Showing papers by "Philippe Gerrienne published in 2010"

Early Middle Ordovician evidence for land plants in Argentina (eastern Gondwana)

Abstract: Summary •The advent of embryophytes (land plants) is among the most important evolutionary breakthroughs in Earth history. It irreversibly changed climates and biogeochemical processes on a global scale; it allowed all eukaryotic terrestrial life to evolve and to invade nearly all continental environments. Before this work, the earliest unequivocal embryophyte traces were late Darriwilian (late Middle Ordovician; c. 463–461 million yr ago (Ma)) cryptospores from Saudi Arabia and from the Czech Republic (western Gondwana). •Here, we processed Dapingian (early Middle Ordovician, c. 473–471 Ma) palynological samples from Argentina (eastern Gondwana). •We discovered a diverse cryptospore assemblage, including naked and envelope-enclosed monads and tetrads, representing five genera. •Our discovery reinforces the earlier suggestion that embryophytes first evolved in Gondwana. It indicates that the terrestrialization of plants might have begun in the eastern part of Gondwana. The diversity of the Dapingian assemblage implies an earlier, Early Ordovician or even Cambrian, origin of embryophytes. Dapingian to Aeronian (Early Silurian) cryptospore assemblages are similar, suggesting that the rate of embryophyte evolution was extremely slow during the first c. 35–45 million yr of their diversification. The Argentinean cryptospores predate other cryptospore occurrences by c. 8–12 million yr, and are currently the earliest evidence of plants on land.

A New Definition and a Lectotypification of the Genus Cooksonia Lang 1937

Abstract: The genus Cooksonia Lang 1937 includes some of the earliest land plants. Specimens of Cooksonia pertoni Lang 1937 are considered the earliest Eutracheophytes. The definition of the genus is thus central to the delineation of the clade. However, the generic diagnosis is problematic. It is not restrictive enough, and most of the few diagnostic characters are plesiomorphic. Observations on new specimens of Cooksonia paranensis Gerrienne et al. 2001, a species very close to C. pertoni, considered along with a compilation of the Cooksonia literature, allow us to propose more precise diagnostic characters. An allometric study was performed on more than 100 specimens of C. paranensis. This study allows discrimination of true morphological variations from growth stages. The growth habit of Cooksonia is discussed. An emended diagnosis including apomorphic characters is given for the genus, as well as a lectotypification of the genus and the type‐species.

Early seed plant radiation: an ecological hypothesis

Abstract: Abstract The earliest steps of seed plant evolution have been extensively studied during the past 25 years. There is a growing body of evidence indicating that the first major spermatophyte radiation occurred during the Late Devonian. At least fourteen Late Devonian species are now recognized, and our knowledge of the diversity of those early seed plants has dramatically increased. Five morphotypes of seeds have been defined, mostly based on cupule morphology and on the number and degree of fusion of the integumentary lobes. In this paper, we critically discuss the abundant environmental information in order to characterize the environment in which this radiation occurred. Sedimentological information indicates that seed plants evolved in disturbed environments. It is suggested that early seed plants thrived in the shade of the dominating Archaeopteris, and that their evolution was canalized by this strong biotic pressure. We also confirm the previous suggestion that the variability of seed morphotypes can be explained by the weak abiotic selective pressure that existed in the Archaeopteris understory.

Palaeogeographic and palaeoclimatic considerations based on Ordovician to Lochkovian vegetation

Abstract: This paper summarizes research on Ordovician to Lochkovian vegetation development in a palaeogeographic framework. A terrestrialization model is described. Palaeogeographic maps are modified to explain the vegetation migrations. Land plants first evolved on the Gondwana plate and colonized Avalonia and later Laurentia and Baltica when those plates were in close proximity (Ashgill–Llandovery). South America was colonized during the Llandovery following the collapse of the ice sheet centred on the southern pole which had previously been an impassable barrier for miospores. An Aeronian–Telychian event related to the global early Silurian transgression modified the vegetation by destroying biotopes where the earliest vegetation was thriving. During the regression, trilete spore-producing plants appear to have been more able to respond to environmental changes and dominated the vegetation from the Homerian. Cryptosporeproducing plants could survive under a wide range of climates, which helped them to survive during the Hirnantian glaciation. On the contrary, the earliest trilete spore-producing plants were probably climatically restricted, as suggested by latitudinal variation in assemblage composition. By the end of the Silurian, there were several phytogeographic units. Information on the earliest vegetation favours palaeogeographic reconstructions where plates are in close proximity. The oldest miospores believed to have been produced by embryophytes (Steemans 1999, 2000; Steemans & Wellman 2004) are Ordovician in age, roughly 465 Ma (Le Herisse et al. 2007; Strother et al. 1996). They have been found in two localities from Saudi Arabia, in the Hanadir Member (Qasim Formation) of Llanvirn age. The assemblage contains monad, dyad and tetrad cryptospores, naked or enclosed within a laevigate membrane. Most of the cryptospore morphologies known in younger Ordovician strata are already present. Despite the absence of a detailed systematic description, around eleven taxa can be identified on the basis of the illustrations. Several tetrads and monads have also been reported from the Sarka Formation of Llanvirn age in the Czech Republic (Vavrdova 1984). A rich assemblage of cryptospores has been described from the Caradoc type area in the Welsh Borderland of south Britain (fig. 1 in Richardson 1988; Wellman 1996). The assemblage contains 13 genera and 17 species. A new assemblage, currently under investigation, has been observed in Saudi Arabia. Its age is still imprecise, however, and is thought to be either Caradoc or earliest Ashgill. Its cryptospore composition is similar to the UK assemblage. Reports of cryptospore assemblages from Ashgillian layers are numerous. On the Gondwana plate they have been described from Libya (Richardson 1988), Chad (Le Herisse et al. 2004), Turkey (Steemans et al. 1996), Czech Republic (Vavrdova 1982, 1984, 1988, 1989) and Saudi Arabia (work in progress). An assemblage has also been published from a Chinese locality (Wang et al. 1997). On the Avalonia plate, cryptospores have been found in Belgium (Steemans 2001) and in south Wales (Burgess 1991). To our knowledge, no Ordovician diversified cryptospore assemblages have been published on the Laurentia and Baltica plates. All Ashgillian assemblages are very similar in composition: tetrads and dyads dominate; monads are not frequent; species enclosed within a membrane are abundant; spores of dyads and tetrads are usually tightly associated together. An important step in the evolution of vegetation is the inception of the first trilete spores. Rare Ambitisporites specimens have been found in the Ashgill (Hirnantian) from Turkey (Steemans et al. 1996). Trilete spores are also observed from Saudi Arabia in the latest Caradoc or earliest Ashgill up to the Ordovician–Silurian boundary (Steemans et al. 2009). Surprisingly, specimens of ornamented Synorisporites trilete spores are present. The earliest From: Vecoli, M., Clement, G. & Meyer-Berthaud, B. (eds) The Terrestrialization Process: Modelling Complex Interactions at the Biosphere–Geosphere Interface. Geological Society, London, Special Publications, 339, 49–58. DOI: 10.1144/SP339.5 0305-8719/10/$15.00 # The Geological Society of London 2010. ornamented trilete spores previously reported were from the Homerian (Burgess & Richardson 1991). The Saudi Arabian material is currently under investigation. Rhuddanian assemblages are numerous on the Gondwana plate (North Africa, Saudi Arabia, South America), and on the Euramerican plate (USA and UK). Assemblages are similar to those from the Ashgill. During the late Aeronian there is an important decrease in cryptospore biodiversity. It continues during the Telychian where the first patinate trilete spores (Archaeozonotriletes spp.) occur (Richardson 1988; Wellman et al. 2000a; Steemans & Pereira 2002; Mendlowicz Mauller et al. 2004a). Data on the Wenlock are scarce, especially concerning the Sheinwoodian. They are mainly from the UK (e.g. Richardson & Lister 1969; Burgess & Richardson 1991; Wellman & Richardson 1993) and Libya (Richardson & Ioannides 1973). Biodiversity is still very low and assemblages are similar to those from the Telychian. Numerous inceptions of new trilete spores and cryptospores occur during the Homerian. Records exist from the UK (e.g. Richardson & Lister 1969; Burgess & Richardson 1991; Wellman & Richardson 1993; Burgess & Richardson 1995), from the Czech Republic (Dufka 1995) and from Libya (Richardson & Ioannides 1973), and so on. For the fist time, the biodiversity of trilete spores is greater than that of cryptospores. In the Ludlow, Pridoli and younger stratigraphic levels, data become more numerous. Trilete spores continue to diversify, and are generally more abundant and diverse than cryptospores. Data are mainly from the UK (e.g. Richardson & Lister 1969; Wellman 1993; Wellman & Richardson 1993; Burgess & Richardson 1995), from Libya (e.g. Richardson & Ioannides 1973; Rubinstein & Steemans 2002), from Turkey (Steemans et al. 1996), from Spain (Richardson et al. 2001), from Brazil (Steemans et al. 2008a) and so on. Clearly, trilete spores continued to rapidly diversify and dominate miospore assemblages. Finally, during the Lochkovian, data are very abundant and will not be exhaustively listed here. Miospore biodiversity evolution A detailed analysis of the literature enables construction of a biodiversity curve from the Ordovician up to the Lochkovian (Steemans 1999, 2000; Steemans & Wellman 2004). Recently, a similar curve compiled by Strother has been published in Traverse (2007). This is generally consistent with the previous curves, although there are some small discrepancies that are probably a result of Strother’s wider cryptospore concept which includes all continental palynomorphs, that is, continental algae (Strother & Beck 2000). The three main events of miospore evolution will be discussed in the following sections: the Hirnantian glaciation, the Aeronian-Telychian event and the early Lochkovian. Hirnantian glaciation The Hirnantian glaciation is known to have been responsible for the strong decrease in biodiversity of most fossil groups (Webby et al. 2004). Ashgillian to lower Aeronian miospore assemblages are very similar in all localities, showing that the glaciation had little effect on miospore assemblages. This appears to be the case whatever the climatic conditions were, for example, close to the southern pole in Paraguay or Brazil (Le Herisse et al. 2001a; Steemans & Pereira 2002; Mendlowicz Mauller et al. 2004a, b) or in temperate regions on the Avalonia plate (Wellman 1996; Steemans 2001). No decrease in miospore biodiversity has been observed after the Hirnantian glaciation (Steemans & Wellman 2004): pre and post-glaciation assemblages are very similar. This has been explained by the ability of the earliest cryptosporeproducing land plants to grow and to reproduce under diverse climates (Steemans 2000; Wellman & Gray 2000). The areas where the vegetation was destroyed during the advancing glaciation were recolonized as soon as the ice retreated. Aeronian–Telychian event The miospore diversity curve shows a minimum value during the Aeronian and the Telychian (Steemans 1999) because of a high rate of Aeronian extinction and the small number of inceptions of new species around the Llandovery–Wenlock transition. The low biodiversity value could reflect the global marine sedimentological conditions after the melting of the Hirnantian glaciers. It is probably partially true that miospore numbers are underestimates as reports from marine sediments dominate this interval. However, assemblages of miospores are very different before and after this ‘event’: (1) trilete spores which were previously rare become increasingly abundant, even if diversity remains low up to the Homerian; (2) cryptospores enclosed in a membrane (e.g. Velatitetras spp, Segestrespora spp., etc.) which were very abundant became rare after the event; (3) cryptospore tetrads and dyads from the mid Ordovician up to the middle part of the Aeronian had tightly adpressed spores (in younger strata these polyads consist of loosely adherent spores); and (4) monads which were previously rare became much more abundant. This strong cryptospore biodiversity decrease which was accompanied by a major change in the vegetation could be considered as a major event in P. STEEMANS ET AL. 50

First record of Rellimia Leclercq & Bonamo (Aneurophytales) from Gondwana, with comments on the earliest lignophytes

Abstract: Abstract The lignophytes (Embryophytes that possess a bifacial cambium) evolved during the Devonian period and include seed plants. Their advent was a major event in the history of life and had a profound impact on terrestrial environments. Recent reinvestigations of a Devonian locality, Dechra Aït Abdallah in Central Morocco, led to the discovery of a rich assemblage of fossil plants and Tentaculita. This paper focuses on a single specimen of the lignophyte Rellimia Leclercq & Bonamo. Rellimia (Aneurophytales) is a monospecific genus reported from a large number of Middle Devonian localities from western Europe (Belgium, Czech Republic, Germany and Scotland) and America. Its fertile organs are highly distinctive and borne helically on branches. They consist of a basal stalk that dichotomizes once near the base, the resulting branches dividing pinnately and bearing elongated sporangia terminally on ultimate divisions. According to the late Emsian age indicated by our sample of Tentaculita, this Moroccan specimen is to date the earliest representative of both the genus and the lignophytes. If confirmed, this occurrence suggests a possible origin of the Aneurophytalean lignophytes in Gondwana and their rapid and widespread colonization in the Middle Devonian towards Laurussia.

The early land plants from the Armorican Massif: sedimentological and palynological considerations on age and environment

Abstract: The Châteaupanne Unit belongs to the South Armorican domain of the Armorican Massif (France), which is part of the Variscan belt. This unit includes two Lower Devonian plant levels and one of them corresponds to the Basal Member of the Chalonnes Formation. A sedimentological and palaeontological analysis of these fossiliferous deposits from the Châteaupanne quarry (Montjean/Loire, Maine et Loire, France) is presented here for the first time. The age determination based on palynology indicates that the locality records the earliest occurrence of plant megafossils in the Armorican Massif. Their presence suggests an emergence event that has never been described before. Our study highlights the promising potential of the Basal Member of the Chalonnes Formation to aid in understanding these occurrences, and provides new insights into the history of the Variscan belt.

Cenomanian sands and clays north of the Vesdre valley: the oldest known Cretaceous deposits in eastern Belgium

Abstract: A number of motored auger holes have been drilled in 2002 and 2006 in four sand-clay deposits preserved in dissolution pockets within the Dinantian limestones of the watershed north of the Vesdre valley. These deposits of unknown age are currently classified as (Tertiary) SBL in the new geological map of Wallonia. We present detailed lithostratigraphic logs of the deposits and describe the results of sedimentological and mineralogical analyses. In particular, K-Ar dating of neoformed Mn oxides found at the base of one augerhole at Rechain yielded ages ranging from Cenomanian to Santonian, allowing us to place the Rechain and Andrimont deposits within the early Late Cretaceous. This is fully consistent with their topographic location very close beneath the trace of the pre-Cretaceous erosion surface and makes them the westernmost remains of the Hergenrath Member of the Late Cretaceous Aachen Formation. To the west, the Magnee deposit is more "typical SBL", probably corresponding to the Late Neogene filling of a dissolution pocket by reworked weathering products of the local Cretaceous cover.

Spatio-temporal natural and anthropogenic environmental variability during the last 1500yrs in an ombrotrophic bog (East Belgium).

Abstract: Peatlands cover ca. 3 % of the Earth’s surface and provide crucial continental archives for deciphering past climatic changes and anthropogenic impacts on decadal to millennial timescales. Numerous studies have demonstrated that peat bogs are excellent archives to investigate past environmental and ecological changes during the Holocene. Studies which focus on intra-site variability at high resolution are rare however, despite their potential to provide constraints on the reliability of the palaeoenvironmental reconstruction and the influence of micro-scale variability. Such variability must be taken into account in any peatland restoration process linked with recent environmental changes, particularly human-derived impact such as peat cutting, drainage and tree cultivation. Four 1m-long Wardenaar monoliths were retrieved from the Misten bog (Hautes-Fagnes, East Belgium). The cores were investigated using chronological (radiocarbon AMS dating of plant macrofossils, 210Pb age modelling), biological (macrofossils, pollen content, testate amoebae), organic (humification level) and geochemical proxies (major and trace geochemistry, Nd and Pb isotopes). The aims of this research were to: (1) to assess whether the bog vegetation and other environmental indicators have changed simultaneously in time and space, (2) identify the most sensitive palaeoenvironmental indicator(s) and (3) assess to what extent variation in peat accumulation rates affects the record of each proxy. Preliminary interpretations show great variability (up to 50%) in peat development on a decimetre depth-scale as assessed by the variation in peat palynological and macrofossils zones from one core to another. In addition, our recent high-resolution records of environmental change have high applied palaeoecological value since they can be used to inform conservation management (‘natural’ changes in the composition of the peat forming vegetation and the range of water table depth variability over a range of timescales).

the oldest known CretaCeous deposits in eastern Belgium

Abstract: A number of motored auger holes have been drilled in 2002 and 2006 in four sand-clay deposits preserved in dissolution pockets within the Dinantian limestones of the watershed north of the Vesdre valley. These deposits of unknown age are currently classified as (Tertiary) SBL in the new geological map of Wallonia. We present detailed lithostratigraphic logs of the deposits and describe the results of sedimentological and mineralogical analyses. In particular, K-Ar dating of neoformed Mn oxides found at the base of one augerhole at Rechain yielded ages ranging from Cenomanian to Santonian, allowing us to place the Rechain and Andrimont deposits within the early Late Cretaceous. This is fully consistent with their topographic location very close beneath the trace of the pre-Cretaceous erosion surface and makes them the westernmost remains of the Hergenrath Member of the Late Cretaceous Aachen Formation. To the west, the Magnee deposit is more "typical SBL", probably corresponding to the Late Neogene filling of a dissolution pocket by reworked weathering products of the local Cretaceous cover.

Etude de spécimens de progymnospermes «type Callixylon » de Ronquières (Dévonien moyen, Belgique).

Abstract: Le terme progymnosperme est un terme inventé par Beck en 1960 pour décrire les fossiles présentant un bois de structure gymnospermique mais avec un feuillage de type Archaeopteris. Le genre Callixylon est inclus dans ces progymnospermes et est classé dans l’ordre des Archaeopteridales. Les Archaeopteridales sont notamment caractérisées par une eustèle et des bandes de ponctuations aréolées sur les parois radiales des trachéides. L’ouverture de ces ponctuations sont en fente et inclinées de 45 degrés.