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| The uppermost Ordovician–lowermost Silurian (Upper Katian–Rhuddanian) is surveyed with respect to the a- and ß-diversity of rhynchonelliformean brachiopods. The survey is based on new collections as well as existing literature, compiled in a large, georeferenced database. The brachiopod faunas are plotted using a Geographical Information System (GIS) and subjected to density analysis to display fluctuations within the different faunas through time. In addition, an analysis is performed on the preferred relative depth ranges of the brachiopod communities through the latest Ordovician–earliest Silurian crisis interval, following the concept of Benthic Assemblage zones. Both analyses support the view of a two-phased late Ordovician (Hirnantian) decline in diversity followed by a radiation in the early Silurian (Rhuddanian). These data show that the main taxonomic loss was geographically located on the peri-Laurentian terranes, in the Laurentian epicratonic seas and on the margins of the Ægir Ocean. Refuges during the survival interval were probably located in the shallow-water zones of especially Baltica, but also Gondwana, the peri-Laurentian terranes and the Kazakh Terranes. Except for Baltica, these refuges may to a large extent be obscured by the success of the Hirnantia fauna and, thus, may not contain many true survivor taxa. South China apparently operated as a refuge for the mid-shelf to deeper-water faunas. Avalonia, Baltica, Laurentia and South China experienced increased diversity in shallowwater settings in the recovery interval. Most regions experienced increased diversity among deeper-water faunas, although this appears to have been delayed until the late Rhuddanian on Laurentia. At this time the deeper-water faunas disappeared from the peri-Laurentian terranes (as did the terranes themselves) probably as a consequence of the progressively narrowing Iapetus Ocean. | |
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