Philippe Gerrienne

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

• 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.

An Early Devonian flora, including Cooksonia from the Paraná Basin (Brazil)

We report here the presence of an extensive flora from an early Lochkovian (Lower Devonian: ∼406 million years) locality in the Parana Basin (Brazil). The exceptional macrofossil assemblage includes several hundred specimens of a new Cooksonia species, and representatives of 10 other taxa, 5 of which at least are new. This plant assemblage illustrates the amplitude of the Siluro-Devonian land plant primary radiation. During the Lochkovian, the Parana Basin was positioned in southern Gondwana, within the southern polar circle. The occurrence of this rich plant assemblage substantiates the hypotheses of a Warm Mode and of an ice-free southern pole during the earliest Devonian. Some taxa exhibit characters interpreted as potentially related to cold hardiness: embedding of the sporangium within the axis, abundance of emergences, dense branching, and protection of the apical meristem.

A Simple Type of Wood in Two Early Devonian Plants

The earliest evolution of wood occurred in plants of surprisingly small stature. The advent of wood (secondary xylem) is a major event of the Paleozoic Era, facilitating the evolution of large perennial plants. The first steps of wood evolution are unknown. We describe two small Early Devonian (407 to 397 million years ago) plants with secondary xylem including simple rays. Their wood currently represents the earliest evidence of secondary growth in plants. The small size of the plants and the presence of thick-walled cortical cells confirm that wood early evolution was driven by hydraulic constraints rather than by the necessity of mechanical support for increasing height. The plants described here are most probably precursors of lignophytes.

The plant Leclercqia (Lycopsida) in Gondwana: implications for reconstructing Middle Devonian palaeogeography

Abundant and well-preserved material of the ligulate lycopsid genus Leclercqia is reported from a new Middle Devonian locality in northeastern Queensland (Australia). The plants occur in a chert horizon in the Storm Hill Sandstone of the Dosey-Craigie Platform. Lithological data and conodont analyses combined with information from in situ spores provide an age for the plant levels ranging from Eifelian, possibly Middle Eifelian, to Early Givetian. Plant taxonomic identification is based on vegetative and fertile stems that display both external morphology and anatomy. This material represents the best documented occurrence of Leclercqia outside Laurussia and possibly the earliest in Gondwana; it provides evidence that colonization of Gondwana by the species L. complexa was contemporaneous to that of Siberia and Kazakhstan. Analysis of the distribution patterns of L. complexa suggests that it was adapted to a wide range of environments, but within certain limits which we hypothesize to be those of a climatic belt. Such considerations support previous studies using other biological data, such as faunas and palynomorphs, for reconstructing Devonian palaeogeography. They favour a close proximity of Laurussia and Gondwana rather than the occurrence of a wide ocean separating the two palaeocontinents in Middle Devonian times.

Lower and middle devonian miospore-based stratigraphy in Libya and its relation to the megafloras and faunas

Abstract A review of miospores, faunas and megafloras of the Lower and Middle Devonian in Libya allows new conclusions to be drawn. The application of the Ardenne-Rhenish Devonian miospore zonation in Libya gives the following results. In the North Hammadah Basin, the base of the Tadrart Formation might well be strongly diachronous depsite the lithostratigraphical correlations shown on the base of “Gamma-Ray/Neutron” logs. At least the lower part of the Tadrart Formation in borehole MG-1 is within the lowermost part of the Lochkovian not at its base. In the southern margin of the Hammadah Basin, the Tadrart and Ouan-Kasa formations are probably not older than late Emsian, maybe early Eifelian. The Ouenine Formation I is often absent there or strongly reduced. The isochroneity of the (discordant) lower limit of the Tadrart Formation across the Hammadah Basin is far to be demonstrated. The Caledonian age of the discordance on the southern margin of the Hammadah Basin can obviously be challenged. In the eastern Murzuk area, the plant-bearing Tadrart-Emi Magri formations have a Middle Devonian age according to the plant fossils themselves. The discordance which separates these formations from the Acacus Sandstone at Dor el Goussa has to be late Early Devonian or Middle Devonian because the Acacus Sandstone “Psilophytes” cannot be older than Pragian and can even be younger (Emsian). There is obviously no available argument which could counteract the fossil floral data and prove that the discordance in the eastern Murzuk has the same age as the Silurian/Devonian transitional beds in the northern Hammadah Basin.

A NEW DEFINITION AND A LECTOTYPIFICATION OF THE GENUS COOKSONIA LANG 1937

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.

The in situ Glyptostroboxylon forest of Hoegaarden (Belgium) at the Initial Eocene Thermal Maximum (55 Ma)

Abstract Hundreds of silicified standing stumps have been discovered within a lignitic horizon in the middle of the Tienen Formation near Hoegaarden in northeast Belgium. The anatomical features of the fossil stumps, as those of the numerous silicified secondary xylem remains collected since the last century from this area, demonstrate that they all belong to a single taxodiaceous taxon. The stumps bear characteristics shared by Taxodioxylon gypsaceum and Glyptostroboxylon tenerum, but affinities with the latter appear closer. They are attributed to Glyptostroboxylon sp. Calibration of the sedimentological, stratigraphical and organic carbon isotope data reveals that these taxodiaceous fossil trees developed in a swampy lowland environment most probably during the Initial Eocene Thermal Maximum at ca. 55 Ma.

Identification of a fossil wood specimen in the Red Sandstone Group of southwestern Tanzania: stratigraphical and tectonic implications

A piece of silicified fossil wood was found in the middle part of the Red Sandstone Group of the Songwe-Kiwira area, north of Lake Malawi (Nvasa) in the Western Branch of the East African Rift System. It is identified as Pahudioxylon Chowdhury et el., 1960. Until now, Pahudioxylon has been strictly restricted to the Cenozoic. The occurrence of Pahudioxylon in the Red Beds Formation supports a Cenozoic (Miocene) age rather than a Mesozoic age for this formation. Comparison with a collection of modern species gives information about the environmental conditions of deposition of the Red Sandstone Group. A discussion follows on the significance of the Red Sandstone Group during the Rukwa-Malawi Rift development.

The fossil plants from the Lower Devonian of Marchin (northern margin of Dinant Synclinorium, Belgium). V. Psilophyton genseliae sp. nov., with hypotheses on the origin of Trimerophytina

Abstract Psilophyton genseliae Gerrienne, sp. nov. is described from the Lower Devonian locality of Marchin (northern margin of the Dinant Synclinorium, Belgium). The age of the locality based on spores is early Emsian (AB Oppel Zone). The new taxon has smooth axes showing anisotomous branching resulting in lateral fertile units. These units consist of dense clusters of fusiform erect sporangia borne in pairs terminally on 4–5 times isotomously divided axes. Neither in situ spores nor anatomical data have been found. The technique of serial degagement was necessary to demonstrate that the plant belongs to the genus Psilophyton Dawson. A constant sinistral torsion of the fertile units, both the sporangial pairs and the sporangia themselves, is shown in the new taxon. Attention is focused on the presence of a similar sinistral torsion of sporangia in many other Trimerophytina, as well as in several other early land plants such as Tortilicaulis Edwards (rhyniophytoid sensu Edwards and Edwards, 1986). Since most Rhyniophytina have been proven to have conducting cells and sporangial organisation fundamentally different from those of the Trimerophytina (Shute and Edwards, 1989, for the sporangia of Uskiella ; Kenrick et al., 1991; Kenrick and Crane, 1991), the former can no longer be considered precursors of the latter. Several characters of Tortilicaulis and Psilophyton (morphology of the spores; position, shape and sinistral torsion of their sporangia) as well as hypotheses about their respective modes of dehiscence suggest the existence of a link between rhyniophytoids such as Tortilicaulis , and the Trimerophytina. This hypothesis is consistent with the stratigraphic range of those taxa.

Early Devonian plants remains from Marchin (North of Dinant Synclinorium, Belgium). I: Zosterophyllum deciduum sp. nov.

Abstract Zosteropyllum deciduum sp. nov. is described from the Lower Devonian (Emsian) of southern Belgium; the species diagnosis is based on a large number of specimens. Z. deciduum is characterized by its small size and the shape of its sporangia and stalks. It is the first Zosterophyllum (subgenus Zosterophyllum) to show bifurcation within the fertile region. Another distinctive feature is that the sporangia appear to be deciduous; often, on fertile axes, only the sporangial stalks remain. The plant is homosporous, the spores being assigned to the dispersed spores species Retusotriletes goensis. The plant has K and H-branchingl; some axes are interpreted as rhizomatous. K and H-branching is discussed along with characters useful in the determination of species within the genus Zosterophyllum.