BIOLOGY OF THE CHENGJIANG FAUNA

澄江動物群生物學

陳均遠(JUNYUAN CHEN) ； 周桂琴(GUIQING ZHOU)

《Collection and Research》 10期 (1997/12) Pp. 11-105

摘要

The Chengjiang fossil fauna from the Lower Cambrian, discovered in 1984 about 52 km SE of Kunming, South China, is equaled only by the Middle Cambrian Burgess Shale fauna (British Columbia) in its excellent state of preservation. The Chengjiang fauna has been described in approximately 50 papers published in both Chinese and foreign journals, by a number of authors. A book providing a general description of this fauna, has been published until recently by Chen et al. (1996) in Chinese. In the present paper we review the biological composition of the Chengjiang fauna with emphasis on its evolution. A new species, Fuxianospira gyrata is erected to receive macroscopic algae having a thread that was cylindrical, unbranched, and helically coiled. We also describe two new ctenophorans, Maotianoascus octonarius and Sinoascus papillatus. M. octonarius was a globular ctenophore with eight petaloid lobes and 16 sets of elongated comb rows. The petaloid lobes converge aborally into a capsule-like dome, and the oral area is dilated. Sinoascus papillatus was a tetraradial ctenophore having four longitudinal organs but no trace of comb rows. The aboral field of this ctenophore exhibits numerous papillae. Acosmia maotiania is erected for a new priapulid having a lager proboscis comprising one-third of the body. Iotuba chengjiangensis is erected to receive a tube worm referred to phonorid. The body of this animal tapered both distally and proximately, was blind at the proximal end and had a tentacular fold at the distal end.It also had a U-shaped digestive tract that was partitioned into a prestomach, stomach and intestine. Tardipolypoda is erected to receive Cambrian lobopodians interpreted as the sister group of Tardigrada. The genus Canadapis, previously known only from the Middle Cambrian Burgess Shale, is reported. Examination of complete Canadaspis specimens from Chengjiang confirms the close relationship of this species to the Burgess Shale species. A new species, C. aucallus is established. A nearly complete, new anomalocarid having 20 pairs of biramous limbs is reported. A new class, Vetulicolida, is erected to receive arthropod-like metazoans having an abdomen consisting of numerous, telescoping segments and a carapace that was fused and had a pair of lateral basket slits. The genus Banffia, previously known only from the Middle Cambrian Burgess Shale, is interpreted as an annelid. Close examination of nearly complete Bunffia specimens from Chengjiang confirms the close relationship of this taxon to Vetulicola. A new species, Banffia confusa, is established for a soft-bodied vetulicolid having an abdomen that is elongate and bears a semicircular terminal region. The anus is situated axially at the center of the terminal region on the ventral surface. Correlation of the Meishucun Formation with the Nemakit-Daldynian is supported by stable carbon isotope data and trace fossils (the occurrence of Phycodes pedumy). Based on analysis of small shelly fossils, the overlying Shiyatou Formation appears to be of Tommotian age. The Chengjiang fauna is early Atdabanian. This new time frame for basal Cambrian strata confirms the rapid pace of evolutionary changes during Cambrian Explosion.

關鍵字

Cambrian Explosion ； Chengjiang fauna ； Evolution

並列摘要

五億三千萬年前雲南澄江動物化石群是研究寒武紀大爆發獨一無二的科學窗口。一九八四年以來，先後找到了近兩萬塊化石標本，爲遠古生物探密提供了依據。這一化石群首次發現於昆明東南52公里，澄江縣境內帽天山。化石保存在下寒武統玉案山組帽天山段內。化石保存的完好程度僅加拿大中寒武統布爾吉斯頁岩可以與相比。有關澄江化石群研究性報導已在國內外學術雜誌發表五十餘篇論文。最近陳均遠等（1996）著的《澄江生物群－寒武紀大爆發的窗口》一書首次對澄江動物作較全面的綜合性介紹。本文對澄江動物群沉積環境和埋藏學特徵作綜合性的討論，認爲澄江動物群生存在近陸二角洲洲坡和洲前。由於停積在陸地，和在三角洲洲坪和洲坡上大量經過風洗的細塵，受到特大風暴的襲擊，形成泥流順著洲坡而下，成爲浩劫性的事件，將生存在洲坡和洲前上生物群活生生的包埋在泥流之中。這些生物在臨死之前，有些似乎曾試圖逃生，有些還飽含著食物，有些攀附在寄主身上正在嚼取寄主體內的體液。本文從演化生物學的角度對澄江生物群進行總評性的描繪。同時還討論了演化生物學的若干問題，認爲多分節液壓類型是古老的體腔造型方式，以節肢動物爲代表的血腔類群，以及以曳鰓類為代表的假體腔類群均可能派生於早期真體腔類群。其根據是：1)潛穴深度在石崖頭期已出現明顯遞增《其深度達二十公分以上》，這一事實表明多分節液壓類群在石崖頭期已經成爲優勢類群，而節肢助物到石崖頭期之後，即五案山期才大量出現；2)個別古蟲具串珠狀保存腸道，這一保存方式表明古蟲類體腔可能具橫隔膜的存在，推測古蟲類可能具液壓型眞體腔類群；3)帽天山蟲和無節蠕蟲具串珠狀保存腸道，這一保存方式表明早期曳鰓類可能爲真體腔類群。形態學特徴表明多腿緩步動物門（Tordipolypoda）爲同一源類群，這一類群具圓柱狀身軀，六對以上帶爪的腿，與現代有爪類共有許多特微，許多作者將多腿緩步類解釋爲有爪類具有口前附肢，屬於節肢動物超門類群。多腿緩步類不具口前附肢，屬於有腿類群中的與節肢動物超門相平行的另一個超門級的分支，這一分支不具有口前附肢，附肢均為單肢型，包括多腿緩步類和有爪類兩個門一級的類群。多腿緩步類個體較大，多分節，為無口前附肢超門的祖先型。現代緩步類可能通過侏儒化起源於多腿緩步類。多分節雙肢型附肢和須狀口前附肢是節肢類的原始特徵，奇蝦的前附肢用於捕食動物，奧帕賓蟲類的前附肢用於游泳，均為鬚狀口前附肢的派生物。奇蝦類外肢特化為游泳功能的槳狀構造，內肢由於退化而消失。作者以為真節肢動物在寒武紀早期已演化成為許多相當亞門一級的分支。澄江真節肢動物包括：1)帶腹雙瓣殼節肢動物，2)無腹雙瓣殼節肢動物，3)撫仙湖蟲類，4)雙分區納羅蟲類，5)單分區Tegopeltid類，6)海怪蟲類，7)三分區三葉蟲狀節肢動物，8)三分區三葉蟲等相當亞門一級不同造型方式。化石證據表明以上不同方式分區化體制是在多分節基礎上通過體節癒合，以突變方式來實現的，這一突變可能與同源基因族間格基因（gap gene）的消失或趨弱化有關。體制的變化除以體節癒合方式來實現之外，以背甲數量減少的方式來實現造型的變化在寒武紀大爆發件中也是一種極為常見的現象。我們認為這一造型方式的突變可能通過改變控制個體發育的調控基因變化來實現的。纖毛還類為一同源類群，是寒武紀大爆發的重要組成部份，在寒武紀早期曾存在比現在還多的相當門一級的類群。澄江纖毛環類包括1)營潛穴生活方式火把蟲，2)營管棲生活的帚蟲類，3)營底棲生活方式的腕足類、4)營漂浮生活方式水母狀依爾東缽類，5)營固著生活方式帶柄的高足杯蟲（其生活方式與原始海林檎很相似）等相當門一級的生物類群。其中火把蟲是纖毛環類群中最爲原始的生物類型，水母狀依爾東缽和管棲狀帚蟲可能分別通過幼態持續從多態蠕形祖先突變而來的。本文建立了許多新的分類系統。Fuxianospira syrata, 是宏體藻的一個新種，藻絲體螺旋狀，不分枝。櫛水母共有兩個新種，其中Maotianoascus octonarius水母體亞球形，八射對稱，由八瓣葉狀體和十六組櫛板系列組成；球體反口面具一囊狀構造；口部大，擴張狀。Sinoascus papillatus水母體亞球狀，四射對稱，由四瓣葉狀體所組成，未發現櫛板構造，反口面具乳突狀構造。Acosmia maotiamia爲曳鰓動物門的一個新種，吻部大，約占身體總長的三分之一。Iotuba chengjiangensises為管棲生物，屬帚蟲動物門，管體向兩端收縮，近基為盲端，遠基具觸手狀構造；消化道U字形，分爲前胃、胃和腸道三部分。新建立的多腿緩步動物門(Tardipolypoda)爲絶滅了的生物群，與現生緩步門為姐妹類型，主要分布於寒武紀早中期，身體圓柱狀，由六對至二十三對腿所支持，頭與軀幹沒有明顯分界，頭部具有一至二對前附肢，最後一對腿位軀幹的末端。本文認爲緩步類可能通過侏儒化起源於多腿緩步類。加拿大蝦(Canadaspis)只在中寒武統布爾吉斯頁岩發現。Canadaspis aucallus這一個新種是加拿大蝦在澄江動物群的代表，個體較小，身體由較多的分節所組成。本文報導了屬於奇蝦類祖先型新的化石類型的發現，該化石頭部特徵與奇蝦類相似（具一對投附肢和一對位於背前端的帶柄複眼），但軀幹分節較多（約20節），腿雙肢型。新建的古蟲綱為已滅絕的生物類型，具有分節的腹部和節肢動物相似，但甲殼十分奇特，閉合狀，兩側具一對鲤裂。斑府蝦(Banffia)發現於加拿大中寒武統布爾吉斯頁岩，曾誤以為環節動物。Banffia confusa是斑府蝦在澄江化石群的代表，身體柔軟，腹部較長，具有半圓形末節，肛門開口位於末節的中腹部。由於分布於世界各地重要化石群時代對比仍存在著重大分歧，寒武紀大爆發的演化速率尚無法準確的斷定。滇東北寒武紀底部碳酸岩鹽碳同位素變化曲線的研究，為澄江及鄰近地區寒武紀底部地層與世界其它地區對比提供了新的依據。古海水碳同位素變化曲線和生痕化石（Phycodus pedum）表明澄江及其近鄰寒武紀底部梅樹村組的時代相當於西伯利亞Nemakit-Daldynia期。小殼化石的研究結果表明梅樹村組上伏地層，石崖頭組可能為Tommotian期的產物。澄江化石群為Atdabanian期的早期。以上關於寒武紀早期時間框架新的解釋，為寒武紀大爆發高演化速率的假設提供了支持。

並列關鍵字

寒武紀大爆發 ； 澄江生物群 ； 演化