Late Ordovician–early Silurian destruction of the Iapetus Ocean: Newfoundland, British Isles and Scandinavia—a discussion

The available data from Newfoundland, the British Isles and Scandinavia suggest that by late Ordovician–early Silurian times the ocean separating Laurentia from Eastern Avalonia and Baltica had partly closed with the consumption of intervening oceanic crust. Marine seaways, however, persisted until the middle or late Silurian. Phases of crustal transtension and transpression, predominantly under a major sinistral shear couple, occurred throughout the Silurian and early Devonian until the remnant Iapetus Ocean was completely destroyed. The most appropriate Recent plate tectonic models for Silurian sedimentation between Eastern Avalonia and Laurentia are probably the deep-marine foreland basins between Timor and the northern Australian margin, or between Taiwan and mainland China.

Early Ordovician Dounans Limestone fauna, Highland Border Complex, Scotland

A diverse, but sparsely distributed silicified fauna of over 30 taxa has been recovered from 7·5 tonnes of acid-etched Lower Ordovician Dounans Limestone from the Highland Border Complex, near Aberfoyle. The 13 trilobite taxa obtained include 3 new formally named species: Distazeris adoceta, Punka aetholiciocorus and Ischyrotoma stubblefieldi. Other elements, representing Carolinites, Illaenus, Goniotelina, Sycophantia, Kawina, Heliomeroides, Strotactinus, Ectenonotus, Ceratocephala and an indeterminate bathyurelline are described under open nomenclature although at least 4 are also probably new and 2 more may be conspecific with previously described species. The brachiopods include representatives of Archaeorthis, Nothorthis, Orthidium, ?Camerella, Idiostrophia, Orthambonites and 4 other indeterminate stocks. Gastropods tentatively assigned to Murchisonia, Subulites, Straparollina, Maclurites and Cyrtodiscus are described, as are bryozoans, an orthocone, crinoids, and Incertae sedis.

This profoundly North American fauna is Late Canadian ( = mid Arenig) in age and equates with the Cassinian Stage: it is key evidence in showing that the Highland Border rocks are not part of the Dalradian Supergroup whose earliest deformations and metamorphism predate the Ordovician. The field relationships of the limestone, together with evidence from derived clasts in Middle Ordovician and Devonian sequences near Girvan in SW Scotland and at Stonehaven in eastern Scotland suggest that it forms part of a widespread sub-Old Red Sandstone carbonate sequence of Early Ordovician age beneath the northern Midland Valley.

Evolutional trends and palaeobiogeography of the Ordovician trilobite Ovalocephalus Koroleva 1959

Ovalocephalus has a long stratigraphic range and wide geographical distribution in Ordovician peri-Gondwana. Based largely on the well-preserved specimens recently collected from China, all known forms are revised and listed. Phylogenetic analysis was conducted on the genus, involving 10 species. As suggested by the strict consensus tree, evolutional trends of the genus include mainly the isolation of the anterior glabellar portion anterior to S1, the forward shifting of eyes and the related lengthening (exsag.) of the posterior fixigena, the reduction of the number of pygidial axial segments and pleural abaxial rounded free tips, the shortening of the pygidial postaxial region, and the development of cranidial genal spines. Ovalocephalus may have originated in shallow-water sites of the South China Plate in the Early Floian, but migrated into the deep-water regions from the Darriwilian onwards. All the records of the genus from the Early Floian to Early Katian were confined to eastern peri-Gondwanan plates and terranes in low-latitude zones. It was only restricted to the South China, Tarim and North China plates until the Middle Darriwilian, but the Late Darriwilian eustatic sea-level rise and especially the Sandbian-Early Katian immense transgression may have brought about its dispersal to Alborz, Sibumasu and central Asian terranes. Following the closure of the Tornquist Sea, the genus was even able to spread to Baltica during the latest Katian, and the pre-Hirnantian warming (the Boda event) may have promoted a wider distribution of Ovalocephalus to western peri-Gondwana (the Taurides and Armorica terranes) in the then-high latitudes.

The morphology, classification and evolution of the Trinucleidae (Trilobita)

The morphology of the trinucleid exoskeleton is reassessed and various new structures and terms defined. On the basis of new interpretations of cephalic morphology, particularly that of the fringe, of all known genera, a new subfamilial classification is proposed. Two new genera,Deanaspis and Whittardolithus, and the subfamily Reedolithinae are proposed, and the genusBotrioidesStetson resurrected. The existing subfamilies, Trinucleinae, Gryptolithinae and Marrolithinae are recast considerably and the Tretaspidinae, Novaspidinae and Incaiinae rejected. The Hanchungolithinae is left unmodified. Evolution within the family is discussed together with the distribution of genera in terms of recent palaeogeographic reconstructions. A review of the relations between phylogeny and ontogeny confirms the essentially recapitulative nature of trinucleid ontogenies. A list of current generic assignments of known species is given

Early-Middle Palaeozoic biogeography of Asian terranes derived from Gondwana

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 'Hexizoulang Terrane'.

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

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.

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

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 'Mediterranean Province' 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

Biogeography of Ordovician and Silurian faunas

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, e.g. 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.

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

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.