Origin and Radiation of the Earliest Vascular Land Plants
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BREVIA although rare specimens have been recorded in Hir- nantian deposits from Turkey (northern Gondwana). Origin and Radiation of the Earliest Ornamented hilate spores and trilete spores have not been reported from strata older than Wenlock. Trilete spores are generally considered to derive Vascular Land Plants from the vascular plant lineage (7) because extant and fossil basal vascular plant groups (nonseed plants) Philippe Steemans,1* Alain Le Hérissé,2 John Melvin,3 Merrell A. Miller,3 Florentin Paris,4 nearly all produce dissociated single spores of trilete Jacques Verniers,5 Charles H. Wellman6*† or the more derived monolete form. A small pro- he earliest land plants (embryophytes) persed in tetrad or dyad configurations (cryptospores). portion of extant bryophytes produce trilete spores, evolved from charophycean green algal an- Dissociation of these units into individual hilate but in these cases the trilete condition is likely non- Tcestors and probably possessed bryophyte- spores (from dyads) or trilete spores (from tetrads) is a functional or convergent (3, 7). Also, the earliest like anatomy and physiology (1–3). Plant megafossils more derived condition. Cryptospores were the most reported vascular plant megafossils are more-or-less are rare, presumably because they lacked fossilizable abundant plant spores recovered from the Late Ordo- coincident with the appearance and diversification tissues, although there is a large micro- of hilate/trilete spores. Thus, hilate/trilete fossil record of dispersed spores, termed spores from the Late Ordovician may cryptospores because they occur in un- represent an earlier emergence and di- usual configurations such as dyads and versification of the vascular plant lineage tetrads (3–6). The oldest uncontroversial in Gondwana. Hilate/trilete spores found cryptospore record is Mid-Ordovician elsewhere suggests that vascular plants (Darriwilian) in age (Fig. 1A). Similar may have subsequently migrated out of cryptospore assemblages have been Gondwana and colonized other conti- reported throughout the globe, suggest- nents where they secondarily diversified. ing that the earliest terrestrial vegetation References and Notes consisted of ecological generalists and 1. D. Edwards, C. H. Wellman, in Plants Invade the on June 26, 2009 was cosmopolitan. The cryptospore Land,P.G.Gensel,D.Edwards,Eds.(Columbia record shows little spatial or temporal Univ. Press, New York, 2001), chap. 2. variation over about 30 million years. 2. C. H. Wellman, P. L. Osterloff, U. Mohiuddin, Nature 425, 282 (2003). Vascular plants are considered to have 3. J. Gray, Philos. Trans. R. Soc. London Ser. originated and adaptively radiated from B 309, 167 (1985). the Early Silurian on the basis of major 4. J. B. Richardson, in Palynology: Principles changes in spore assemblages (from and Applications,J.Jansonius,D.C. McGregor, Eds. (AASP–The Palynological cryptospore to hilate/trilete spore dom- Society, Dallas, TX, 1996), vol. 2, chap. 18A. inated) and the appearance of vascu- 5. P. Steemans, Rev. Palaeobot. Palynol. lar plant megafossils. 113, 189 (2000). www.sciencemag.org 6. P. K. Strother, in Phanerozoic Terrestrial We examined palynological prep- Ecosystems, R. A. Gastaldo, W. A. arations from 37 core samples from DiMichele, Eds. (Palaeontological Society Ordovician-Silurian strata penetrated Papers No. 6, Pittsburgh, PA, 2000), pp. 3–20. in the Qusaiba-1 corehole, northern 7. Materials and methods, age dating, spore 7 descriptions, and discussion of trilete Saudi Arabia ( ). They yielded abun- spore affinities are available as supporting dant, well-preserved, and thermally material on Science Online. immature marine palynomorphs (chi- 8. P.S. is a National Fund for Scientific Research Downloaded from tinozoans and acritarchs) that were Research Associate. Saudi Aramco provided used to determine the age of the cores material for study and publication. Work was 7 undertaken as part of the Commission ( ). Subsidiary land plant spores were Fig. 1. (A) Ordovician to Lower Devonian time scale. Black bar represents the Internationale de Microflora du Paléozoïque/ present in all samples. The upper part studied interval. Stratigraphic position of 1, the oldest cryptospores; 2, the oldest Saudi Aramco joint project “Understanding ” of the Qasim Formation is a shallow- mesofossils of sporangia containing cryptospores; 3, the previous oldest laevigate the Palaeozoic palynology of Saudi Arabia. ingupwardsuccessionofoffshoreshelf trilete spores from Turkey; 4, the previous oldest ornamented trilete spores; 5, the Supporting Online Material sandy silt and mudstones topped by a oldest megafossils of the vascular plant lineage; and 6, the oldest vascular plant www.sciencemag.org/cgi/content/full/324/5925/ 353/DC1 sandstone. The oldest samples from megafossils with preserved conducting tissues. (B to I) Fossil trilete spores from Materials and Methods this unit are no younger than Mid- the Qusaiba-1 corehole. Descriptions of spores provided in (7). Scale bars SOM Text Katian in age, and succeeding samples indicate 10 mm. References from the uppermost Qasim Formation 11 December 2008; accepted 17 February 2009 are Late Katian in age (Fig. 1A) on the basis of vician sediments of the Qusaiba-1 corehole and are 10.1126/science.1169659 chitinozoan and acritarch biostratigraphy. The Qasim similar to forms in other coeval assemblages found 1 Formation was unconformably overlain by glacial- worldwide. Surprisingly, all samples also contained Palaeobotany, B-18, University of Liège, 4000 Liège, Belgium. 2Université de Brest, UMR 6538 du CNRS, Institut Universitaire marine sediments of the Sarah Formation. These plant spores naturally dissociated from polyads: one Européen de la Mer, 6 Avenue le Gorgeu, 29238 Brest-cedex 3, sediments are characterized by Hirnantian acritarch species of hilate spore and seven species of trilete France. 3Saudi Aramco, Dhahran, 31311, Saudi Arabia. 4Université species with reworked earlier Ordovician forms. The spores (Fig. 1, B to I). Two trilete spore species were de Rennes 1, UMR 6118 du CNRS, 35042 Rennes-cedex, France. 5 Qalibah Formation (Qusaiba Member) overlies the unornamented, whereas the other five had an orna- Research Unit Palaeontology, Ghent University, Krijgslaan 281/S8, B-9000 Ghent, Belgium. 6Department of Animal and Plant Sarah Formation and consists of marine shelf deposits ment of verrucae and/or muri, indicating high species Sciences, University of Sheffield, Sheffield S10 2TN, UK. of Early Silurian age. diversity. Neither hilate spores nor trilete spores have *These authors contributed equally to this work. The ancestral condition among embryophytes been reported from strata of this age (3–6). The oldest †To whom correspondence should be addressed. E-mail: may have been meiotically produced spores dis- unornamented hilate spores are from the Llandovery, [email protected] www.sciencemag.org SCIENCE VOL 324 17 APRIL 2009 353.