructions of the Atlantic-bordering continents and intervening terranes are presented for the Siluro-Devonian boundary, Early Devonian, and Late Devonian, based on incorporation of new palaeomagnetic results that have become available from Laurentia and Gondwana. The key features of the palaeogeographic model are the transpressive collision between the eastern margin of Laurentia and the northwest South American margin of Gondwana in the Siluro-Devonian, and the subsequent Devonian retreat of the north African margin and the development of a wide ocean between Africa and Europe by the Late Devonian. Although rather complex and involving rapid motions especially of Gondwana, the revised Devonian plate tectonic evolution as indicated by palaeomagnetism is not inconsistent with biogeographic and palaeoclimatological evidence.

Palaeogeographic evolution of southwestern Europe during Early Palaeozoic times

Abstract: In the Late Ordovician, both SW Europe and Africa were situated in high latitudes. Sedimentary fades and biogeography suggest that these regions were separated by the Rheic (Mid-European) Ocean from Baltica, with the suture lying between the Armorican Massif and the Ardennes. During the Silurian and the Devonian, palaeobiogeographic affinities persisted between S Europe and N Africa. It is concluded that S Europe consisted of several discrete blocks or microplates prior to the Hercynian Orogeny.

Brachiopod zoogeography across the Ordovician-Silurian extinction event

Abstract: The Extinction event at the close of the Ordovician, one of the largest of the Phanerozoic, had a profound effect on the zoogeographic distribution of brachiopods. Strong endemism in the Ashgill was particularly apparent in the epicontinental seas covering the primary lithospheric plates. Areas in the open ocean and around the margins of the continents had more widely distributed faunas. Glacio-eustatic decline in sea level and climatic deterioration caused by the developing North African glaciation have been widely invoked as the cause of the extinction event. The Hirnantian Stage roughly corresponds to the glacial maximum, and Hirnantian faunas were somewhat more cosmopolitan than were early-middle Ashgill faunas. At the end of the Hirnatian, sea level rise and climatic amelioration accompanying the end of the glacial maximum apparently caused another wave of extinctions. Early Silurian brachiopods were far more cosmopolitan, especially in the epicontinental seas, than they had been in the Late Ordovician. There is a strong correlation between wide geographic distribution of genera and survival of the extinction event. Provincial patterns are consistent with the symposium maps.

A review of Palaeozoic palaeomagnetic data from Europe and their palaeogeographical implications

Abstract: Recent palaeomagnetic studies on Devonian and Carboniferous rocks have resulted in a time re-calibration of the Apparent Polar Wander Path (APWP) for Europe, and revision of the shape of the APWP for North America Differences between previously published versions of these paths are now much reduced The APWP for southern Britain is different from those for North America and Armorica, thus southern Britain is believed to have been an isolated block within the pre-Hercynian ocean New continental reconstructions are presented to take account of these conclusions A lack of sufficient reliable palaeomagnetic data from Baltica make its position on the map uncertain, and hence the significance of the Tornquist Sea between Baltica and Palaeo-Europe remains incompletely understood

Palaeomagnetic constraints on the position of Gondwana during Ordovician to Devonian times

Abstract: The currently available palaeomagnetic data base for Gondwana is reviewed and using revised rotation parameters for the fit of the southern continents an attempt has been made to construct an apparent polar wander path for Gondwana during Ordovician to Permo-Carboniferous times using a cubic spline fitting technique. Although the density of the data set is still rather sparse and the quality of the data is variable, our approach seems to be justified when tying the apparent polar wander path to selected palaeopoles of high quality. The palaeogeographic scenario based on our results is rather complex. Rapid northward shift of Gondwana during the Ordovician to the Early Silurian and subsequent collision with Laurentia is followed by divergence and the formation of a wide intervening ocean during the Devonian. The final closure of this ocean did not begin before the Late Devonian and was completed by the Late Carboniferous. If the apparent polar wander path presented in this paper is correct, then extremely high drift rates of about 23 cm a -1 have to be postulated for Gondwana during the Late Ordovician-Silurian.

Constraints on Siluran and Early Devonian phytogeographic analysis based on megafossils

Abstract: The detection of phytogeographic patterns in Silurian and Early Devonian land vegetation is hampered by the dearth of occurrences of megafossils, the absence of consistently precise correlation, unreliability of identification, and the lack of sufficient detailed sampling and information on sediments to permit evaluation of taphonomic influences on composition of assemblages. Interpretation is further complicated by the exceptional evolutionary position of the plants themselves and the lack of extant representatives. There is little information on whole plants, their life histories, their ecological and climatic tolerances. The composition of Ludlow, Pridoli and Lochkovian/Gedinnian assemblages is analysed. Those from Kazakhstan and Siberia are particularly enigmatic. The assemblages are plotted on the appropriate continental reconstructions. Most are clustered on the southeast margin of Laurussia and very few occur at high latitudes. In Ludlow time, Baragwanathia dominated assemblages in Australia in contrast with the rhyniophytoid assemblages of Laurussia. New information from Kazakhstan and northwest China adds to the data base in the Pridoli. Differences in Gedinnian assemblages from South Wales, the Welsh Borderland and Scotland, where sediments have been intensively studied and correlation is based on fish faunas and, more usefully, on palynomorphs, are related to taphonomy, to local vegetation distribution pattern, and to evolutionary changes. World-wide analysis of occurrences shows four distinct assemblages, based on Laurussia, Kazakhstan, Australia and Siberia, the latter being a rare example of high latitude vegetation.

The Silurian and Early Devonian biogeography of ostracodes in North America

Abstract: Silurian and Early Devonian ostracode associations in North America represent at least three ecotypes, a leperditicopid association, a large beyrichiacean association, a mixed association, and possibly a fourth, spinose podocopid association, or Thuringian ecotype. Comparison of the large beyrichiacean association and mixed association ecotypes indicates the presence of three informal ostracode provinces, the Appohimchi, Baltic-British and Cordilleran, which remained relatively constant in geographical position throughout the Silurian and Early Devonian. Plotting the provinces on palaeogeographic maps suggests that temperature was not an important factor in delimiting the provinces, and land barriers, or possibly deep-water troughs as barriers, were the cause of provincial development. Benthic ostracodes differed from other benthic invertebrates such as brachiopods and corals in developing provincialism in the late Llandovery and having it start to decline in the Pragian through the Emsian.

Ordovician sedimentary facies and faunas of southwest Europe: palaeogeographic and tectonic implications

Abstract: The term st European Platform is introduced for an Ordovician shelf now within the western Variscides. The geometry of the sedimentary facies belts and their faunal variation suggest that it formed a promontory on the Gondwanan margin. The palaeogeography of the platform is discussed with particular reference to the Centro-Iberian/Armorican area, especially central Portugal. The sedimentary history of the Centro-Iberian/Armorican area was controlled by basement structures. Rapid local subsidence during Early Ordovician extension was followed by later differential subsidence along these early structures. The thin sequences indicate low overall rates of subsidence. The dominantly clastic shallow marine sediments show strong storm influence. Individual sediment packets occur over wide areas indicating little syndepositional relief. The linear nature of facies belts allows reconstruction of the relative orientation of some blocks within the Variscides, and indicates strike-slip faulting during early Variscan events, with subsequent (Carboniferous) rotation of most of Iberia. Faunas from SW Europe from a homogeneous group related to those of the classic 9Mediterranean Province9 areas of Bohemia and Morocco, and more distantly to those of the Avalonian terrane. Faunal migration into the West European Platform from Avalonia accompanied progressive northward drift of the platform, occuring particularly during periods of eustatic sea-level rise in the early Llanvirn, the early Caradoc and the early Ashgill

Early and Middle Devonian gastropod biogeography

Abstract: Early and Middle Devonian gastropods show biogeographic patterns remarkably similar to those of better studied faunal groups of this same interval, notably articulate brachiopods, rugose corals, and trilobites. Three biogeographic realms are recognized: Old World, Eastern Americas, and Malvinokaffric Realms. Gastropod diversity is highest in the Old World Realm and lowest in the Malvinokaffric Realm. The diversity patterns and the high degree of shell ornamentation suggest that the Old World Realm was generally warmer than the Eastern Americas Realm, and that both were considerably warmer than the cool temperate to cold polar waters of the Malvinokaffric Realm. The utility of gastropods for fine-scale delineation of biogeographic units is illustrated for the Eifelian of western North America. At least two subprovincial units (the Alaska-Yukon and Nevada subprovinces) can be recognized. Eifelian gastropods from interior and southeastern Alaska belong to a single unit (the Alaska-Yukon Subprovince), and are most closely related to coeval faunas of northwestern Canada, suggesting little displacement of most of Alaska9s so-called 9suspect9 terranes. Plotting the data on the Devonian palaeogeographic maps of Scotese results in several suggested emendations: (1) North America should be moved south by 10-20°; (2) Australia is too far south on the Emsian and Givetian reconstructions, it should be in a more palaeotropical position; and (3) Siberia is too far north, it too should also be placed in a palaeotropical position.

Late Palaeozoic provinciality in the marine realm

Abstract: Biogeographic units commonly recognized in large-scale palaeontological studies are usually related to realms, not provinces as recognized in modern zoogeography. In this study the geographic distribution of the Rugosa, Tabulata, Bivalvia, Ammonoidea, Strophomenida, pedunculate Articulata, Bryozoa, and Crinoidea are used to recognize a set of realms and provinces (based on 30% or more generic endemism in the combined fauna within a province) for the Early and Late Carboniferous and Early and Late Permian. Between 12 and 18 provinces in four or five realms are identified in each interval of the Late Palaeozoic. The recently stated view than the Late Palaeozoic was a time of increasing provinciality is related to a decrease in the average range of regionally distributed genera, not an increase in either endemism or in the number of recognizable biogeographic units.

Late Palaeozoic bryozoan biogeography

Abstract: Late Palaeozoic (Carboniferous and Permian) distributions and times of dispersal of bryozoans, fusulinids, and other marine shelf invertebrates were greatly influenced by the effects of land masses converging to form a supercontinent. This convergence modified oceanographic circulation patterns as well as environmental, climatic, and temperature patterns. Sea level fluctuations of 75 to 100 metres having about two million years duration also strongly affected faunal dispersal patterns. At the end of the Early Carboniferous (late Visean through early Bashkirian), the assemblage of Lesser Pangaea disrupted the tropical, subtropical and warm temperate marine shelf faunas which included cryptostomes, trepostomes, and cystoporates. In the middle of the Early Permian (Leonardian), the assemblage of Greater Pangaea further disrupted the marine shelf faunas giving rise to biogeographic provinces and finally realms. In the assembling of Pangaea, the northward transport of plates and cratons caused the northern marine shelf of Pangaea to be moved into cooler and cooler waters. Tectonic movements caused cool ocean currents to be redirected toward the equator along the western marine shelves of Pangaea. Faunal diversity greatly increased apparently as a result of the flow of consistently warm equatorial currents along the eastern marine shelves of Pangaea.

Rugose coral distribution as a test of Devonian palaeogeographic models

Abstract: A data bank, based on stage by stage distributions of 420 rugose coral genera in 25 regions of the world is analysed, mostly by means of Otsuka coefficients, to test an Emsian reconstruction of the world proposed by Scotese. Devonian rugose corals inhabited a narrower range of facies than some other benthic groups, and even without regard to facies, provide a tool for testing geographic reconstructions. Basin dwelling coral genera typically have longer temporal and broader geographic ranges than corals living in shallower environments, and are less suitable for palaeogeographic studies. They are treated separately in this work. For the most part, conclusions drawn from the analysis are either consistent with, or positively supportive of, the Scotese reconstruction. However, large but poorly known rugose coral faunas from Mongolia and the Amur Basin are at about 60° N in the reconstruction, and other well known coral faunas, from Altai-Sayan, are at 45°-50°N. In the light of known distributions of both modern corals and Devonian southern hemisphere corals, in all recently proposed palaeogeographic reconstructions, it is questionable that the original latitude of any large northern hemisphere Devonian coral fauna would have exceeded 45°.

Givetian-Frasnian phytogeography of Euramerica and western Gondwana based on miospore distribution

Abstract: The Givetian and Frasnian miospore distributions in western Gondwana and southern Euramerica show a rather uniform vegetation prevailing from palaeo-polar to palaeo-tropical regions. Similar climatic conditions are certainly required to explain this but it is concluded from a discussion on the dispersal of homosporous vegetation that no wide oceans separated these regions at the time. Frasnian northern Euramerica vegetation seems different and might correspond to an equatorial belt. Heckel & Witzke9s palaeogeographical reconstruction fits much better with the miospore distribution than other maps.

Graptolite biogeography: implications for palaeogeography and palaeoceanography

Abstract: Two faunal regions, a cool-water, Atlantic and a tropical-water, Pacific, may be distinguished among Tremadoc into Ashgill planktic graptolite faunas. The zenith of graptolite provincialism was during the Arenig-Llanvirn when Laurentia, Australia-New Zealand, North China, South China, Siberia, Argentine Precordillera, and parts of Kazachkstan were provinces within the Pacific Region. Coeval, Southern Hemisphere, Atlantic Region provinces were: England-Wales, Baltoscania, Bolivia-Peru-Northern Argentina, western Europe, North Africa, and possibly part of modern Tien Shan. South China Arenig-Llanvirn faunas include many incursions of Atlantic Region taxa, probably reflective of current circulation changes linked to development of seasonal monsoons. Mid-Ordovician plate motion included northward movement of Baltoscania into the tropics. That motion resulted in Baltoscania becoming a province in the Pacific Region in the Late Ordovician. Late Ordovician glaciation led to restriction of graptolites to the tropics (Pacific Region) during the latest Ordovician. Deglaciation was followed by re-establishment of a tropical, Pacific Region fauna and a cool-water, Atlantic Region fauna during the Llandovery and Wenlock. Graptolite faunas appear to have been only tropical from the Ludlow to their extinction in the Pragian (Early Devonian). The graptolite regions and provinces are essentially consistent with plate positions suggested by palaeomagnetic and lithofacies data.

Palaeobiogeography of Middle Palaeozoic organic-walled phytoplankton

Abstract: Potential advantages to the use of Palaeozoic organic-walled phytoplankton over other fossil groups are their presence in large numbers in small samples (particularly cores) and their partial independence from lithofacies. Reasons why this potential has not been fully realized are discussed, and the history of study is briefly reviewed. The 9latitude parallel9 distribution model proposed for Silurian phytoplankton does not hold up when plotted on a more recent plate reconstruction, although some important insights are forthcoming. Ordovician microfloras appear broadly consistent with proposed plate models, and the diachronous distribution of the genus Frankea can be explained by combining the plate model with a restricted, high southern latitude distribution. Ten Frasnian microfloras are compared in detail using G. G. Simpson9s Index and cluster analysis. Microfloras from Brazil, Ghana and Algeria cluster together, distinct from outlying areas, while the Carnarvon Basin microflora in Australia is considered relatively endemic. The Frasnian phytoplankton data support a plate reconstruction in which a large seaway separated Africa and South America from North America and Europe.

Biogeography of the Devonian stromatoporoids

Abstract: Stromatoporoids were a common component of shallow carbonate environments of North America, Eurasia, and Australia during the Devonian. They were least abundant during the Early Devonian. After that time abundance increased, and remained high steadily through the Frasnian. At the Frasnian-Famennian boundary the number of stromatoporoids was greatly diminished, but they did not become extinct until the end of the Devonian (at the end of the Strunian). The geographic extent of stromatoporoids expanded and contracted concurrently with increases and decreases in total population size. Provincialism at the genus level prevailed during the Early Devonian, with stromatoporoids inhabiting the Old World and Eastern Americas Realms; none are known from the Eastern Americas during the Siegenian. For the remainder of the Devonian stromatoporoids were cosmopolitan at the genus level. The abundance of stromatoporoids varied directly with eustatic sea level during the Devonian. Variations in depositional conditions apparently controlled the local distribution of genera.

Devonian palaeogeography and palaeobiogeography of the Central Andes

Abstract: During Devonian time, western Gondwana (northern Chile, Bolivia, and Peru) witnessed a period of thick clastic rock deposition. The sialic Arequipa Massif, apparently a stable terrane since Precambrian time, supplied the bulk of the Bolivian intracratonic basinal sediments. Late Ordovician orogeny in northern Chile-northwestern Argentina included the emplacement of the Puna magmatic arc, with attendant plutonism. The resulting highland supplied coarsegrained sediments to northern Chile. Late Devonian(?) orogenesis deformed Early to Middle Devonian rocks in northern to central Peru. It is suggested that mid-Palaeozoic convergence was 9buffered9 by the Arequipa Massif, giving little evidence of orogeny in the Bolivian Devonian-Carboniferous sequence. A highly endemic (Malvinokaffric) shelly fauna, indicative of high palaeolatitudes, occupied the region in Early Devonian (Emsian) time, while allochthonous (Eastern Americas Realm) faunas entered southern Peru. During Middle Devonian time a much lower diversity 9post-Malvinokaffric9 fauna (consisting of the brachiopods Tropidoleptus, Globithyris , and other taxa) arrived, although a Malvinokaffric stock remained. These taxa arrived via the Amazonas and other eastern basins during a rotation of the region to lower latitudes and presumably warmer waters in Middle Devonian time.

Stratigraphic and palaeogeographic distribution of Palaeozoic oolitic ironstones

Abstract: The record of Palaeozoic oolitic ironstones reflects second-order Phanerozoic sea-level changes. Middle Cambrian- earliest Ordovician high stand: scattered ironstones developed on low-latitude Laurentia and locally on high southern-latitude NW Africa and Nova Scotia. Early-early Late Ordovician high stand: major and scattered minor ironstones were widespread on high southern-latitude N Africa and Peri-Gondwanan blocks. Minor ones on C Laurentia, NE and SW Baltica, SW Kazakhstan, and NW and SW Siberia, NW Malaysia, and C Australia accumulated in middle and low latitudes, as did all later Palaeozoic ironstones. Latest Ordovician-earliest Silurian low stand: during southern-latitude glaciation minor ironstones were limited to WC Algeria and low-latitude C and SE Laurentia. Early-early Late Silurian high stand: minor ironstones accumulated on NC, WC, and SE South America, N Africa, and Peri-Gondwanan blocks, while major ones developed on NW Africa and CE Laurentia. Latest Silurian-Early Devonian low stand: major ironstones developed on WC Algeria; minor ones on CE Laurentia, several Peri-Gondwanan blocks, N and W Africa, NC South America, and South China. Middle-early Late Devonian high stand: major ironstones accumulated on N Africa, SE Baltica, NW Iberia, and South China; minor ones on SW and CE Laurentia, and NW and SE Middle Europe. Latest Devonian-earliest Carboniferous low stand: well-developed latest Devonian ironstones were limited to N Africa and NW Middle Europe, minor ones to SE Middle Europe and CE Laurentia; minor earliest Carboniferous ones to NW Middle Europe and NC Africa.

Cambro-Devonian biogeography of nautiloid cephalopods

Abstract: The biogeography of Cambro-Devonian nautiloid cephalopods is documented from their origin during Late Cambrian through to the end of the Devonian. The biogeography developed as the consequence of two related sets of events with different magnitudes. Events controlling the biogeography of first order magnitude were geotectonic, either Gondwana glaciation due to plate movement or contraction in size of oceanic ridges related to decreased rates of sea floor spreading, manifested as first and second order eustatic fall in sea level. These first order events separate nautiloid biogeography into four episodes: (1) Late Cambrian; (2) Ordovician; (3) Silurian through Early Devonian; and (4) Middle through Late Devonian. Each began with an expansion in terms of generic diversity and ended with a crisis of some proportion. Only the end of the Late Cambrian episode cannot be explained by geotectonic events. Events of the second order are both biologic and geotectonic and involve the interaction between the dispersal characteristics of nautiloid cephalopods and plate movements. Due to lack of a planktonic larva and depth restrictions as a function of shell design, the ability of nautiloids to disperse and colonize shallow shelf seas separated by expanses of ocean or depths exceeding shell design limits was minimal. When the width of oceans or depths decreased through some form of geotectonics, nautiloids rapidly radiated and colonized regions as they became available. Thus second order biogeographic episodes tend to evolve from a collection of disjunct patterns with high faunal similarity among faunas of particular landmasses, but with little similarity among separate landmasses, to rapid development of high similarities both among and within faunas of converging landmasses once the physical barrier of distance and water depth is eliminated.

The relationship of Ordovician graptolite provincialism to palaeogeography

Abstract: Graptolite biogeography is examined for the late Arenig-Llanvirn when provincialism was at its greatest and the Llandeilo-Caradoc when graptolites were cosmopolitan. This examination incorporates new data from China and recently revised palaeogeographic base maps. Water-mass specificity is considered to be the primary factor affecting graptolite biogeography. On the recently revised base maps, the distribution of Pacific and Atlantic provinces in the late Arenig-Llanvirn is in general consistent with the model of water-mass specificity and the hypothesis that climate cooling associated with the onset of continental glaciation lead to the development of provincialism. However, a small clockwise rotation of South America and a slight northward shift in the position of North America is recommended. The climatic-cooling hypothesis includes the proposal that continued cooling confined graptolites to the tropics where the uniform environment precluded provincialism. The distribution of the cosmopolitan fauna of the Nemagraptus gracilis Zone in relationship to the Llandeilo-Caradoc palaeogeography is consistent with this hypothesis, except for the positions of the European platform, Britain, Armorica, and the Carpathians in the mid to high southern latitudes. Either the latitudinal positions of these areas must be shifted substantially to the north, or the climatic-cooling hypothesis mnst be in error.