Evolution of Laurentian brachiopod faunas during the Ordovician Phanerozoic sea level maximum

Abstract

• The distribution of brachiopod faunas around Laurentia during the Ordovician period was influenced by the dynamicity of the surrounding palaeo-continents. Laurentia seemed to have been somewhat static, straddling the palaeo-equator, whereas the late Ordovician was characterised by the later stages of the closure of the Iapetus Ocean, with Baltica, eastern Avalonia and a variety of exotic terranes docking the southern and south-eastern coasts of Laurentia. In the earliest Ordovician, ‘primitive’ taxa that originated in the Cambrian were major components of brachiopod faunas. Later in the early Ordovician some major, first appearances (plectambonitoids) contributed to increase the diversity of the faunas, and by the end of the middle Ordovician, a very diverse fauna had conquered the continent: strophomenoids first appeared, plectambonitoids increased importance, and overall rhynchonelliform diversity exploded. During the tectonically active Ordovician, orogenies created geographical barriers that accentuated the faunal differences between peri-Laurentian terranes and cratonic assemblages. The late Ordovician diversification of brachiopods is the result of several gradual events spanning the Dapingian to the Sandbian. From the Sandbian to the Katian, the Scoto-Appalachian fauna, which is characteristic of terranes in the vicinity of the south-eastern margin of Laurentia, retained its peculiarity and did not invade the epicontinental seas of Laurentia. By the Hirnantian, two main groups, characterised by well-defined climatically controlled provinces, can be differentiated. Although the Hirnantian was marked by drastic extinction events, part of Laurentia (notably the intracratonic platform) may be identified as a place of hospitality for brachiopod faunas as many taxa thrived across the extinction boundary, into the Silurian period.