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Pb-682 Kj Singh1 The Palaeobotanist 57(2008) : 69-88 0031-0174/2008 $2.00 Early land plant developments: Global progress and Indian priorities KAMAL JEET SINGH Birbal Sahni Institute of Palaeobotany, 53 University Road, Lucknow 226 007, India. Email: [email protected] (Received 06 December, 2007; revised version accepted 21 May, 2008) ABSTRACT Singh KJ 2008. Early land plant developments: Global progress and Indian priorities. The Palaeobotanist 57(1-2) : 69-88. This review paper is primarily aimed to furnish a general account on the origin, evolution, radiation and the proliferation of early land plants on the earth. The story of the emergence of the plant life on the land and its subsequent development during Silurian and Devonian times has been narrated as per Edwards and Selden’s (1993) theory of gradual colonization, which shows that the land plants established themselves involving four stages of continuous growth and the diversification. The paper demonstrates how the simplest land forms (Cooksonia, rhyniophytes) in the middle Silurian have given risen to much evolved progymnosperms (Archaeopteris) during the late Devonian within a span of 48 million years of time (from 423-375 ma). An account of the records of land plants from their earliest occurrences in the Ordovician to the end of Lochkovian in the Lower Devonian has been provided to show the current status of such studies on the global platform. The palaeo-phytogeography concerning spores and megafossils from Llanvirn (middle Ordovician) to Lochkovian (Lower Devonian) has also been depicted on the base maps of the globe. Key-words—Early land plants, Ordovician, Silurian, Devonian, Cryptospores, Cooksonia, Rhyniophytes, Zosterophylls, Trimerophytes. izkjafHkd LFkyh; ikni fodkl% oSf’od izxfr ,oa Hkkjrh; izkFkfedrk,a deythr flag lkjka'k ;g leh{kkRed 'kks/k&i= eq[;r;k i`Foh ij izkjafHkd LFkyh; ikniksa ds mn~xe] fodkl] fofdj.k ,o Øe izlj.k dk O;kid fooj.k nsus ds mn~ns’; ls gSA tehau ij ikni thou ds vkfoHkkZo dk fooj.k rFkk blds mÙkjorhZ ,MoMZ ,oa lsYMe (1993) ds Øfed mifuos’kui ds fln~/kkarkuqlkj] flY;wfj;u ,oa fMoksuh dky ds nkSjku fodkl of.kZr gS tks n’kkZrk gS fd LFkyh; ikniksa us Lor% dks fujarj o`n~f/k dh pkj izkoLFkkvksa vkSj fofo/k:i.k esa lfEefyr djds LFkkiuk dhA 'kks/k&i= n’kkZrk gS fd e/; esa ljyre LFkyh; ikni ¼dqdlsfu;k]j;fu;ksQk;V~l½ esa fdl izdkj vafre fMoksuh ds nkSjku 48 fefy;u o"kksZa ds le; ¼42.3 fefy;u o"kksZa ls 37.5 djksM+ o"kZ½ ds varjky esa flY;wfj;u ds vfr fodflr izkd vuko`rchth ¼vkfdZ;ksIVsfjl½ dks mfRFkr fd;k gSA oSf’od iVy ij bl rjg ds v/;;uksa dh orZeku n’kk dks n’kkZus gsrq fuEu fMoksuh esa vkWMksZfo’ku ls ykWddksfo;u ds var rd muds izkphure izkfIr;ksa ls Hkwfe ikS/ kksa ds vfHkys[kksa dk o.kZu miyC/k djok;k x;k gSA xksyd ds vk/kkj ekufp= ij ykuouZ ¼e/; vkWMksZfo’ku½ ls ykWddksfo;u ¼fuEu fMoksuh½ rd chtk.kqvksa ,oa LFkwy thok’eksa ls lacaf/kr iqjk&ikniHkwxksy Hkh fpf=r fd;k x;k gSA laDsÿr&'kCn—izkjafHkd LFkyh; ikni] vkWMksZfo’ku] flY;wfj;u] fMoksuh] xw<+chtk.kq] dqdlksfu;k j~;fu;ksQk;V~l] t+ksLVsjksQk;V~l vkSj VªkbeSjksQk;V~lA Challenges in Indian Palaeobiology: Current Status, Recent Development and Future Directions © Birbal Sahni Institute of Palaeobotany, India Editors: N.C. Mehrotra & Mukund Sharma Publisher: Birbal Sahni Institute of Palaeobotany, Lucknow 70 THE PALAEOBOTANIST INTRODUCTION produced by the cyanobacteria was used up as quickly as it was produced in oxidizing the soluble ferrous salts that were HE Earth came into existence about 4,600 million years abundant in the primitive oceans forming banded iron T ago by the consolidation of the accretion disc of debris formations. About 2,300 million years ago the supply of ferrous orbiting the Sun. Nitrogen, hydrogen and carbon dioxide salts began to become depleted causing the ceasing of banded dominated the earliest atmosphere. The earliest bacteria relied iron formations in the sea. The oxygen being produced by the on hydrogen and some carbon dioxide while extracting energy cyanobacteria therefore started to accumulate in the oceans from inorganic compounds and this had a marked impact in and then in the atmosphere. This coincided with the reducing the atmospheric levels particularly of hydrogen. There appearance of eukaryotes that ultimately gave rise to all are no fossils of these early life forms. The oldest known fossils multicellular organisms. At the beginning of Palaeozoic era date from about 3,500 million years ago, with the remains of (570 million years ago) the chemical composition of the oceans small filamentous organisms looking like cyanobacteria. Also and atmosphere was broadly similar to that of present (Fig. 1). dating from this time are structures known as stromatolites, Relatively high levels of atmospheric oxygen were therefore, which are carbonaceous mounds, formed by cyanobacteria. already available for life outside the oceans. It was not until Cyanobacteria initially used hydrogen and carbon dioxide for about 430 million years ago that life, led by plants slowly started their metabolism, but as molecular hydrogen became depleted to take hold on the land. The geography of the world at that from atmosphere they adapted to use hydrogen present in the time was different from present. In the middle Silurian, there water. A major by-product of photosynthesis is oxygen, which were two main land areas, separated by the Rheic Ocean, the was absent in the early atmosphere. Most of the oxygen being Laurentia, which includes much of Europe and North America, Fig. 2—Reconstruction of Cooksonia, showing slender, naked stems Fig. 1—Figure showing evolution of atmosphere and the main biotic with terminal sporangia (S). They are thought to be the most changes that influenced it (after Thomas and Cleal, 2000). primitive type of land vascular plants (after Thomas and Cleal, 2000). SINGH-EARLY LAND PLANT DEVELOPMENTS GLOBAL PROGRESS AND INDIAN PRIORITIES 71 {: Fig. 3-Photograph of Capel Horeb Quarry, mid-Wales. The middle Silurian strata here have yielded the oldest known examples of stem with vascular tissue preserved in-situ. They arc the oldest known unequivocal examples of the land vascular plants. Prof. Dianne Edwards. F.R.S., (in black Jacket) of United Kingdom is also seen in the picture. and the Gondwana including South America, Africa, central The present paper covers the initial radiation of the and east Asia and Australia. The best known early land plant embryophyes in the Ordovician; the emergence of fossils come from Laurentia but they have also been recorded tracheophytes, including the lycophytes in the Silurian; the from other parts of the world coming under Gondwana proliferation ofaxial plants with terminal sporangia around the realm. Silurian-Devonian boundary; the first major radiation of The gradual process of colonization of the land by the Zosterophylls at the end of Lochkovian and lastly the plants has been summarized herewith taking into consideration diversification and development of the trimerophytes during the views ofa number ofeminent workers in this field. Edwards middle-upper Devonian. The records ofearly land plants (from and Selden (1993) suggested four stages ofcolonization. The Ordovician-Lower Devonian) including spores and first stage begins in the Precambrian and extends into Early megafossils have been compiled to show their age wise Phanerozoic and later. It involved prokaryotes and occurrences in different localities located across the earth (Figs photosynthesizing protists (algae) but there is no direct fossil 21-23). These records have also been depicted on the global evidence of this stage. The second stage, based on maps (Figs 16, 17, 18, 19) to show the palaeo-phytogeography microfossils (spores and cuticles) possibly from bryophyte and the phytoprovincialism during Ordovician, Silurian and origin started in the Ordovician and en'ded in the Lower the Lower Devonian times. Sketches ofa number ofearly land Devonian. The third one beginning early in the Silurian, plants (Figs 2, 4-5, 7-15) have been included to strengthen the consisted of small plants of axial organization with terminal base of the paper. Similarly the photograph of a very famous sporangia, belonging to ?tracheophytes. The fourth and the plant locality (Capel Horeb, Fig. 3) and the reconstruction of final pioneering stage, responsible for the advent of taller the early Devonian volcanic landscape at Rhynie, Scotland vascular plants ofvaried organizations, begins around Silurian­ (Fig. 6, commonly known as Rhynie Chert) are also given to Devonian boundary and it gave rise to the highest evolved have an idea ofsuch important localities. Fig. 20 gives a quick plant groups including the extant ones. glance to the geological time scale. 72 THE PALAEOBOTANIST THE EMBRYOPHYTES AND THE DEVELOPMENT OF structures that can be related to sediment binding in these EARLY TERRESTRIAL ECOSYSTEM palaeosoils, but also based on the distribution of organic carbon and profiles of minerals (Phosphate) cycled by Four stages of colonization of the land by the plants are organisms in these soils. Another example of such vegetated recognized (Edwards & Selden, 1993): palaeosols is one of the horizons in the 1,200 Ma Mescal Limestone of central Arizona, based on depleted δ13C and Stage 1 (? Precambrian extending into Early Phanerozoic δ 18O values (Beeunas & Knauth, 1985). Such surface mats and later) might not be having good sediment binding capabilities but As per the current research findings, there existed a they could have anchored different kinds of substrates. This terrestrial ground cover in the Precambrian around 2,200 Ma might have resulted in the development of thin humus layers, ago (Edwards & Selden, 1993). Although the sub aerial thereby creating microenvironments for the development of surfaces in the Precambrian rocks were found to be carbon habitats for multicellular terrestrial green algae during early rich, yet no direct fossil evidence for such a covering is Palaeozoic.
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