川南布拖县乌科志留纪早期介形类丽足介目(Myodocopida)

MYODOCOPIDA(OSTRACODA)FROM THE EARLY SILURIAN IN WUKE AREA OF BUTUO,SOUTHERN SICHUAN PROVINCE

乌科剖面位于四川布拖县乌科乡境内。在此剖面的石门坎组上碎屑岩段中产有相当丰富的介形类丽足介目,特别是球茎介科的化石。经研究、鉴定,初步划分出2个属和6个种,其中包括3个新种(Entomozoe bispinosa sp.nov.,E.suboblongasp.nov.,E.wukeensis sp.nov.),1个比较种和2个未定种。在分析上述属、种,特别是相关种的地质、地理分布特征的基础上,并参考其他门类化石(如腕足类动物群)和岩石性质,提出石门坎组上碎屑岩段可能要低于笔石M.crispus带上部,大致相当于志留纪兰多维列世特列奇期早期。Entomozoe和球茎介科的最早代表推测是产自石门坎组上碎屑岩段Entomozoe的那些分子,如E.suboblongasp.nov.,而非E.tuberosa(Jones),1861(或其亲缘种E.aff.tuberosa)(Siveter et al.,1990,2006)。文中还就上述丽足介目动物群的生态环境进行了分析,指出石门坎组上碎屑岩段可能是在陆棚浅水到相对较深水、能量较低的、温暖的开阔海环境下沉积的。

The Upper Clastic Member of the Shimenkan Formation at the Wuke section in Wuke area of Butuo coun- ty, southern Sichuan consists mainly of yellow-green calcarious, argillaceous siltstones and mudstones. The member yields relatively abundant fossils, including ostracods, brachipods, trilobites, gastropods, bivalves, conulariids, etc. Ostracoda contains myodocopids and a few non-palaeocopids. Myodocopids comprise six species, of which five species [Entomozoebispinosa sp. nov. , E. suboblonga sp. nov. , E. wukeensis sp. nov. , E. cf. decora Wang et Siveter sp. nov. （MS）, Entomozoe sp. ] are assigned to the Bolbozoidae and one species （Paraentomozoe sp. ） to the Entomozoi- dae. The myodocopid fauna from the Ningqiang Formation and upper Xiushan Formation is composed of boIbozoids and entomozoids, of them E. tuberosa （Jones）, 1861 （or E. aff. tuberosa） is a main element. E. tuberosa （or E. aff. tuberosa） is known from the Wether Law Linn Formation in Pentland Hill of Scotland, the Samuelson HOj For- mation of North Greenland and Sredninskaya Formation of Severnaya Zemlya in Russian Arctic respectively （Siveter et al. , 1990, 1999, 2006）, with a range of the graptolite upper M. crispus Zone to M. crenulata Zone. We infer that the Ningqiang Formation and upper Xiushan Formation, particularly the E. tuberosa （or E. aff. tuberosa）-bearing beds of the Ningqiang Formation and upper Xiushan Formation may be correlated approximately to the upper M. cris- pus Zone to M. crenulata Zone, representing the middle and late Telychian. The myodocopid fauna from the Upper Clastic Member of Shimenkan Formation differs from that of Ningqiang Formation （or upper Xiushan Formation） be- cause of the absence of E. tuberosa （or E. aff. tuberosa）, probably representing different age from the latter. The age of Upper Clastic Member of Shimenkan Formation may be older than the upper M. crispus Zone Esimilarity of as- sociated biota （for example, brachiopod fauna） and lithological characters to those of the lower Xiushan~, approxi- mately corresponding to the early Telychian. There{ore, the first appearance of Entomozoe or Bolbozoidae in China may also be earlier than the upper M. crispus Zone （or the middle and late Telychian age ）. Palaeoecologically, the myodocopid fauna from the Ningqiang Formation and the upper Xiushan Formation con- sists mainly of endemic and benthic taxa and may mainly inhabit an open shelf-marine area, including shallow to rela- tively deeper water invironments. This fauna may also be preserved in the environments with a lower energy condition because 1） some specimens of this fauna are preserved as ＂butterfly-shaped＂ partten~ and 2） in general, myodocopids with carapace consisiting mainly of chitin can not be preserved as fossils （including internal and external moulds） in near-shore, shallow-water and high-energy areas as seen in the Devonian and Early Carboniferous strata. The Upper Clastic Member of Shimenkan Formation is generally comparable with the Ningqiang Formation （or the upper Xiushan Formation） in sedimentary environments because both resemble in the ostracod eco-assemblage, lithological charac- ters and associated biota.