湖北武汉地区志留系下红层的古鱼类化石及其生物古地理意义*

依据在湖北武汉古姆山志留系浅海红层中新发现的古鱼类化石,确证了兰多维列统特列奇阶下部红层——志留系下红层在鄂东南地区广泛存在,并在前人工作基础上对武汉地区原坟头组进行了重新厘定,将其一分为二,上部仍为坟头组,下部则称清水组。新厘定后的坟头组以黄绿色、灰绿色、灰黄色泥岩、泥质粉砂岩、粉砂岩为主,富含腕足类、双壳类、腹足类、三叶虫和遗迹化石; 清水组下部以紫红色、红褐色泥岩、泥质粉砂岩为主,夹少量石英砂岩,上部以紫红色、黄绿色石英砂岩为主,夹泥岩、泥质粉砂岩、粉砂岩。在清水组上部新发现的鱼类化石包括盔甲鱼类锅顶山汉阳鱼(Hanyangaspis guodingshanensis)、意外洪山鱼(Hongshanaspis inexpectatus)、后棘江夏鱼(Jiangxialepis retrospina)及软骨鱼类中华棘鱼(Sinacanthus)、新中华棘鱼(Neosinacanthus)。这些新发现的古鱼均为张家界脊椎动物群温塘鱼类化石组合中的成员,其中,后棘江夏鱼可以跟产自江西武宁地区下红层清水组中的九江江夏鱼(J. jiujiangensis)直接对比,二者同属于真盔甲鱼目曙鱼科江夏鱼属,而曙鱼科成员则可以作为志留系下红层的标志性分子。根据鱼群面貌和地层层序,武汉地区的志留系下红层可以与皖、苏地区的侯家塘组、浙西北地区的唐家坞组、赣西北地区的清水组、湘西北地区的溶溪组以及新疆塔里木地区的塔塔埃尔塔格组进行很好的对比,其时代为志留纪兰多维列世特列奇期早期。华南板块志留系下红层的分布特征表明, 在特列奇期早期,上、下扬子海之间存在一狭长的浅海区域,这为盔甲鱼类在湖北武汉和江西武宁两地之间的扩散与交流提供了便利条件和可能。

Fossil fishes from the Silurian Lower Red Beds in Wuhan,Hubei and their biogeographic significances

Based on the new findings of fossil fishes from the Silurian shallow marine red beds at the Gumushan section,Wuhan,Hubei Province,the lower Telychian(Llandovery)red beds are confirmed to be developed in southeastern Hubei. The original Fentou Formation in Wuhan area is emended and subdivided into two parts: the upper part is the emended Fentou Formation,while the lower part is called Qingshui Formation. The redefined Fentou Formation is characterized by yellowish-green,gray-green,and gray-yellow mudstones,argillaceous siltstones,and siltstones bearing abundant brachiopods,bivalves,gastropods,trilobites,and trace fossils. The lower part of the Qingshui Formation consists of purple-red,reddish-brown mudstones and argillaceous siltstones intercalated with quartz sandstones,while its upper part is dominated by purple-red and yellowish-green quartz sandstones intercalated with mudstones,argillaceous siltstones and siltstones containing lots of fossil fishes. Fishes from the upper part of the Qingshui Formation include the galeaspids Hanyangaspis guodingshanensis,Hongshanaspis inexpectatus,Jiangxialepis retrospina,and the chondrichthyans Sinacanthus and Neosinacanthus,all belonging to the Wentang Fish Assemblage of the Zhangjiajie Vertebrate Fauna. The galeaspid J. retrospina can be compared directly to J. jiujiangensis from the Qingshui Formation in Wuning area,Jiangxi Province,as members of the genus Jiangxialepis assignable to the family Shuyuidae,whose members are elements of the Silurian Lower Red Beds in South China. According to comprehensive analysis of the fish assemblage and stratigraphic sequence,the Lower Red Beds in Wuhan,with an age of early Telychian,can be correlated with the Houjiatang Formation in Anhui and Jiangsu,the Tangchiawu Formation in Zhejiang,the Qingshui Formation in Jiangxi,the Rongxi Formation in Hunan,and the Tataertag Formation in Tarim Region. In addition,the distribution pattern of the Silurian Lower Red Beds in the South China Block indicates that the Upper and Lower Yangtze seas were connected by a narrow shallow epeiric water during the early Telychian,which probably provides a precondition for the galeaspids dispersing and communicating between Wuning of Jiangxi and Wuhan of Hubei.