COMPARATIVE TAPHONOMY OF TWO NONMINERALIZED ARTHROPODS: NARAOIA (NEKTASPIDA; EARLY CAMBRIAN, CHENGJIANG BIOTA, CHI A) AND LIMULUS (XIPHOSURIDA; HOLOCENE, ATLANTIC OCEAN)

沒有礦化外殼的節肢動物拿嬈蟲及鱟蟲的比較埋藏學研究

LOREN E. BABCOCK ； 張文堂(WENTANG CHANG)

《Collection and Research》 10期 (1997/12) Pp. 233-250

摘要

The nektaspid arthropod Naraoia, which lacks a mineralized exoskeleton, is preserved in various states of disarticulation in the Lower Cambrian Chengjiang Biota of Yunnan Province, China. Specimens range from fully articulated (including appendages) through appendageless, and finally, separated anterior and opsterior shields. Actualistic taphonomic experiments conducted on a modern, nonmineralized analog, the xiphosurid Limulus, suggest that the internal soft parts of Naraoia decayed rapidly prior to burial, but the tougher chitinous exoskeleton was considerably more resistant to disarticulation. Before burial, appendages and the anterior and posterior shields of Naraoia may have remained articulated for years in anoxic water, or for weeks to months in oxic water, assuming that scavengers and bioturbators were excluded by some means. The exoskeleton of Naraoia, if it remained in water, evidently became pliable soon after death. As such, it was easily compressible or foldable without breakage at the margin. In assessing the early taphonomic history of nonmineralized arthropod s, it is necessary to make a clear distinction between preburial and postburial circumstances. The inferred slow rates of preburial exoskeletal disarticulation of Limulus and Naraoia suggest that many nonmineralized arthropods, even mostly articulated ones, were perhaps buried days to months after death or moulting. However, their exceptional preservation is also related to postburial factors including burial in mud, clay-organic interactions, early mineralization, resistance of chitin to breakdown, and the exclusion of scavengers and sediment bioturbators from the postburial depositional site. Anoxia is evidently an important means by which scavengers and bioturbators were excluded from postburial depositional sites, especially in Cambrian marine Lagerstätten. Other factors, such as salinity fluctuations or intermittent subaerial exposure, also may have excluded such organisms from some Lagerstätten, especially some deposited in marginal marine settings during the middle and late Palaeozoic.

關鍵字

arthropoda ； Chengjiang biota ； China ； Naraoia ； Limulus ； taphonomy

並列摘要

屬於節肢動物門Nectaspida目的拿嬈蟲*(Naraoia)沒有礦化的外殼，在中國雲南省早寒武世澄江生物群中呈各種關節分離狀態的保存，包括個體完整並帶有附肢、個體完整但附肢脫落、及頭部和尾部分離的標本等。用現生的類似物劍尾類鱟蟲作實際的埋藏學實驗，得出在埋藏之前，拿嬈蟲內部軟組織很快腐爛，堅韌的幾丁質外殼在關節分離時有相當大的耐力。埋藏之前，在沒有食腐動物及生物擾動者的情況下，拿嬈蟲的附肢、頭部及尾部可在缺氧的水中保持完整狀態達數年之久，或在有氧的水中保持數周或數月。該蟲死後，牠的外殼留在水中顯然是柔韌的，在受壓或被擠壓時，殼的邊緣處不會破裂。要想探討沒有礦化的節肢動物外殼的早期埋藏歷史，必需要弄清楚埋藏之前或之後環境的區分。推斷是慢速埋藏前的鱟蟲及拿嬈蟲外殼關節分離，暗示許多非礦化外殼的節肢動物，甚至關節是連接的，或許是在死後或蛻皮後已埋藏有數天至數月之久。特殊情況下的保存，仍然與埋藏後的因素有關，如在泥中埋藏，泥質與有機質的相互作用、早期的礦化作用、幾丁質的韌力被破壞以及在埋藏後沉積地區沒有食腐動物及擾動生物的存在等。缺氧是一種重要因素，可排除食腐動物及生物擾動者在埋藏後的沉積區內的存在，尤其是在寒武紀海生動物埋藏的富集區內。其它因素如海水含鹽度的變化，間歇的地面上的曝露等，也可以排除生物體在原埋藏後富集區的存在，尤其是在中、晚古生代海相邊緣環境的沉積內。

並列關鍵字

節肢動物 ； 澄江生物群 ； 中國 ； 拿嬈蟲 ； 鱟蟲 ； 埋藏學