黔北上奥陶统宝塔组头足类多样性初探

Biodiversity of Late Ordovician cephalopods from the Pagoda Formation in Northern Guizhou

上奥陶统宝塔组(桑比阶–凯迪阶)是华南分布广泛的标志性地层单元,以发育特殊的网纹状灰岩并产出大量头足类化石为特征。黔北桐梓县尧龙山地区的宝塔组厚度较大,出露极好,产出数量丰富、保存精美和特征典型的头足类化石。本文在研究区酒店垭、集镇采坑和天域极地3个宝塔组剖面采集到大量原域埋藏的头足类化石,共8属11种及少量属种未定类型,以直形壳或盘形壳、体管很小且适合远洋深水游泳的类型为主,整体上符合扬子台地深水陆棚Sinoceras–Michelinoceras–Disoceras生物相特征。基于壳体扩大率、壳径与体管直径比、气室高与隔壁颈长比、隔壁下凹度、壳径与气室高比等参数变量的主成分分析表明,在外部特征不显著的情况下,Sinoceras和Michelinoceras的直观差别是壳体扩大率,而Michelinoceras属不同种的差异主要体现在气室高与隔壁颈长之比,其他特征的区分度并不明显。层面上单位面积的化石定量统计显示,尧龙山地区宝塔组中头足类的多样性较高,并以Sinoceras和Michelinoceras具有极高的个体丰度为特征。扬子板块在奥陶纪桑比期–凯迪期之交处于高水位的陆表海环境,栖息环境均一化和生态系统复杂化可能是促进该时期头足类动物的复苏和多样性演化的重要原因。

The Upper Ordovician Pagoda Formation(Sandbian-Katian),a stratigraphic marker unit paleogeographically widespread in South China,is exemplified by its pervasive limestone with network structures throughout the formation and its abundant cephalopod fossils.The Pagoda Formation,with a thickness of around 25 meters,crops out extensively in the Yaolongshan area of northern Guizhou.It is perfectly exposed at the Jiudianya,Jizhen quarry and Tianyujidi sections in the study area.Here we report on a study of a relatively productive cephalopod fossil assemblage from which 11 species in eight genera and several taxa incertae sedis are documented.These cephalopod fossils,with ortho-or gyro-ceracones and small siphuncles,have long been cited as evidence for a pelagic migrant lifestyle and are consistent with the deep-water shelf Sinoceras–Michelinoceras–Disoceras cephalopoda biofacies in the Yangtze Platform during the Late Ordovician.Principal Component Analysis(PCA)based on the parametric variations including rate of conch tapering,diameter ratio of shell to siphuncle,ratio of camerae height to septal neck width,concavity of septa in camerae and ratio of conch diameter to camerae height shows that Sinoceras and Michelinoceras can be readily distinguished by the rate of conch tapering,and that species of Michelinoceras differ notably from each other in their camerae height and septal neck ratio.Other parametric variations,however,are limited in taxonomic classification.Quantitative analysis shows that the Pagoda cephalopod assemblage in Yaolongshan area has a relative higher biodiversity and is dominated in richness and abundance by large orthoconic genera,i.e.,Sinoceras and Michelinoceras.Potentially,a nutrient-rich deeper neritic environment and an increasing ecosystem complexity may give rise to the recovery phase of the cephalopods of the Yangtze Platform and their diversity climax during the Ordovician Katian age.