Thomas Servais

Ordovician organic-walled microphytoplankton (acritarch) distribution: the global scenario

Abstract A number of palaeobiogeographical models for Ordovician organic-walled microphytoplankton (acritarchs, prasinophytes, and related groups) have been published during the past 30 years. A modern synthesis of Ordovician acritarch palaeobiogeography, based on previously published acritarch ‘provinces’ and global distribution models, as well as new plots on recently compiled palaeogeographical maps is presented. Review of the literature and new plots indicate that a number of preliminary conclusions can be drawn. Following minor biogeographical differentiation of acritarch assemblages during the Cambrian, ‘provincialism’ started at the Cambrian–Ordovician boundary. In the late Tremadocian a warm-water assemblage, containing the genera Aryballomorpha , Athabascaella and Lua , but no diacrodians, seems to be limited to low-latitude localities such as Laurentia and North China. From the late Tremadocian and throughout most of the Arenig a peri-Gondwana acritarch assemblage with the easily recognisable taxa Arbusculidium filamentosum , Coryphidium , and Striatotheca is present on the southern margin of Gondwana, and its distribution corresponds almost exactly with that of the Calymenacean–Dalmanitacean trilobite fauna. It seems reasonable to consider the acritarchs of Baltica as belonging to a temperate-water ‘province’, which was probably not restricted to the palaeocontinent of Baltica but had a wider distribution at about the same latitude, as some of the elements recorded from Baltica also occur in South China and Argentina. The maximum separation of the continents during the Arenigian, reflected by a pronounced biogeographical differentiation of most Ordovician fossil groups, led to the development of geographically distinct acritarch assemblages. Data from the late Middle Ordovician and the Late Ordovician remain too poor to elucidate global palaeobiogeographical patterns. The biogeographical distribution of Ordovician acritarchs appears similar to that of the resting cysts of modern dinoflagellates, primarily controlled by latitude but also following the continental margins.

THE ACRITARCH GENUS VERYHACHIUM DEUNFF 1954: TAXONOMIC EVALUATION AND FIRST APPEARANCE

Abstract Veryhachium Deunff 1954, originally described from the Ordovician of western France, is one of the most frequently recorded acritarch genera. Over 250 species and subspecies, from the Cambrian to the Neogene, have been attributed to the genus. This genus has a simple morphology; it displays a triangular, rectangular, or polygonal central vesicle, with a few, simple processes drawn out from the angles of the vesicle in a single plane, and sometimes with supplementary or auxillary processes arising from the vesicle body. Veryhachium has been emended and revised numerous times. The number of valid species is excessive: most are probably synonyms. To facilitate effective classification, only a few morphological categories should be retained. For the Lower Paleozoic, the use of two informal groups is proposed. These are the Veryhachium trispinosum group for triangular specimens, and the Veryhachium lairdii group for rectangular forms. Although generally abundant and widespread throughout the Phanerozoic, Veryhachium is of limited biostratigraphic, paleoecologic, or paleogeo-graphic value. However, its First Appearance Datum (FAD) is of great importance for Ordovician stratigraphy; the first Veryhachium morphotypes appear in the Tremadocian Stage, making the genus an important biostratigraphic marker.

The messaoudensis—trifidum acritarch assemblage and correlation of the base of Ordovician Stage 2 (Floian)

The Global Stratotype Section and Point (GSSP) for Stage 2 of the Ordovician System, now the Floian Stage and approximately equivalent to the lower and middle Arenig of England and Wales, is defined by the first appearance datum (FAD) of the graptolite Tetragraptus approximatus in the Diabasbrottet Quarry section at Mount Hunneberg, Sweden. One of the issues this raises is how to correlate the base of Stage 2 at the GSSP with areas and successions that do not contain a correlative graptolite fauna. The distinctive Cymatiogalea messaoudensis–Stelliferidium trifidum acritarch assemblage is present in the upper Tremadocian Araneograptus murrayi Graptolite Biozone of NW England and ranges across the Tremadocian–Stage 2 boundary there (the Tremadoc–Arenig boundary of Anglo-Welsh nomenclature). It also occurs widely at other high southern Ordovician palaeolatitudes around the margin of Gondwana, being reported from Ireland, Wales, the Isle of Man, Belgium, Germany, Spain and Turkey, and may also be present in Bohemia and Argentina (Eastern Cordillera). It therefore has the potential to contribute towards the recognition and correlation of the base of Stage 2 in those areas. Of particular interest are the First Appearance Datums of various taxa within the stratigraphical range of the messaoudensis–trifidum assemblage, notably that of Aureotesta clathrata simplex , which is considered to be close to the base of Stage 2 in NW England.nnElements of the messaoudensis–trifidum assemblage also occur in Baltica, the palaeoplate on which the GSSP for the base of Stage 2 is located. However, many of the taxa used to subdivide the messaoudensis–trifidum assemblage around Gondwana have not been recorded from Baltica and may be restricted palaeobiogeographically to the Gondwanan margin. Furthermore, acritarch microfloras have not been reported from the Diabasbrottet Quarry section itself, and there are hiatuses across the base of Stage 2 in the two sections from Baltica considered in this paper. Hence, direct correlation of the base of Stage 2 between the GSSP and other sections using acritarchs is not yet possible. Nevertheless, some taxa, for example the genera Peteinosphaeridium and Rhopaliophora , are shown to have FADs at similar stratigraphical levels in the late Tremadocian Stage of both Baltica and Gondwana, and therefore have the potential to correlate time slices in the late Tremadocian Stage between palaeoplates.

NEW SEM OBSERVATIONS OF KERIOTHECAL WALLS: IMPLICATIONS FOR THE EVOLUTION OF FUSULINIDA

This study presents new scanning electron microscopy (SEM) observations of Paleozoic foraminifera. It focuses especially on the wall of three Fusulinida, Sakmarella moelleri, Paraskinnerella skinneri, and Taiyuanella furoni, that belong to the Schwagerinoidea, which are fusulinids with keriothecal test walls. In addition, Eostaffella sp. is investigated as an example of the microstructure of the microgranular imperforate tectum. The fillings of the keriothecal alveoles of Sakmarella, Paraskinnerella, and Taiyuanella yield diverse, characteristic flower-like structures that indicate a gradual evolution of the microstucture of the tests. Sakmarella and Paraskinnerella have flowers with a narrow center, and Taiyuanella has a larger center. The true keriothecal structure, described, for example, in Triticites, is distinguished from these structures by simply filled alveoles. The term ‘‘anthotheca’’ is introduced for the structure with narrow-centered flowers. It can be distinguished from the large-centered flower structure identified as Zhuang’s (1989) stalactotheca. Following a literature review of previous work on keriothecal structures, several morphofunctional hypotheses are discussed, including the housing of symbiotic algae or cyanobacteria, which leads to the new interpretation that the flowers of the antho- and stalactothecal structures are induced by algal endosymbionts. In addition, we discuss the monophyly of keriothecal fusulinids. The transition from diaphanothecal to keriothecal forms recognized since Thompson (1964) is reinterpreted in terms of relationships with symbiotic algae or cyanobacteria. The Schwagerinoidea constitute an important group in understanding the evolution of larger foraminifera in the late Paleozoic.

Cosmopolitan arthropod zooplankton in the Ordovician seas

Abstract Evidence is presented here for a zooplanktonic component in Ordovician marine ecosystems, namely the caryocaridid arthropods, that add to other well-documented midwater organisms such as graptolites, cyclopygid and telephinid trilobites, orthoconic cephalopods and the microphytoplankton (e.g. acritarchs). Although the soft anatomy of caryocaridids is largely hypothetical, their carapace design and ultrastructure, and their phyllocarid-like abdominal morphology (flattened furcal rami, telescopic segments) indicate a swimming lifestyle in midwater niches. Both functional and ecological interpretations are supported by their palaeogeographical and facies distributions and by analogies with modern pelagic ostracods. Caryocaridids occur at numerous localities on the palaeo-plates of Laurentia, Baltica, Avalonia, Perunica, Gondwana and South China and are recurrent faunal components of graptolitic black shales (mainly Tremadoc to Llanvirn). Typical faunal associates are the didymograptid and isograptid graptolites, pelagic cyclopygid and deep-sea benthic atheloptic trilobites. Their depositional environments suggest that the caryocaridids and their pelagic associates (graptolites) most probably thrived in waters above the distal shelf margins, where upwelling-controlled primary productivity possibly reached its maximum. Their exact bathymetrical range within the water column cannot be inferred from fossil evidence. However, their feeding strategies may have led them to exploit food resources across the mesopelagic–epipelagic boundaries as do numerous midwater crustaceans in present-day ecosystems. Caryocaridids represent a significant step in the post-Cambrian colonisation of midwater niches by arthropods and in the construction of complex modern foodwebs.

The messaoudensis-trifidum acritarch assemblage (Ordovician: late Tremadoc-early Arenig) of the Barriga Shale Formation, Sierra Morena (SW-Spain).

The messaoudensis-trifidum acritarch assemblage is currently considered to be characteristic of latest Tremadoc-earliest Arenig cold-water environments on the periphery of Gondwana, at high latitudes in the southern hemisphere. An integrated biostratigraphical study on both acritarchs and graptolites was until now only available for localities in northwest England. Reinvestigation of the messaoudensis-trifidum acritarch assemblage from the Barriga Formation (Sierra Morena, southwestern Spain), which contains some graptolite horizons that can be attributed to the latest Tremadoc (pre-phyllograptoides and pre-approximatus graptolite biozones), strengthens the biostratigraphical potential of the messaoudensis-trifidum assemblage, and the importance of some of the acritarch taxa recovered from these levels. It is concluded that the acritarch genera Coryphidium Vavrdová, 1972, Peteinosphaeridium Staplin et al., 1965 emend. Playford et al., 1995, Striatotheca, Burmann, 1970, and the Veryhachium lairdii group (rectangular veryhachiids) appear in the latest Tremadoc, and should not be considered as indicators of the base of the Arenig, as previously suggested.

Chinese Paleozoic acritarch research: review and perspectives

Abstract Paleozoic acritarchs from China have been reported in more than 100 articles, most of which are published in Chinese journals and in Chinese. One half of the papers concern the Ordovician, and the majority of the investigated sections are located in the provinces of southern China. This review includes the first complete bibliographical listing of all publications on Chinese Paleozoic acritarchs.

A statistical approach to classification of the Cambro–Ordovician galeate acritarch plexus

Abstract The investigation of large populations of galeate acritarchs recovered from the Late Cambrian to Early Ordovician of the Algerian Sahara allows the definition of 11 morphological criteria which may be useful for the differentiation of individual morphotypes. These parameters have been used for statistical analyses to understand better the classification of this important acritarch plexus. Following a critical evaluation of all parameters, five of them can be retained for multivariate and cluster analyses. The current taxonomic model, with a differentiation into the four genera Caldariola, Cymatiogalea, Priscogalea and Stelliferidium, cannot be maintained. The most important variables appear to be the process length and the presence/absence of ramifications of the distal end of the processes. A provisional four cluster model is proposed to classify the galeate acritarchs from the Algerian assemblages. This study is a first step in the process of investigating the potential use of multivariate statistics in galeate acritarch classification and may serve as a model for future studies to understand acritarch taxonomy.

Ordovician palynology: balance and future prospects at the beginning of the third millennium

Ordovician palynologic studies started in the 1930s when Eisenack first described Palaeozoic hystrichospheres (later named acritarchs), and defined the chitinozoans and melanosclerites. During the ensuing two decades, Ordovician palynologic investigations were mostly descriptive. It was the rise of the oil industry in the 1950s and 1960s, which accelerated palynologic research, particularly with the recognition that acritarchs and chitinozoans were biostratigraphically important groups for Ordovician stratigraphy. Today, more than 700 publications deal with Ordovician acritarchs, and about 400 papers concern Ordovician chitinozoans. In addition to these two palynomorph groups, other less important organic-walled microorganisms have been studied. These include plant remains (spores, cuticles), scolecodonts and such enigmatic groups as the melanosclerites and the mazuelloids. This paper summarises the research on Ordovician palynomorphs during the 20th century and looks ahead to the types of research that may be important and most fruitful for Ordovician palynology at the beginning of the new millenium. Particular attention is paid to the C.I.M.P./I.G.C.P. no. 410 joint meeting Ordovician Palynology and Palaeobotany, held in Prague during the 8th International Symposium on the Ordovician System. A brief account is given concerning the global Ordovician chronostratigraphy and the correlation of the main regional series and stages.

The acritarchs of the South Chinese Azygograptus suecicus graptolite Biozone and their bearing on the definition of the Lower–Middle Ordovician boundary

Abstract In southern China, well preserved acritarch assemblages have been recovered from numerous sections in the Yangtze Platform and the Jiangshan–Changshan–Yushan (JCY) area crossing the interval where the Lower–Middle Ordovician boundary should be defined, i.e., roughly at the base of the Chinese Azygograptus suecicus graptolite biozone. The acritarch taxa Aureotesta clathrata and Arbusculidium filamentosum first appear below the suecicus zone, while the genus Ampullula and the species Barakella felix have their First Appearance Datum (FAD) in the suecicus zone. These latter taxa, that are also present in Baltica and peri-Gondwana, respectively, thus probably indicate the base of the Middle Ordovician. To cite this article: J.xa0Li et al., C.xa0R. Palevol 1 (2002) 75–81.