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An unusually rich and diverse suite of virgianid brachiopods, hitherto poorly known, is 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; Borealoides balderi gen. et sp. nov. shows extreme thickening of the shell wall and internal structures, approaching the extravagant calcification of Katian virgianids. The highly distinctive mid‐Aeronian stricklandioid brachiopod genus, Kulumbella, characterized by a shell with criss‐cross (divaricate) ribbing, also occurs in North Greenland, represented by K. heimdali 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. A palaeoecological preference for warm‐water carbonate settings would explain the unusual abundance and richness of the virgianid faunas in North Greenland. [ABSTRACT FROM AUTHOR]

ABSTRACT Middle Ordovician iron ooids in the Upper Yangtze region of South China are composed mainly of hematite and/or chamosite, found in mixed siliciclastic and carbonate successions, with hematitic ooids occurring in the west, and predominantly chamositic ooids in the east of the study area. In the three iron ooid‐bearing Middle–Upper Ordovician successions, 19 microfacies are recognized and grouped into eight facies associations, representing a shallow‐water mosaic comprising restricted and semi‐restricted lagoons, and open marine subtidal deposits interfingering with tidal flat and shoal facies. Hematitic ooids with well‐sorted and well‐rounded quartz grains formed in the transgressive shoal setting when the depositional environments changed from restricted lagoon to bioclast–quartz shoal and open marine subtidal. Episodic stasis and erosional intervals during transgression controlled the formation of hematite‐rich and mixed hematite–chamosite laminae within the cortices of hematitic ooids. In contrast, chamositic ooids formed in a semi‐restricted lagoonal environment, under long‐term transgressive condensation. Alternating episodes of relatively oxic conditions with thriving organisms and eutrophication‐driven anoxia resulted in the alternation of porous and dense laminae consisting mainly of chamosite in chamositic ooids. The stromatolite‐like cauliflower structures associated with chamositic ooids suggest microbial activity that promoted iron concentration, similar to the origin of approximately coeval iron‐rich oncoids in South China. Various iron ooid types demonstrate that these coated grains could form in a range of depositional setting and palaeooceanographic conditions on a generally shallow‐marine platform during the Ordovician. [ABSTRACT FROM AUTHOR]