Ecology, biofacies, biogeography and systematics of micromorphic lingulate brachiopods from the Ordovician (Darriwilian–Sandbian) of south-central China

Ordovician (Darriwilian to Sandbian) micromorphic linguliform lingulate brachiopods are described from the Guniutan Formation at the Fenxiang section in Hubei province, and the Maocaopu and Cili sections in Hunan province of south-central China, situated on the Yangtze Platform. A total of 7560 specimens from 155 limestone samples (within the interval of Lenodus variabilis – Pygodus anserinus biozones) are assigned to 22 species, representing a low taxonomic diversity and low abundance fauna. The fauna is dominated by the Acrotretoidea, mainly species of the Torynelasmatidae, with Torynelasma the most abundant (more than 40% of total number of specimens), along with the Eoconulidae and Eoconulus (18% of total) representing the second most common forms. Species of the Ephippelasmatidae are also common (16% of total) diverse, and include representatives of Myotreta and Numericoma, as well as Ephippelasma, whereas species of the Scaphelasmatidae are somewhat less common (13% of total). All three investigated sections represent outer shelf environments, but the Maocaopu section is situated in a relatively deeper position, in proximity of the south-eastern outer margin of the Upper Yangtze Platform, close to its boundary with the Jiangnan Slope. A quantitative analysis of the relative abundance data was carried out in order to investigate the biofacies distribution of the micromorphic brachiopod communities across the Yangtze Platform, something that has not been attempted before with Palaeozoic linguliforms. Six lingulate microbrachiopod communities could be recognized in the sections. The major biofacies shift, which occurred in the Cili section in the upper part of the Microzarkodina ozarkodella Zone, at the time of the onset and initial rise of the Mid Darriwilian Carbon Isotope Excursion (MDICE) suggests that these biofacies may have been depth controlled.

本文系统记述了产自中国中南部湖北省分乡剖面崮沱组（Guniutan Formation）、湖南省茅草坪（Maocaopu）与慈利（Cili）剖面（均位于扬子地台（Yangtze Platform））的奥陶纪（Ordovician）达瑞威尔阶（Darriwilian）至桑比阶（Sandbian）微型舌形贝型舌形腕足类（micromorphic linguliform lingulate brachiopods）。本次研究共纳入产自155块灰岩样品（对应多变伦多牙形石（Lenodus variabilis）-鹅形矮牙形石（Pygodus anserinus）生物地层带区间）的7560件标本，这些标本被归入22个物种，构成一个分类多样性与丰度均较低的生物群。该生物群以阿克雷托超科（Acrotretoidea）占绝对优势，主要类群为托氏贝科（Torynelasmatidae）物种，其中托氏贝属（Torynelasma）最为繁盛，占总标本数的40%以上；其次为始锥贝科（Eoconulidae）及其代表的始锥贝属（Eoconulus），占比约18%。鞍形贝科（Ephippelasmatidae）物种同样常见且多样性较高，包含肌贝属（Myotreta）、数壳贝属（Numericoma）以及鞍形贝属（Ephippelasma）的代表类群；而剑形贝科（Scaphelasmatidae）物种相对少见，占总标本数的13%。
本次研究涉及的3个剖面均形成于外陆棚沉积环境，其中茅草坪剖面沉积水体相对更深，紧邻上扬子地台东南外缘，临近其与江南斜坡（Jiangnan Slope）的分界处。为探究扬子地台范围内微型腕足类群落的生物相分布格局，研究人员针对相对丰度数据开展了定量分析——此前针对古生代舌形贝型腕足类尚未开展过此类研究。本次研究在各剖面中共识别出6个舌形微型腕足类群落。在微牙形石属（Microzarkodina）ozarkodella带上部，即中达瑞威尔阶碳同位素漂移（MDICE）起始与初始上升阶段，慈利剖面发生了显著的生物相更替，这表明该类生物相可能受水深调控。