TRILOBITE BIOGEOGRAPHY IN THE MIDDLE DEVONIAN - GEOLOGICAL PROCESSES AND ANALYTICAL METHODS

Phylogenetic patterns of trilobite clades were used to deduce biogeogr aphic patterns during the Middle Devonian, a time of active plate coll ision between North America (Laurentia) and other plates, coincident w ith several major episodes of sea-level rise and fall. The mapping of biogeographic states onto phylogenies for asteropyginid and proetid tr ilobites indicated that during their history these trilobite clades of ten shifted the areas they occupied, and also underwent vicariant diff erentiation, followed by range expansion, followed by subsequent vicar iance. Biogeographic patterns in these individual phylogenies were eva luated and synthesized using a modified version of Brooks Parsimony An alysis, which is discussed. This method makes it possible using cladis tic methods to distinguish between episodes of vicariance and episodes of dispersal. Two types of dispersal are recognized herein: (1) the i ndividualistic responses of certain taxa in a single clade that cannot be generalized, i.e., traditional ad hoc dispersal, and (2) those pat terns of congruent range expansion that are replicated across several clades. The latter are not treated as true dispersal, expansion of a t axon's range over a barrier accompanied by diversification, but rather as a result of the temporary removal of barriers to marine taxa, due either to relative sea-level rise or to the collision of formerly disj unct plates. These are interpreted as changes in the structure of area s, and this type of dispersal is referred to as geo-dispersal. Geo-dis persal was found to have occurred in the Middle Devonian trilobite fau na of Eastern North America.Biogeographic analysis indicated that East ern North America is a strongly supported area, with the Appalachian a nd Michigan Basins as sister areas. Armorica and the Canadian Arctic a re also sister areas. Congruence was found between area cladograms pro duced by vicariance and dispersal analyses for Middle Devonian trilobi tes, suggesting that in some cases the geological processes governing vicariance, such as sea-level changes, were the same as those that cau sed dispersal.