ABSTRACT
The earliest rhynchonellids, Rostricellula, Dorytreta, Ancistrorhyncha, and Sphenotreta, evolved during Middle Ordovician (Llanvirn) time and became widespread in shallow marine environments of Laurentia, Siberia, Kazakhstan, and other paleoequatorial continents. Intercontinental migration of these precursor rhynchonellids is interpreted to have been facilitated by dispersal through island faunas as stepping stones during sealevel lowstand, and reduced oceanic deep-water barriers. During the Caradoc, rhynchonellids began to show provincialism. Among the five genera (Rhynchotrema, Oligorhynchia, Drepanorhyncha, Orthorhynchula, and Orthorhynchuloides) that emerged in Laurentia, only Rhynchotrema occurred widely in other paleocontinents. This marked the last successful intercontinental migration of rhynchonellids during the Middle and Late Ordovician. Provincialism became most pronounced in the latest Caradoc and early Ashgill, when three rhynchonellid genera (Hiscobeccus, Lepidocyclus, and Hypsiptycha) evolved and spread rapidly to nearly every part of Laurentia but rarely succeeded in migrating to other continents. Elsewhere, only sporadic and questionable Lepidocyclus and Hypsiptycha have been reported from Baltica and Kazakhstan. Similarly, the Siberian genera Lepidocycloides and Evenkorhynchia rarely established themselves in other continents: only one occurrence of Lepidocycloides is known from the western margin of Laurentia (Canadian Rocky Mountains). Onset and peak of rhynchonellid provincialism coincided with Caradoc-early Ashgill sealevel rise, which may have caused drowning of island faunas, enhance by further separation of Laurentia and Siberia. By the latest Ordovician, intercontinental mixing of taxa increased, paradoxically as a result of substantial sealevel drop and closure of the lapetus Ocean, as shown by distribution of Plectothyrella, which became widespread as a common element of the Hirnantia fauna.
