Late Ordovician and Early Silurian virgianid and stricklandioid brachiopods from North Greenland: Implications for a warm-water faunal province

An unusually rich and diverse suite of virgianid brachiopods, hitherto poorly known, are systematically described here for the first time from the Ordovician–Silurian boundary interval (late Katian–Aeronian) of North Greenland. The Late Ordovician virgianids comprise typical taxa of the warm-water Tcherskidium fauna (e.g. Tcherskidium tenuicostatum, Proconchidium schleyi, Holorhynchus giganteus, and Deloprosopus dawesi sp. nov.). Among the early Silurian taxa, Virgiana hursti sp. nov. occurs as abundant shell beds, similar to other congeneric species in Laurentia, but has somewhat larger internal skeletal structures, albeit not as extravagantly developed as in the late Katian virgianids; Boraeloides balderi gen. et sp. nov. shows extreme thickening of shell wall and internal structures, approaching the extravagant calcification of Katian virgianids. The highly distinct mid-Aeronian stricklandioid brachiopod genus, Kulumbella, characterized by a shell with criss-cross (divaricate) ribbing, also occurs in North Greenland, represented by K. heimdalli sp. nov., which has the largest and most strongly biconvex shells for the genus. Palaeogeographically, the Late Ordovician virgianid fauna of Laurentia was highly distinct, confined to the low–mid tropical latitudes north of the palaeoequator. In comparison, the early Silurian (Rhuddanian) Virgiana and some related taxa in Laurentia spanned the tropics of both hemispheres, forming extensive shell beds in carbonate basins, although Borealis and Borealoides gen. nov. remained confined largely to the northern hemisphere, suggesting a certain level of provincialism extending into the earliest Silurian. The unusual abundance and richness of the virgianid faunas in North Greenland is likely explained by a palaeoecological preference for warm-water carbonate settings.

本文首次系统记述了产自北格陵兰奥陶纪-志留纪界线（Ordovician–Silurian boundary）地层中此前鲜有报道的维尔吉亚科腕足类（virgianid brachiopods），其类群异常丰富且多样。
晚奥陶世的维尔吉亚科类群包含暖水切尔基迪姆动物群（Tcherskidium fauna）的典型类群，如细肋切尔基迪姆（Tcherskidium tenuicostatum）、施莱前锥孔贝（Proconchidium schleyi）、巨型全吻贝（Holorhynchus giganteus）以及新种道氏德洛普罗索普斯贝（Deloprosopus dawesi sp. nov.）。
早志留世类群中，新种赫斯特维尔吉亚贝（Virgiana hursti sp. nov.）以富集的介壳层产出，与劳伦大陆（Laurentia）其他同属物种的产出特征类似，但其内部骨骼结构稍显宽大，不过并未如凯迪阶晚期（late Katian）维尔吉亚科类群般极度发育；新属新种鲍尔德鲍雷尔贝（Boraeloides balderi gen. et sp. nov.）的壳壁与内部结构极度增厚，接近凯迪阶（Katian）维尔吉亚科类群的夸张钙化程度。
极具辨识度的埃隆阶中期（mid-Aeronian）斯特里克兰德类腕足属库伦贝（Kulumbella），以具交叉（分叉，divaricate）壳纹的壳体为典型特征，同样发现于北格陵兰，其代表物种为新种海姆达勒库伦贝（K. heimdalli sp. nov.），该种为本属中壳体最大且双凸程度最强的类群。
从古地理学角度分析，劳伦大陆晚奥陶世的维尔吉亚科动物群特征鲜明，仅分布于古赤道（palaeoequator）以北的低—中热带纬度区域。与之相较，早志留世鲁丹阶（Rhuddanian）的维尔吉亚贝及劳伦大陆其他相关类群横跨南北两半球热带区域，在碳酸盐盆地中形成广泛分布的介壳层；但北格陵兰贝属（Borealis）与新属北奥勒贝（Borealoides gen. nov.）仍主要局限于北半球，这表明至早志留世初期仍存在一定程度的生物地理分区性（provincialism）。
北格陵兰维尔吉亚科动物群异常富集且多样的现象，大概率可由其对暖水碳酸盐沉积环境的古生态偏好得到解释。