STEPPED MIGRATIONS AND THE DEVELOPMENT OF THE FRASNIAN (UPPER DEVONIAN) COSMOPOLITAN BRACHIOPOD (SPIRIFERID) FAUNAS IN CENTRAL AND WESTERN NORTH AMERICAN CARBONATE PLATFORMS

By the end of the Middle Frasnian North American brachiopod faunas were remarkably cosmopolitan throughout the tropics of Euramerica. Sea level changes during the Frasnian controlled the timing of migration and speciation events in spiriferoid brachiopod clades in central and western North American carbonate platforms. Following the latest Givetian sea level rise of Devonian Transgressive-Regressive (T-R) Cycle IIb-1 and subsequent Early Frasnian sea level highstand spiriferoid brachiopod faunas were dominated by the widely dispersed spiriferids including Eleutherokomma, Allanella, Acutatheca, Eosyringothyris (Iowa Basin only) and Orthospirifer. Spiriferinids Cyrtina and Tecnocyrtina are widespread. Allanella, Eosyringothyris, and Tecnocrytina are extinct by the end of the Early Frasnian.

Middle Frasnian sea level rise allowed migrations of cyrtospiriferids into western and central North American shelves. These include Tenticospirifer, Regelia (endemic to NT of Canada) and distinctive species groups of Cyrtospirifer. Conispirifer migrated into reef platforms in western Canada. Iowa Basin Orthospirifer migrated into the Appalachian Basin, and Platyrachella (Iowa Basin only) evolved from Orthospirifer. Species of Eleutherokomma occur in western and central (Iowa Basin) North American shelves. A second seal level rise (T-R cycle IIb-2) led to the migration of Cyrtospirifer into the Iowa Basin, and Rigauxia migrated from European shelves into western North American shelves at this time.

The most significant sea level rise of the Devonian (T-R cycle IId-1) lead to peak cosmopolitanism of shelf brachiopod faunas in the Late Frasnian. Elita and Tylothyris dispersed into carbonate platforms at this time. Acutatheca is widespread in equatorial and subtropical shelves. New immigrants include Thomasaria, and Theodossia (Canada only). Two distinct groups of Cyrtospirifer occur in most North American basins. Conispirifer and Pyramidaspirifer expanded their ranges from equatorial to subtropical shelves, and Regelia migrated into eastern North America from the equatorial tropics. The Lower Kellwasser Event (LKE) led to the collapse in diversity of spiriferids in subtropical shelves where post-LKE survivors include Cyrtospirifer, Conispirifer, and Tylothyris. Equatorial faunas feature Cyrtospirifer, Conispirifer, Pyramidaspirifer, and Theodossia. All are extinct prior to or during the Upper Kellwasser Event, except Cyrtospirifer and Tylothyris, that occur in post-extinction faunas.