- 期刊
- OpenAccess
EARLY CAMBRIAN CHORDATE FROM CHENGJIANG, CHINA
中國寒武紀早期脊索動物
摘要
The lower Cambrian problematicum Yunnanozoon lividum, currently known from the Maotianshan Shale Member of the Yu'anshan Formation (Chengjiang fauna), is described and illustrated. This soft-bodied organism has been variously interpreted as a metazoan of unknown affinities (Hou et al., 1991), a chordate (Chen et al., 1995, 1996), or a hemichordate (Shu et al., 1996a). Shu et al. (1996b) recently interpreted a dorsally compacted specimen from Chengjiang as a new chordate, called Cathaymyrus diadexus. However, the anatomical resemblance with Yunnanozoon lividum raises questions regarding the validity of this taxon. A chordate affinity for Y. lividum is supported by the presence in most specimens of a notochord (preserved as a pair of axial lines) and a region showing segmented muscles. The hemichordate hypothesis (Shu et al., 1996a) is based primarily on misinterpretation of the two axial lines as a gut, misinterpretation of muscle traces flanking the notochord as valve structures, and misinterpretation of gonads in a subventrally exposed specimen as gut contents. Both its old age and anatomical characteristics suggest that Y. lividum was a very primitive chordate. The most recent common ancestor of this group was a free-swimming, Yunnanozoon-like filter feeder having the following plesiomorphic features: (1) body laterally compressed and lancet-shaped; (2) notochord large and extended anteriorly; (3) myomeres nearly straight; (4) gill slits few in number; (5) gonads asymmetrically metameric; and (6) endostyle present. A Yunnanozoon-like ancestor may have given rise independently to urochordates, cephalochordatcs, conodonts, and agnathans (primitive vertebrates).
並列摘要
雲南蟲為軟軀幹生物，一九九一年侯先光等發現於中國雲南澄江下寒武統玉案山組帽天山段（距今五億三千萬年），並將它解釋爲分類未定。一九九五年陳均遠等提出雲南蟲爲脊索動物最古老的化石代表。舒德干等（1996）將雲南蟲解釋爲半索動物；同時他們將有僅有的一塊背壓，蠕形標本定名爲“華夏魚”，並認爲“華夏魚”才是脊索動物。“華夏魚”與雲南蟲不僅在解剖學諸多特徵相同，而且個體大小十分相似，並産於同一地點和相同的層位，因而“華夏魚”極可能是雲南蟲的背壓標本。舒德干等在文章中將“華夏魚”背中部的脊狀隆起解釋爲脊索。脊索爲內部構造，這一表面脊狀隆起作爲脊索的解釋顯然是缺乏根據的；我們認爲這一構造根本不是脊索，而可能是雲南蟲身上的背鳍。筆者通過對二十多塊化石標本作詳細的觀察，認爲雲南蟲解剖學特徵包括以下幾個方面：身體側扁，紡錘形，有一背中鰭和一對腹摺，脊索粗大，位於亞腹部，縱貫首尾；消化器官包括口，咽喉，中腸和後腸等部份；口位於漏斗狀前器之後端，咽喉位於鰓腔的後部：咽喉之前具兩對以上的齒狀構造（可能屬於消化器的一部份）；後腸螺旋狀，位於脊索的腹部；背部由22～24肌節所組成，肌節爲近平直的肌隔所分開；鰓腔較大，具七對鰓弓和一中柱、鰓腔末端具一圍鰓孔。以上雲南蟲解剖學特徵進一步肯定了雲南蟲屬於脊索動物門的分類地位。雲南蟲是脊索動物門唯一的最古老的化石代表。脊索動物的祖先可能與雲南蟲相似，爲營自由游泳和濾食生活方式。筆者同時認爲脊索動物祖先型特徵可能包括以下幾個方面：1)身體側扇，紡錘形，2)脊索粗大，位於亞腹部，縱貫身體的首尾，3)肌隔正直，4)鰓弓數量較少，5)生殖腺分節排列，6)具內柱。並認爲脊索動物各主要分支，如無頷綱(最原始的脊椎動物)，尾索動物，頭索動物，牙形動物等均可能直接起源於雲南蟲狀的祖先。雲南蟲爲半索動物之說顯然是基於錯誤的觀察。