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Ediakara Ediacara Ediakara Stratotyyppi ja elämän varhaiskehityksen problematiikka Ediacara Stratotype and the problematics of the Metazoan origins ANNA HAAPANIEMI Abstract tavasti viimeaikoina. Tämä on saattanut olla yksi niistä syistä, miksi aikakaudesta usein käytetty nimi The new geological period Ediacara has been ”Vendi” korvattiin nyt Ediakaralla. Mutta samalla discussed vividly ever since its formal ratification kun kambrikautisten fossiilien joukosta on löytynyt in 2004. Although Ediacara is a considerable leap merkkejä ediakaraeliöstöstä, niin nyt myös ediaka- in the construction of an international geological rakautisista kerrostumista on raportoitu löydetyn timetable, it is also connected to several theories entuudestaan tunnettuja kambrikautisia eläimiä. and concepts central to the geological community, Nämä löydöt siirtäisivät kiistattomasti monisoluis- but sometimes little understood. The Vendobionta ten kehityksen prekambriin. Näin ollen komplek- theory created by Seilacher is now rivalled by sisen elämän synnyn tutkimuksen painopiste on the “Ediacaran survivors” –hypothesis of Con- siirtynyt kambrikauden radiaation syistä ediakara- way Morris. Supporting evidence of Ediacarans kauden alkuun. Eliöstön mahdollinen kehittyminen in Cambrian strata seems to be piling up, but at Rodinian hajoamisen jälkeisten jäätiköiden keskelle the same time Cambrian animals appear to have on problemaattinen; jäätiköitymiskauden on arvioi- their origin deep in ediacaran times. Thus, the tu olleen tuona aikana epätavallisen laaja. Mikäli focus of current research has partly shifted from elämän kehittyminen on ollut mahdollista, on voi- the causes of Cambrian radiation to the beginning makas valintapaine ja isolaatio saattanut olla radiaa- of Ediacaran. However, the first appearance of the tion taustalla. Vaihtoehtoisesti kyseessä on saattanut biota in the glacial conditions of the post-Rodinian olla ’punctuated equilibrium’ -teorian mukainen world is problematic; the glacial extent has been evoluutionopeuden kiihtyminen; viimeisimmän jää- estimated unusually large. If this situation truly has tiköitymisvaiheen ja ensimmäisen ediakarafossiilin allowed development of complex life, it could have välillä on vain 5 miljoonaa vuotta. been a result of intense natural selection pressure and isolation. The speed of evolutionary changes could have grown to levels corresponding “punctu- Johdanto ated equilibrium” –situation; the time between the – Ediakara korvaa Vendin last glaciation period and first ediacaran fossil is IUGS:n (International Union of Geological only 5 Ma according to current knowledge. Sciences) stratigrafisen komission ICS:n (Interna- tional Commission on Stratigraphy) tavoitteena on luoda maailmanlaajuinen stratigrafinen aikataulu Abstrakti eri aikakausiin erikoistuneiden työryhmien avulla. Uusi geologinen aikakausi Ediakara on herättänyt Komission julkaisema geologinen aikataulu luo paljon keskustelua. Vaikka se onkin suuri askel pohjan geologien käyttämälle terminologialle ja geologisen aikataulun kokoamisessa, on Ediakaran sitoo nimistön tarkasti ajoitettuihin ikiin. Viimeisin hyväksyminen myös yhteydessä moniin keskeisiin versio julkaistiin vuonna 2005, ja mielenkiintoista teorioihin ja käsityksiin, jotka usein jäävät vähem- kyllä, se toi mukaan uuden aikakauden: ediakaran mälle huomiolle. Seilacherin Vendobionta-teoria on (IUGS, 2005) (kuva 1). Virallistamista on kuiten- saanut varteenotettavan kilpailijan Conway Morri- kin edeltänyt yli vuosikymmenen tutkimusrupea- sin ”ediakaran selviytyjät” -hypoteesista, jota tu- ma, joka huipentui vuoden 2003 äänestyksiin ja kevien fossiililöytöjen määrä on kasvanut huomat- viimein IUGS:n ratifiointiin maaliskuussa 2004 GEOLOGI 58 (2006) 117 (Knoll et al. 2004a, 2004b). Uusi kausi sijoittuu tyyppikohteen alueen mukaan: Nuccaleena-muo- kambrikauden alapuolelle, ollen ensimmäinen dostuma sijaitsee Ediacara-vuoristossa Flinders geokronologisesti määritelty prekambriin kuuluva Ranges-alueella (kuva 2). Muita nimiehdotuksia ajanjakso. Ediakara on siten vanhin kultaisen piikin olivat venäläisten kannattama Vendi ja kiinalaisten määrittämä geologinen kausi! Piikki iskettiin lo- ehdottama Sinia, vanhakantaisempi Ediakaria ei pulta eteläaustraliaisen Nuccaleena-muodostuman sen sijaan saanut kannatusta (Knoll et al. 2004b). ”lakkikarbonaattiin”, mutta päätös ei ollut täysin Mielenkiintoista kyllä, äänestystulokset tuntuvat yksimielinen (ks. alla). Ediakaran ylärajan sen si- heijastavan pitkälti kansallisia näkemyksiä, kylmän jaan määrittää toinen vastaava kultainen piikki: sodan ilmapiiri on käsinkosketeltava. Erityisesti kambrikauden stratotyyppi löytyy Newfoundlan- vendin kannattajat Fedonkinin johdolla kieltäyty- dista Treptichnus pedum -jälkifossiilien ilmestyes- vät hyväksymästä päätöstä, sillä heidän mielestään sä stratigrafiaan. Uusi kausi on saanut nimensä päätös on vastoin tieteen traditiota: Vendi-nimeä on käytetty kirjallisuudessa jo 50 vuoden ajan. He nä- kevät myös uuden Ediakara-nimen ongelmallisena, sillä se assosioituu voimakkaasti sen edustamaan eliöstöön – päätettiinhän samanaikaisesti asettaa kauden alaraja, ei eliöstön ilmestymisajankohtaan, vaan Marinoa-jäätiköitymisen loppuun. Kuitenkaan Kuva 1. Ediakarakausi virallistettiin maalis- itse vendin stratotyyppiä ei koskaan ehdotettu ko- kuussa 2004 ollen ensimmäinen geokronolo- gisesti määritetty prekambriin kuuluva kausi. missiolle. Itä-Euroopan kratonialueelta on vaikea IUGS (2005) mukaan. löytää maailmanlaajuisesti korreloitavaa kerrostu- (GLDFDUD3HULRGZDVUDWL¿HGLQ0DUFK maa, korreloituvuus on kuitenkin yksi tärkeimmistä WKXVEHLQJWKH¿UVWJHRFKURQRORJLFDOO\GH¿QHG ja oleellisimmista GSSP kriteereistä (Knoll et al. 3UHFDPEULDQSHULRG$IWHU,8*6 2004b). Toisaalta myös Vendi-nimi, niin kuin Edia- Kuva 2. Ediakarakauden määrittelevä uusi GSSP sijaitsee Ediacara-vuoriston maisemissa Flinders Ranges -alueella (vasemalla). ”Kultaisen piikin” paikka on merkitty tyyppikohteen ”lakkikarbonatiit- ti”-kerrostumaan (oikealla). Kuva: David Wood ja Antti Kallio. 7KHQHZ*663LVORFDWHGLQWKH)OLQGHUV5DQJHVDUHDRI(GLDFDUDPRXQWDLQV OHIW 7KH³*ROGHQ 6SLNH´LVPDUNHGRQWKHFDSFDUERQDWLWHOD\HU ULJKW 3KRWRE\'DYLG:RRGDQG$QWWL.DOOLR 118 GEOLOGI 58 (2006) karakin, voidaan yhdistää kyseenalaiseen asiayh- mantereen noin 750 Ma sitten tapahtuneen ha- teyteen; Vendobionta-teoria (ks. alla) on ajautunut joamisen synnyttämät tektoniset muutokset ovat vastatuuleen. Tämä saattaa hyvinkin olla yksi tekijä ainakin välillisesti luoneet ne haasteet ja mahdol- äänestystulosten taustalla. Näin ollen käy selväksi, lisuudet, joihin elämänkehityksen oli vastattava. että mielipidettä uudesta aikakaudesta on vaikea Samanaikaisesti jäätiköitymiskaudet yleistyivät, muodostaa ymmärtämättä sen taustalla olevaa pit- ja maailmanlaajuisesti tavatut tilliittikerrostumat kää paleontologista tutkimusta. (kuvat 3 ja 4) antavat ymmärtää, että kyseessä oli erityisen kylmä ajanjakso. On jopa tulkittu, että jääpeite ulottui aina päiväntasaajalle saakka. Vendobionta-teoria syntyy Teoria, joka kantaa nimeä ”Snowball Earth” (Lu- – Ediakara ainutlaatuinen mipallo-Maa) on kuitenkin vielä kyseenalainen Meistä ne, jotka ovat lukeneet ediakara-faunas- (Hoffman ja Schrag 2002). Donnadieu et al. (2004) ta, ovat mitä oletettavimmin siinä käsityksessä, esittävät, että nämä kaksi ilmiötä, supermantereen että kyseessä oli ns. evolutiivinen kokeilu, joka hajoaminen ja jäätiköitymisjakso, olisivat olleet päättyi jättämällä jälkeensä ainoastaan mielen- yhteydessä toisiinsa. Heidän mukaansa tektonisten kiintoisen fossiiliaineiston. Sukupuuttoon kuollut muutosten aiheuttama voimakas rapautuminen olisi ”Vendozoa” (1989) ja myöhemmin Vendobionta johtanut huomattavaan CO2-pitoisuuden laskuun. (1992) oli alun perin Adolf Seilacherin esittämä Tämä olisi ollut riittävä muuttamaan ilmaston jää- teoria. Vendobionta koostui pehmytkudoksisista tiköitymisille suotuisaksi aikana, jolloin auringon ja sessiileistä organismeista; sen sukupuuton ai- säteily ei vielä ollut yhtä voimakas kuin nykyään. heuttanut ’kambrin räjähdys’ saattoi Seilacherin Aiheutti jäätiköitymisjakson sitten mikä tahansa, (1992) mukaan olla seurausta biomineralisaation ei sen olemassa olosta kuitenkaan kiistellä. ja liikkumiskyvyn kehittymisestä. Kumpikin omi- Hiilen isotooppikoostumuksen positiiviset ano- naisuushan mahdollistaisi tehokkaan bioturbaation maliat kertovat etenevistä jäätiköistä. Jäätiköitymis- – ja predaation. Ediakara olikin niin sanotusti para- vaiheet voidaan jakaa Sturtia-, Marinoa- (Varange- tiisi McMenaminin (1998) ’Garden of Ediacara’ ri-) ja Gaskiers-vaiheisiin (kuva 5), joista erityisesti -kirjan mukaan. Niinpä Seilacherin mukaan ky- Marinoa oli poikkeuksellisen laaja. Mutta samalla seessä oli erillinen radiaatio, joka hävisi kilpailussa isotooppikoostumuken vaihtelu kertoo orgaanisen kookkaille monisoluisille (Metazoa). Vaikka edia- aineksen poistumisesta hiilen kiertokulusta. Mie- karafossiilien huomattava koko, desimetreistä yli lenkiintoiseksi ilmiön tekee sen yhteys happipitoi- metrin halkaisijaan, tuntuisikin perustellulta argu- suuden kasvuun, muutokseen, joka on olennainen mentilta monisoluisuuden puolesta, oli Seilacherin kookkaampien ja monimutkaisempien elämänmuo- mukaan kuitenkin kyse yksisoluisista organismeis- tojen kehitykselle. Ediakaraeliöstön synnyn reuna- ta. Poimuttunut, patjamainen rakenne (ks. esim. ehto on ollut ilmakehän hapen kasvu, ja jäätiköity- kansikuvan Dickinsonia sp.) poikkesi aiemmin miskausilla on saattanut olla tässä merkittävä rooli vallinneesta yksinkertaisen solun pyöreästä muo- (Kirschvink et al. 2000,
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