Cephalopods from reef limestone of the Vasalemma Formation, northern Estonia (latest Sandbian, Upper Ordovician) and the establishment of a local warm-water fauna

The cephalopods of the reef limestones of the Vasalemma Formation, northern Estonia, are highly diverse and comprise 22 species belonging to 10 families and seven orders in a sample of &gt;300 specimens. Most of the specimens were collected from shell concentrations in synsedimentary cavities and are interpreted as parautochthonous, washed in from nearby habitats. Nearly all of the shells are fragmented and nearly 15% are partially encrusted by epibionts. The assemblage is dominated by small (mostly less than 30 mm wide), straight-shelled actinocerids and orthocerids; in addition, coiled tarphycerids are common. The high-level taxonomic composition of the Vasalemma cephalopod assemblage, with a dominance of actinocerids and an absence of endocerids, is in agreement with deposition in a warm-water (tropical or subtropical), shallow, subtidal regime. At the species level the assemblage is highly endemic, but the generic composition allows for a statistical comparison with other faunas. A cluster analysis of contemporary assemblages reveals a high degree of similarity with late Sandbian cephalopod faunas of epicontinental Laurentia. The palaeogeographical distribution pattern is similar to that of brachiopods, which supports earlier interpretations of these clusters as mainly controlled by water temperature and depositional depth. Several of the Vasalemma genera became conspicuous elements of epicontinental Laurentia during the Katian, which emphasizes that immigration towards Laurentia was an important factor in Late Ordovician diversity dynamics. Of the described taxa, the following are new: <i>Beloitoceras cautis</i> sp. nov., <i>Curtoceras abditus</i> sp. nov., <i>Hemibeloitoceras arduum</i> sp. nov., <i>H. molis</i> sp. nov., <i>Hoeloceras muroni</i> sp. nov., <i>Isorthoceras cavi</i> sp. nov., <i>I. maris</i> sp. nov., <i>I. padisense</i> sp. nov., <i>I. vexilli</i> sp. nov., <i>Ordogeisonoceras tartuensis</i> sp. nov., <i>Orthonybyoceras isakari</i> sp. nov., <i>O. moisense</i> sp. nov., <i>Pleurorthoceras organi</i> sp. nov., <i>Rummoceras rummuensis</i> gen. et sp. nov. and <i>Trocholites gennadii</i> sp. nov. http://zoobank.org/urn:lsid:zoobank.org:pub:E5211305-A5D0-4366-AAB1-08F96F817122

爱沙尼亚北部瓦萨莱马组礁灰岩中的头足类（cephalopods）物种多样性极高，在超过300件标本的采样中，共包含隶属于10个科、7个目的22个物种。绝大多数标本采自同沉积洞穴中的壳体堆积体，被认为是准原地埋藏（parautochthonous）的类群，由邻近生境经搬运后沉积于此。几乎所有壳体均已破碎，近15%的壳体表面部分被表栖生物（epibionts）所附着。该组合以小型（多数壳宽不足30毫米）直壳内角石类（actinocerids）和直角石类（orthocerids）为主；此外，旋壳的塔飞角石类（tarphycerids）也较为常见。瓦萨莱马组头足类组合的高级分类单元组成以内角石类占优且无内壳角石类（endocerids），与暖水（热带或亚热带）浅海潮下沉积环境的特征相符。在物种层面，该组合具有高度的地方性特有性，但属级组成可用于与其他动物群开展统计学对比。对同期组合的聚类分析显示，其与劳伦大陆（Laurentia）陆表海的晚桑比阶头足类动物群具有高度相似性。其古地理分布模式与腕足类（brachiopods）相似，这支持了此前关于此类聚类主要受水温和沉积深度控制的解释。瓦萨莱马组的多个属在凯迪阶（Katian）成为劳伦大陆陆表海的显著类群，这表明向劳伦大陆的移民是晚奥陶世生物多样性动态变化的重要影响因素。在所描述的分类单元中，以下为新发表类群：<i>Beloitoceras cautis</i> sp. nov.、<i>Curtoceras abditus</i> sp. nov.、<i>Hemibeloitoceras arduum</i> sp. nov.、<i>H. molis</i> sp. nov.、<i>Hoeloceras muroni</i> sp. nov.、<i>Isorthoceras cavi</i> sp. nov.、<i>I. maris</i> sp. nov.、<i>I. padisense</i> sp. nov.、<i>I. vexilli</i> sp. nov.、<i>Ordogeisonoceras tartuensis</i> sp. nov.、<i>Orthonybyoceras isakari</i> sp. nov.、<i>O. moisense</i> sp. nov.、<i>Pleurorthoceras organi</i> sp. nov.、<i>Rummoceras rummuensis</i> gen. et sp. nov. 以及<i>Trocholites gennadii</i> sp. nov.。相关分类单元的注册信息：http://zoobank.org/urn:lsid:zoobank.org:pub:E5211305-A5D0-4366-AAB1-08F96F817122