Marine Ostracod Provinciality in the Late Ordovician of Palaeocontinental Laurentia and Its Environmental and Geographical Expression

BackgroundWe examine the environmental, climatic and geographical controls on tropical ostracod distribution in the marine Ordovician of North America. Methodology/Principal FindingsAnalysis of the inter-regional distribution patterns of Ordovician Laurentian ostracods, focussing particularly on the diverse Late Ordovician Sandbian (ca 461 to 456 Ma) faunas, demonstrates strong endemicity at the species-level. Local endemism is very pronounced, ranging from 25% (e.g. Foxe basin) to 75% (e.g. Michigan basin) in each basin, a pattern that is also reflected in other benthic faunas such as brachiopods. Multivariate (ordination) analyses of the ostracod faunas allow demarcation of a Midcontinent Province and a southern Marginal Province in Laurentia. While these are most clearly differentiated at the stratigraphical level of the bicornis graptolite biozone, analyses of the entire dataset suggest that these provinces remain distinct throughout the Sandbian interval. Differences in species composition between the provinces appear to have been controlled by changes in physical parameters (e.g. temperature and salinity) related to water depth and latitude and a possible regional geographic barrier, and these differences persist into the Katian and possibly the Hirnantian. Local environmental parameters, perhaps operating at the microhabitat scale, may have been significant in driving local speciation events from ancestor species in each region. Conclusions/SignificanceOur work establishes a refined methodology for assessing marine benthic arthropod micro-benthos provinciality for the Early Palaeozoic.

研究背景：本次研究聚焦北美奥陶纪海相地层内热带介形类（ostracod）的分布规律，探讨其受环境、气候与地理因素的调控机制。研究方法与主要发现：通过对奥陶纪劳伦大陆（Laurentia）介形类的区域间分布格局展开分析，重点关注晚奥陶世桑比阶（Sandbian，约4.61亿至4.56亿年前）的多样化石组合，结果显示介形类在物种水平上存在极强的地方性特有现象。各盆地的局地特有性极为显著，特有物种占比介于25%（如福克斯盆地Foxe Basin）至75%（如密歇根盆地Michigan Basin）之间，这一分布模式同样见于腕足类（brachiopods）等其他底栖生物群落。对介形类化石组合开展的多元排序分析（ordination）显示，劳伦大陆可划分为中陆省（Midcontinent Province）与南部边缘省（southern Marginal Province）两个生物地理分区。尽管这两个分区在双角笔石生物带（bicornis graptolite biozone）的地层水平上区分最为显著，但对全部数据集的分析表明，在整个桑比阶时期内，这两个生物省始终保持分界清晰。两省间的物种组成差异似乎受与水深、纬度相关的物理参数（如温度、盐度）变化，以及潜在的区域地理屏障所调控，且这种差异延续至凯迪阶（Katian）甚至可能的赫南特阶（Hirnantian）时期。局地环境参数（或许作用于微生境尺度）可能是推动各区域内祖先物种形成局地特有物种的重要因素。研究结论与意义：本研究建立了一套精细化的研究方法，用于评估早古生代海相底栖节肢动物微底栖生物的生物地理分区性。