Philippe Steemans

Spore stratigraphy and correlation with faunas and floras in the type marine devonian of the Ardenne-Rhenish regions

A spore zonation scheme comprising fifty-one zones is proposed for the marine Devonian strata of the Ardenne-Rhenish regions of Western Europe. The zonation comprises a series of Oppel and interval-type zones and these are closely intercalibrated with the associated marine faunal zonations to give a seventy-five level scale of correlation for the Devonian succession. The spore zonation provides stratigraphical dating of the Devonian megafloras of the region, particularly those from the Lower and Middle Devonian. The proposed spore zonation is closely compared with that erected for the Devonian of the Old Red Sandstone Continent.

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.

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.

Ordovician and Silurian cryptospores and miospores from southeastern Turkey

Ordovician and Silurian samples from southeastern Turkey have yielded a large number of well-preserved palynomorphs including cryptospores and miospores, which are described here. Two new species are described (Segestrespora burgessii Steemans, Le Herisse et Bozdogan sp. nov. and Velatitetras anatoliensis Steemans, le Herisse et Bozdogan sp. nov.), two new combinations are proposed (Velatitetras retimembrana Steemans, le Herisse et Bozdogan comb. nov. and Velatitetras rugosa Steemans, le Herisse et Bozdogan comb. nov.), and two genera are emended (Dicryptosporites and Segestrespora). The morphon concept is used for the species with very close morphologies (Van der Zwan, 1979). Using these sporomorphs, four Oppel zones are erected and compared with other similar studies. Ordovician samples contain biostratigraphically useful chitinozoans and acritarchs which precisely correlate the level of appearance of the oldest trilete miospore Morphon Ambitisporites avitus—dilutus with middle to upper Ashgillian.

Miospore evolution from the Ordovician to the Silurian

The definition of the term cryptospore is amended to include only spores thought to be produced by embryophytes and to exclude all enigmatic palynomorphs. Cryptospores are included here in the miospore group.The oldest published assemblage of typical cryptospores is Llanvirn in age. The evolution of cryptospore assemblages is very slow from the Llanvirn to the early Llandovery. The Telychian and Sheinwoodian stages are periods of drastic impoverishment in cryptospore biodiversity, followed by an important modification in miospore assemblages. This step in the evolution of the vegetation is correlated with the rapid transgression of seas on the continent during the early Llandovery. On the other hand, the Hirnantian glaciation does not affect diversity of the miospore assemblages.

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.

Miospore assemblages from the Silurian–Devonian boundary, in borehole A1-61, Ghadamis Basin, Libya

Abstract Well preserved and diversified miospore assemblages have been recorded from a relatively continuous sequence in borehole A1-61 which spans the Silurian–Devonian boundary in the northwestern part of the Ghadamis Basin, Libya. The sequence is represented by early Devonian Lochkovian beds of the Tadrart Formation that transgress onto the Silurian Ludlow-Pridoli beds of the upper part of the ‘Alternances Argilo-greseuses’ Formation. The present work demonstrates a succession of miospore assemblages from closely sampled layers that have been stratigraphically dated as Ludlow–middle Pridoli and early Lochkovian by chitinozoans and acritarchs. Over 80 species of cryptospores and trilete spores have been identified. Modified detailed morphological interpretations are given. The miospore assemblages are correlated with miospore zonation schemes established for the type sequences of the Welsh Borderland, and those previously described from Libya. Early occurrences of some species as Streelispora newportensis on the western Gondwana plate, are put forward by comparison with the Old Red Sandstone continent. Phytogeographic and palaeobotanic implications based on these observations are discussed.

Silurian and Lower Emsian spores in Saudi Arabia

Silurian cryptospores and Silurian to Lower Devonian miospores are identified in cuttings from three wells situated in Saudi Arabia between longitudes 42 and 51°E and latitudes 23 and 29°N. All samples are contaminated by caved younger spores and pollen. The biostratigraphic scheme defined in western Europe is tentatively applied. The identical succession of first occurences of the key-species approximately dates the levels sampled. A discordance is demonstrated between the Silurian and the Lower Devonian deposits in each examined well. The ages of the first Lower Devonian sediments in these wells suggest a transgressive direction from SE to NW. Two new taxa are formally introduced: Cymbosporites dammanensis Steemans, sp. nov. and Chelinospora arabiensis Steemans, sp. nov.

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.