review article The origin and early evolution of plants on land Paul Kenrick Peter R Crane The origin and early evolution of land plants in the mid Palaeozoic era between about 480 and 360 million years ago was an important event in the history of life with far reaching consequences for the evolution of terrestrial organisms and global environments A recent surge of interest catalysed by palaeobotanical discoveries and advances in the systematics of living plants provides a revised perspective on the evolution of early land plants and suggests new directions for future research The origin and early diversification of land plants marks an interval of unparalleled innovation in the history of plant life From a simple plant body consisting of only a few cells land plants liverworts hornworts mosses and vascular plants evolved an elaborate twophase life cycle and an extraordinary array of complex organs and tissue systems Specialized sexual organs gametangia stems with an intricate fluid transport mechanism vascular tissue structural tissues such as wood epidermal structures for respiratory gas exchange stomates leaves and roots of various kinds diverse spore bearing organs sporangia seeds and the tree habit had all evolved by the end of the Devonian period These and other innovations led to the initial assembly of plant dominated terrestrial ecosystems and had a great effect on the global environment Early ideas on the origin of land plants were based on living groups but since the discovery of exceptionally well preserved fossil plants in the Early Devonian Rhynie Chert research has focused almost exclusively on the fossil record of vascular plants1 2 During the 1970s syntheses of palaeobotanical and stratigraphic data emphasized the Late Silurian and Devonian periods as the critical interval during which the initial diversification of vascular plants occurred1 2 and identified a group of simple fossils rhyniophytes such as Cooksonia and Rhynia as the likely ancestral forms2 They also supported earlier hypotheses of two main lines of evolution one comprising clubmosses Fig 1f and extinct relatives the other including all other living vascular plants ferns horsetails and seed plants Fig 1g j and related fossils1 2 During the past two decades the discovery of fossil spores from as far back as the mid Ordovician period3 improved knowledge of living green algae4 5 renewed interest in the phylogenetic position of other relevant groups such as mosses and liverworts5 and advances in molecular systematics5 14 together with unexpected new data on the structure and biology of Silurian and Devonian fossils15 25 have provided a broader perspective on the origin of a land flora26 These new data indicate that the early diversification of land plants substantially pre dates the Late Silurian to Early Devonian and suggest that the main basal lineages originated over a period of more than 100 million years Myr Patterns in the early fossil record Evidence on the origin and diversification of land plants has come mainly from dispersed spores and megafossils Gray recognized three new plant based epochs Eoembryophytic Eotracheophytic and Eutracheophytic spanning the origin and early establishment of land plants each is characterized by the relative abundance of spore types and megafossils3 This synthesis highlights diversification and floral change in the Ordovician and Silurian3 27 28 and emphasizes a major discrepancy between evidence from spores and megafossils unequivocal land plant megafossils are first recognized in the fossil record roughly 50 Myr after the appearance of land plant spores NATURE VOL 389 4 SEPTEMBER 1997 Eoembryophytic mid Ordovician early Llanvirn 476 Myr to Early Silurian late Llandovery 432 Myr 3 Spore tetrads comprising four membrane bound spores Fig 2d appear over a broad geographic area in the mid Ordovician and provide the first good evidence of land plants3 26 29 The combination of a decay resistant wall implying the presence of sporopollenin and tetrahedral configuration implying haploid meiotic products is diagnostic of land plants The precise relationships of the spore producers within land plants are controversial but evidence of tetrads and other spore types such as dyads in Late Silurian and Devonian megafossils16 30 as well as data on spore wall ultrastructure25 and the structure of fossil cuticles31 support previous suggestions of a land flora of liverwort like plants Fig 1c 3 Some early spores and cuticles may also represent extinct transitional lineages between charophycean algae Fig 1a b and liverworts Box 1 but precise understanding of their affinities is hindered by the dearth of associated megafossils Eotracheophytic Early Silurian latest Llandovery 432 Myr to Early Devonian mid Lochkovian 402 Myr 3 The early Silurian latest Llandovery marks the beginning of a decline in diversity of tetrads and a rise to dominance of individually dispersed simple spores which are found in several basal land plant groups such as hornworts some mosses and early vascular plants 3 Although tetrads remain dominant in some Early Devonian localities from northwestern Europe32 the elaboration of simple spores and turnover of spore species 3 provide evidence of increasing land plant diversity and vegetational change Although spores have been observed in Silurian megafossils the affinities of most dispersed forms remain unknown indicating that substantial land plant diversity is currently undetected in the megafossil record30 The earliest unequivocal land plant megafossils are from the midSilurian of northern Europe33 and lowermost Upper Silurian of Bolivia34 and Australia35 and the uppermost Silurian of northwestern China36 Early assemblages include clubmosses such as Baragwanathia and related early fossils such as zosterophylls some species of Cooksonia and various other plants of uncertain affinity such as Salopella and Hedeia Fig 3 These data document an influx into land plant communities of diverse but generally small usually less than 10 cm tall organisms related to vascular plants Fig 3 Exceptions to the generally small size include the clubmoss Baragwanathia37 and the large and much branched Pinnatiramosus from the early Silurian of China38 The habit and branching of Pinnatiramosus is similar to that of green algae in the Caulerpales but the presence of tracheid like tubes is inconsistent with this interpretation39 Additional details including conclusive data on
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