Alain Le Hérissé

Origin and Radiation of the Earliest Vascular Land Plants

Colonization of the land by plants most likely occurred in a stepwise fashion starting in the Mid-Ordovician. The earliest flora of bryophyte-like plants appears to have been cosmopolitan and dominated the planet, relatively unchanged, for some 30 million years. It is represented by fossilized dispersed cryptospores and fragmentary plant remains. In the Early Silurian, cryptospore abundance and diversity diminished abruptly as trilete spores appeared, became abundant, and underwent rapid diversification. This change coincides approximately with the appearance of vascular plant megafossils and probably represents the origin and adaptive radiation of vascular plants. We have obtained a diverse trilete spore occurrence from the Late Ordovician that suggests that vascular plants originated and diversified earlier than previously hypothesized, in Gondwana, before migrating elsewhere and secondarily diversifying.

Paleoecology, biostratigraphy and biogeography of late Silurian to early Devonian acritarchs and prasinophycean phycomata in well A161, Western Libya, North Africa

Abstract Diverse and well preserved acritarch and prasinophycean phycomata assemblages were recovered from the late Silurian to Lower Devonian strata of well A161 in western Libya, and four distinct acritarch biozones are recognized, based on the stratigraphic distribution of 156 species. The palynoflora is independently dated by means of chitinozoans, and allows discussion of the evolution of acritarchs and prasinophyte phycomata across the Ludlow–Přidoli boundary in relation to probable major climatic change, as well as in the early and middle Přidoli, and the lower Lochkovian. Correlations are proposed with the British Isles, Baltica, and Algerian Sahara. Sedimentation occurred in shallow high-energy conditions throughout, but with periodic rise of sea level. The changes in marine to terrestrial palynomorph ratios through the section document the relationship between marine palynomorph assemblages and sea surface conditions in these marginal marine environments. The major drift of Gondwana towards low latitudes during the Ludlow–Přidoli transition seems to have been the driving force behind homogenization of assemblages on the two sides of the Rheic ocean, and explains the similarities between phytoplanktonic assemblages of the north Gondwanan margin and the South of Baltica. The data suggest that the Rheic ocean was almost closed by the late Silurian, and had become restricted to a moderately deep sea. In the Lochkovian the microflora are strongly facies-dependent and delineate more restricted provinces such as the Ibarmaghian domain in the sense of Plusquellec (1987) including the Maghreb and Ibero–Armorican areas. Nine new species are described: Arkonia nova, Arkonia paulumstriata , ? Cymatiosphaera florida, Cymatiosphaera nimia, Dactylofusa hispidusa, Disparifusa quasibernesgae, Evittia areolata, Multiplicisphaeridium verticisum , and ? Villosacapsula steemansii . In addition, three new combinations are suggested: Visbysphaera bonita (Cramer) comb. nov., Visbysphaera jardinei (Cramer) comb. nov. and Visbysphaera albanega (Cramer et al.) comb. nov.

Biogeography of Late Silurian and Devonian acritarchs and prasinophytes

Abstract The palaeobiogeography of Late Silurian-Devonian acritarch and prasinophyte microfloras is assessed using qualitative data, similarity indexes and cluster analysis. Based on the recent palaeogeographic reconstructions, the northwards movement of Gondwana initiated during the Ludlovian with the progressive narrowing of the Rheic ocean, resulted in North Africa and south European regions being closer to the equatorial zone and hence in a warmer climate. An apparent homogeneity of Late Silurian assemblages from regions of southern Baltica and the North Gondwanan margin could be explained by such a configuration. The phytoplanktonic associations could also have benefited from the equatorial currents system for trans-Rheic connections. In addition, the qualitative study and the cluster analysis allow for delineation of a biogeographic unit, including part of South America, North Gondwanan and perigondwanan regions, which is comparable to the cold Malvinokaffric realm of the southern hemisphere, based on invertebrates. During the same time, major differences recognized in the distribution of acritarchs from near shorelines to deep oceans, emphasize the potential of mazuelloids (previously referred to as acritarchs) as indicators of deep-water facies and outer margin sites. The Early Devonian shows a geographic restriction of several acritarch genera and species, with a pronounced lateral differentiation between North Gondwanan and eastern North American microfloras, that seems to have been controlled by physical barriers rather than climatic differences. The similarities in acritarch assemblages between northern Europe and the North Gondwanan margin argue for a decrease in acritarch provinciality on both sides of the Rheic ocean, and precludes the presence of a wide Devonian ocean north of western Gondwana.

GLOBAL PATTERNS OF ORGANIC-WALLED PHYTOPLANKTON BIODIVERSITY DURING THE LATE SILURIAN TO EARLIEST DEVONIAN

Abstract Numerous environmental factors as well as oceanic circulation patterns and geographic constraints all contribute to the abundance, distribution, and diversity of present-day marine phytoplankton assemblages. These same factors presumably affected the Paleozoic marine phytoplankton, which was dominated by organic-walled acritarchs and prasinophytes. During the Late Silurian (Gorstian, Ludfordian, and Přídolí) and earliest Devonian (Lochkovian), important paleogeographic, paleooceanographic, and geochemical changes were occurring as well as major compositional changes and diversity fluctuations in the marine organic-walled phytoplankton. Innovative morphologies appeared during the Late Silurian, in both low and high latitude assemblages, but with significant quantitative differences. This was followed by a turnover in assemblage composition during the Silurian/Devonian transition, and an initial radiation of new acritarch and prasinophyte taxa in the Early Devonian. Observed changes in total phytoplankton diversity during the Gorstian through earliest Lochkovian are based on organic-walled microphytoplankton data derived from published and unpublished key stratigraphic sections where independent age control has been firmly established. These key sections are from: Missouri and Oklahoma, U.S.A. and western Newfoundland, Canada (Laurentia); Gotland, Sweden, and Podolia, Ukraine (Baltica); the Welsh Basin and Borderland (Avalonia); northern France and northern Spain (Armorica); and Libya in northern Africa, and Argentina and Bolivia, South America (Gondwana). Regional biodiversity changes for the organic-walled microphytoplankton were determined for the warm low latitude areas (Baltica, Laurentia, and Avalonia) and temperate to cool higher latitude areas (northern and southern Gondwana). The Late Silurian—earliest Devonian organic-walled phytoplankton was divided into three major categories to facilitate comparison of compositional fluctuations, both within stratigraphic sections as well as between geographic areas. The three categories, based on overall morphology, are marine chlorophytes and prasinophytes, marine acritarchs, and nonmarine types, including coenobial forms. This triparate grouping is both broad and detailed enough to mark critical changes in both the phytoplankton assemblages, as well as the paleoenvironment. In general, high phytoplankton diversity peaks occurred during the Early and Late Gorstian in the warm low latitude areas, followed by varying fluctuations during the Ludfordian and Přídolí for both the warm low latitude and cool high latitude areas. An initial radiation of new phytoplankton taxa and the appearance of more cosmopolitan assemblages mark the beginning of the Lochkovian.

Stratigraphic and paleogeographical significance of Silurian acritarchs from Saudi Arabia

Abundant and well-preserved acritarchs of Early Silurian and late Middle to Late Silurian age have been obtained from marine subsurface sequences in seven exploratory wells drilled by the Saudi Arabian Oil Company in central and northwestern parts of Saudi Arabia. Preliminary results on their stratigraphic distribution allow an informal zonation of 9 regional acritarch assemblages zones to be proposed. Correlations are made between the different local wells studied, and with contemporaneous assemblages described elsewhere. During the Silurian, Saudi Arabia occupied an intermediate to high latitudinal position, on the eastern part of the Gondwana margin. This paleogeographic situation may explain the mixture of taxa known from Western European (Balto-Scandinavia) and North American sequences, with species that have hitherto been recorded only from North Africa or South America in the high-latitude province. A local distinctive endemic microflora is also represented. This additional record of Silurian acritarchs from Northern Gondwana is of considerable importance concerning the biostratigraphic and paleogeographic implications. The distribution of acritarchs shows distinct geographic patterns that may be attributed to climatic factors, oceanic circulation pattern, and/or related to local environmental parameters. In the systematic part, eleven new species of acritarchs, Anomaloplaisium johnsium, Baltisphaeridium diabolicum, Buedingiisphaeridium incertum, Dactylofusa horrida, Goniosphaeridium versatile, Goniosphaeridium sp. A, Leiofusa cucumis, Multiplicisphaeridium circumscriptum, Neoveryhachium carminae constricta, Veryhachium strangulatum, Visbysphaera glabra, and one new genus and species of coenobial Algae, Kahfia arabica, are described and illustrated.

Biogeography of upper Llandovery and Wenlock acritarchs

Abstract The world-wide spatial distribution of upper Llandovery and Wenlock acritarch microfloras is assessed using qualitative data, similarity indexes and cluster analysis. The database consists of comprehensive lists of taxa available essentially from eastern North America, South America, western Europe, Baltoscandia, Siberia, North Africa and adjacent regions, and a few additional data from China. The data are plotted on the most recent palaeogeographic map. The results of qualitative and quantitative analysis clearly show that patterns of distribution of the acritarchs are under the interdependence of continental arrangement, latitudinal position, environmental conditions and oceanic currents. While climate was undoubtedly a significant factor governing the distribution of microfloras, the associations do not necessarily coincide with palaeolatitudes as predicted in Cramers provincialism model proposed for Silurian acritarchs. Thus the suggestion of a low latitude Deunffia-Domasia realm near the Llandovery-Wenlock boundary has to be modified; several new data from the North Gondwana margin indicate a relation of the D-D association with outer-shelf environments independent of latitudinal control; in the same way, Neoveryhachium carminae appears to be facies-controlled in relation to clastic facies and turbid environments; the supposed benthic cysts belonging to the Pulvinosphaeridium-Estiastra group coincide with the distribution of climatically sensitive carbonate lithofacies in the equatorial zone. These conclusions are supported by the results of quantitative analysis. In addition, the spatial distribution of other taxa is discussed, in order to provide new tools for paleogeographical reconstructions.

Microplancton a paroi organique du silurien de Gotland (suede): Observations au microscope electronique de structures de desenkystement

Abstract Different types of excystment structures of Silurian organic-walled microplankton (Acritarcha and Prasinophyceae) probably assignable to unicellular algal cysts are described, based on scanning electron microscopy studies. Several genera previously reported to open by simple rupture have shown preformed excystment mechanisms with complex ornamented borders. The excystment structure is considered as an important morphologic feature and may be used in addition to other characters for a more comprehensive classification and in order to determine the relationships of these cysts with other organisms.

THE END OF A MYTH: ARPYLORUS ANTIQUUS PALEOZOIC DINOFLAGELLATE CYST

Abstract Arpylorus antiquus, erected by Calandra in 1964, was isolated from upper Silurian sedimentary rocks from the Mechiguig 1 borehole in southern Tunisia, with other palynomorphs. The folded vesicle and the quadrangular form of the aperture break down into platelike fragments, resembling the tabulation of dinoflagellates. The presence of these elements has been used to interpret A. antiquus as a dinoflagellate cyst. The morphology and affinity of A. antiquus is reinterpreted herein based on investigation of larger sets of samples, including material from the type locality, together with material of Algeria, Saudi Arabia, and Brazil. More complete specimens than those previously described have been observed using gentle laboratory techniques, showing a large development of a fine membrane at the periphery of vesicles. This element was destroyed using classical palynological treatments, implying that the holotype is an incomplete specimen. The membrane at the periphery of vesicles and dorsoventral differentiation of these vesicles suggest that A. antiquus is a part of a more complex biological structure. We suggest a possible relationship with eurypterids, arthropods related to phyllocarids, represented by abundant fragments in the assemblages. Arpylorus antiquus is possibly a structure of storage. The chemical composition of A. antiquus using a Fourier transform infrared FTIR microspectroscopy analysis, reveals a wall composed of biopolymer that is not consistent with dinosporin. We conclude that Arpylorus antiquus is definitively not a dinoflagellate cyst. Although dinoflagellates may have older Paleozoic or even Proterozoic ancestors as the biomarker record may suggest, the dinoflagellate tabulation evolved only in the early Mesozoic.

DESCRIPTION AND MICROSCALE ANALYSIS OF SOME ENIGMATIC PALYNOMORPHS FROM THE MIDDLE DEVONIAN (GIVETIAN) OF LIBYA

Abstract Large acritarchs were recovered from the Awaynat Wanin II Formation in the A1-69 borehole, western Libya at 394.4 m. This sample has been dated by acritarchs and spores as Givetian (Middle Devonian). Two species of acanthomorph acritarchs were recovered that are unusually large for the Paleozoic. The first type, represented by a single specimen, is a new occurrence of a previously described but unnamed form from the Middle Devonian of the Sahara. The second type, which is more abundant, is named as Vanguestainidium cucurbitulum gen. et sp. nov. Biogeochemical analysis using infrared microspectroscopy shows that the wall composition of this new taxon is close to algaenans isolated from the green alga Botryococcus braunii Kützing 1849 although these forms are not morphologically similar. The presence of well-preserved fragile palynomorphs, abundant terrestrially-derived palynomorphs including large megaspores, and rare marine palynomorphs suggests deposition in a low-energy nearshore environment such as a coastal swamp.

Lochkovian (Early Devonian) acritarchs and prasinophytes from the Solimões Basin, northwestern Brazil

Eighteen core samples from PETROBRAS well 1-JD-1-AM, comprising the upper part of the Jutai Formation, Jandiatuba area, Solimoes Basin, northwestern Brazil, have been studied for marine microphytoplankton. Forty-seven taxa of acritarchs and prasinophytes have been identified. The presence of Demorhethium lappaceum, Duvernaysphaera actinota, Riculasphaera fissa, Thysanoprobolus polykion, Winwaloeusia distracta, ?Cymatiosphaera florida, and numerous Schizocystia spp., among others, suggests a Lochkovian age for the assemblage. Moreover, the occurrence of Polyedryxium fragosulum and Pterospermella chiquitina indicates a possible late Lochkovian age, in agreement with previous age determinations based on chitinozoans and miospores. The significance of Schizocystia as a biostratigraphic marker for the beginning of the Devonian in Gondwana is discussed. The Solimoes Basin assemblage shows close compositional similarity with coeval assemblages from North America, north Gondwana, and the peri-Gondwana region.