--> Late Silurian-Early Devonian Paleogeography Part 4 - Explaining Devonian Brachiopod Provinciality and Distributions

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Late Silurian-Early Devonian Paleogeography Part 4 - Explaining Devonian Brachiopod Provinciality and Distributions

Abstract

Based largely upon the work of Boucot, Johnson and Talent (1967) it has been recognized that during most of Silurian time, ‘cosmopolitan’ brachiopod genera were much the same everywhere. Beginning in latest Silurian, however, isolation of eastern America began to allow brachiopod evolution there to diverge from evolutionary developments elsewhere. Upon the North American craton west of the structural high referred to as the Continental Backbone, and in most other areas, brachiopod cosmopolitanism continued. However, in eastern North American, isolation continued through Early Devonian, and the distinct character of the ‘Appalachian Provincial Fauna’ intensified. Johnson (1970), in Taghanic Onlap and the end of North American Devonian Provinciality (GSA bulletin 81, p 2077-2106) attributed resumption of a world-wide cosmopolitan brachiopod fauna in Late Devonian to rising sealevels. This implies, of course, that the cause of Appalachian Provincialism had been the falling sealevels of Latest Silurian and Early Devonian. Brachiopods, as benthic marine organisms, are restricted to relatively shallow waters their dispersal patterns necessarily follow shorelines, and their spread into new areas would be blocked both by deep water and emergent land areas. With severely falling sealevels, newly emerging structural highs constituted increasingly formidable barriers to brachiopod dispersal. Eventually, a complete isolation of eastern America was attained. Johnson reluctantly suggested that water temperature boundaries may have also played a role. Referencing studies of faunas along the western coast of the Americas, he noted that equatorial faunas could be dispersed only so far north or south before encountering waters too cold to allow their survival. Another very important aspect of these authors’ work is that the Appalachian Provincial Fauna has also been recognized in northeastern Mexico, Columbia and Venezuela. Presented herein is a paleo-reconstruction of the positions of continental landmasses that meets paleo-latitude requirements of experts working in this field, but has the advantage of also explaining the isolation and extent of the Eastern America Realm. Because the proposed configuration presents a ‘Mediterranean’ type sea in eastern America, it also eliminates the need for water temperature boundaries because the area of isolation is surrounded by landmasses.