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Origin and Evolution of Lycopods Origin and evolution of lycopods C. G. K. Ramanujam Ramanujam CGK 1992. Origin and evolution of lycopods. Palaeobotanist 41 51·57. The lycopods are known from as early as Sieginean Stage of the Lower Devonian. Lower and Middle Devonian lycopods were all herbaceous. Arborescent taxa appeared by Upper Devonian (e.g., Cyclostigma and Lepidosigillaria). The microphyllous foliage of lycopods seem to have originated from enations as well as telomic trusses. The lycopods auained peak of their evolution during the Upper Carboniferous. Towards the close of the Carboniferous and dawn of the Permian, with gradual dWindling and disappearance of swamps, the lepidodendrids suffered drastic decline numerically and phytogeographically. General aridity of the Triassic resulted in acute dwarfing as evidenced by Pleuromeia. This trend continued funher resulting in the highly telescoped Nathorstiana during the Cretaceous. The earlier lycopods were homosporous; heterospory appeared by Upper Devonian. Heterospory ran rampant in the Lepidodendrales. The ultimate in heterospory and the approach to seed habit could be witnessed in Lepidocarporz. Four discrete types of strobilus organization could be recognized by the Lower Carboniferous, viz., 1. Lepidostrobus type, 2. Mazocarpon type, 3. Achlamydocarpon type, and 4. Lepidocarpon type. Recent studies point towards the origin of lycopods along rwo different pathways, with both Zosterophyllopsida and Rhyniopsida representing the progenirors All available evidence show that Lycopsida constitutes a "Blind Alley" in the evolution of vascular plants. Key-words-Lycopsida, Origin, Geological hisrory, Evolution. C. G. K. Ramanujam, Department oj Botany, P. G. College oj Science, Osmania University, Saijabad, Hyderabad 500 004, India. riu ~ ~ <'II,," qllU tlltit <tT ~ ftr<mf ~o ~o cFo ~ ~ ~ ~ ~ ~ ~ ~ ~I ~ ~ ~ ~ ~ <11\\<iltiltl '!tm cF ar<rm 3iah cF f11<;rff 3iah 1Tm <11 \\<ilGltl ~ ~I 'l"~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ '!til m (..l,...."ffe.qj t:J;cf "'fIlilft1hi"'h:QI) if I if '!tm ~~~3lT1 ~~~<tT~cr~~~cF~if~~~<tT~:~:~cF~~ ~ ~ ~ ~ ~ ~ ~ ~ ~3lT1 ~ ~ q\,{if"l~lll ~ ~ ~ ~ ~ tt im'I mtQT if cF <m:'lT mer 1J1lT ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ \ll1TflJrcr 3lh: -tmi't if m 3lTim: 1J1lT1 ~ ~ ~ ~ ~q'1 '1"'lol~i¥ ~~ ~ ~q'1 '1"'ioi~i¥ ~ cr~ <11\\<ilGltl '!til ~ f1'1af1"'lol~i¥ ~ <M4' if .. 1Tt1 .. ~ ~ ~I ~ ~ ~ ~ ~ ~ ~ ~-l. ~ ~ ~ "'fIlill.#!'" ~""1T'1T 3iah ""'"'ill{ cF 3fNf.fuffta fu;lf ~ ~ cr~ ~ ~<>U ~ ~ ~ "'fIlilf\?l.... ll1PT, 2. ll1PT, 3. .."'ql'il4ijc\., ll1PT, 4. ",fIlil4ijc\., ll1PT1 Il1fTOif o:!f'fO itcrT fcl; ~ ~ ~ ~ ~I '!tm cF <11\\<ilGlt'1 '!til 'l:!.'i¥ "w!ft TTtft" cF UNEQUIVOCAL members of Lycopsida could be environment. The Permian Period, which witnessed recognized from as early as Sieginean Stage of the rapid dwindling and disappearance of swamps, Lower Devonian coexisting with the zosterophyllop· heralded their decline. The overall arid Triassic sids and rhyniopsids. They soon underwent rapid Period triggered drastic physical, numerical and multiplication and radiation qualitatively and phytogeographic diminution of the lycopods. The quantitatively all through the Devonian, and by geological history of the Lycopsida closely parallels Carboniferous Period attained developmental that of the Sphenopsida which, however, suffered climax, both vegetatively and reproductively, as exemplified by numerous herbaceous and Essays In Evolutionary Plant Biology arborescent taxa that dominated the swampy Editors, B. S. Venkatacha12, David L Dllcher & Harl K. Maheshwarl Publisher' , Blrbal S.hnl Institute of PaJ.eobolany, Lucknow 52 THE PALAEOBOTANIST much more drastic decline. Thomas (1978), Phillips Lepidodendrales - Upper Devonian to (1979) and DiMichele and Phillips (1985) have Permian provided superb overviews of the morphology, Pleuromeiales - Triassic to Cretaceous reproductive biology and ecological adaptation of Lycopodiales - Upper Carboniferous the Palaeozoic arborescent lycopods. to Recent The extant Lycopsida comprises five genera, Selaginellales - Upper Carboniferous viz., Phylloglossum, Lycopodium (Lycopodiaceae), to Recent Selaginella (Selaginellaceae) and Isoetes and Stylites lsoetales - Triassic to Recent (Isoetaceae). If Stylites is considered as a subgenus of Isoetes (see Kramer & Green, 1991), only four VEGETATIVE CHARACTERS genera constitute the extant lycopods. In the geological past, however, the lycopods enjoyed The Lower and Middle Devonian lycopods were better representation at the genus, family and order all herbaceous (e.g.) Baragwanathia, levels. Drepanophycus, Protolepidodendron, Colpodexylon, etc.). Shrubby or moderately arborescent habit GENERAL CHARACTERS AND appeared by the Upper Devonian as evidenced by CLASSIFICATION Lepidosigillaria and Cyclostigma which often attained a height of 8 m and a girth of 30 em. During The lycopods are a well·knit, homogeneous the Carboniferous the arborescent lycopods had group of lower vascular plants. The essential features their best expression in forest giants such as of lycopods are: plants usually herbaceous but Lepidodendron, LepidophloiOs, Sigillaria and occasionally arborescent with dichotomous or a Bothrodendron. It is generally suggested that the combination of dichotomous and monopodial Upper Devonian taxa, such as, Lepidosigillaria and branching; leaves helically disposed microphylls Cyclostigma constitute the transitional forms and the with a prominent unbranched midvein that does not possible precursors of the Carboniferous leave any leaf gap in the stele of stem; vasculature of arborescent lycopods. Banks (1960) opined that stems protostelic or siphonostelic with protoxylem there was a group of diverse herbaceous Devonian in exarch position; arborescent taxa with limited lycopods with eligulate foliage, lack of leaf cushions secondary xylem; a single sporangium borne in the and homospory which subsequently gave rise to axil or on adaxial surface of sporophyll; sporophylls arborescent forms along one line and newer generally aggregated to form discrete strobili; plants herbaceous forms along the other. Andrews (1961) homosporous or heterosporous. visuali zed a common stock from which all the Drepanophycales, Protolepidodendrales, arborescent taxa developed. The lycopods of the Lycopodiales, Selaginellales, Lepidodendrales, Carboniferous and Permian furnished' greatest Pleuromeiales and lsoetales are usually recognized diversity of form incorporating small, insignificant in various systems of classification proposed since herbaceous taxa referable to Lycopodiaceae and 1960 (Delevoryas, 1962; Sporne, 1966; Bierhorst, Selaginellaceae (e.g., Lycopodites, Selaginellites, 1971; Stewart, 1983; Taylor, 1983; Meyen, 1987). Paurodendron, etc.), moderately arborescent (some Bierhorst (1971) included Asteroxylales also under species of Lepidodendron, Paralycopodites), and lycopods, and Meyen's (1987) lsoetales huge, majestic arborescent Lepid0dendrales often encompasses the Lepidodendrales and attaining a height of 50 m and occupying extensive Pleuromeiales of others. The author, however, tracts of swamps (Lepidodendron, LepidophlOiOS, agrees with Stewart (1983) that Asteroxylon Sigillaria, etc.) in the Euramerica. notwithstanding its similarities with lycopods, The branching of the axes was almost deserves inclusion only under Zosterophyllopsida. exclUSively dichotomous in the earliest forms and a Further, the traditional treatment of lsoetales is combination of dichotomous and monopodiaI followed in this contribution. Lycopodiales, nature in the later forms. The foliage of lycopods Selaginellales and lsoetales consist of both extinct should be technically designated as microphylls and extant members; the remaining taxa are irrespective of their size as they possess a single represented by only extinct members. The following unbranched midvein not associated with any leaf are the geological ranges of the various lycopod gap in the central vascular cylinder of the stem. The orders: leaves disposed in a lax or close-knit helix or Drepanophycales Devonian pseudowhorls may be scale-like, linear, deltoid and Protolepidodendrales Devonian to Lower more or less laminar (Fairon-Demaret & Banks, Carboniferous 1978). They may be entire or as in some Devonian RAMANUJAM-ORIGIN AND EVOLUTION OF LYCOPODS 53 taxa forked (Protolepidodendron), trifurcate primeval condition in some lycopods. It now (Colpodexylon) or split up into five lobes appears probable that the microphylls of lycopods (Leclereqia). Further. the leaves may be perennial or could have evolved along two divergent pathways, show abscission leaving prominent leaf cushiuns of one involving enations and the other, telomic varied shapes (e.g.. Lepidodendrales), trusses. Concomitantly, the microphylls- of all the The stelar anatomy of the lycopod stems shows lycopods need not necessarily be considered as well-defined trends of evolution from various types homologous structures. of protostele to mixed protostele and ultimately the With the dWindling and large scale siphonostele. The protosteles of the Devonian disappearance of swampy habitats by the end of the lycopods may be lobed. as in, Drepanopbyclls, Upper Carboniferous and the dawn of the Permian, Colpodexylon and Archaeosigillaria, or with many of the arborescent lycopods such as numerous small marginal, teeth-like ridges, as in, Lepidodendron and Lepidopbloios suffered drastic Protolepidodendron and Leclereqia. The lobed decline but Sigillaria, and smaller lepidodendrids protostele resembles the steles in some of. the such as Paralycopodites survived
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