GIS AND PHYLOGENETICS, A COMBINED APPROACH TO UNDERSTANDING BIOGEOGRAPHIC CHANGES IN THE LATE DEVONIAN

The Frasnian-Famennian (Late Devonian) biodiversity crisis is known to be a time of both pervasive species invasions and biodiversity decline. To unravel the faunal dynamics of this complex crisis, it is critical to understand both the spatial and temporal patterns associated with biodiversity decline. The connection between species invasions and biodiversity decline during the Late Devonian is examined in a cross-faunal analysis focused primarily on the brachiopod and bivalve components of the biota.

Species ranges are reconstructed using GIS temporally and spatially in time increments approximating conodont zones for the fauna of the epicontinental seas of North America during the Givetian through Famennian stages (late Middle to terminal Devonian). This provides a framework to interpret the timing and extent of events such as species invasions into new tectonic basins and the relative importance of geographic range in determining species survival through the crisis interval. Combining GIS analysis with phylogenetic information on species ancestry provides additional insight into Late Devonian faunal dynamics and the interplay between evolution and environmental factors.

Two major patterns are recovered when the whole fauna is analyzed: times of congruent geographic range expansion and invasion into new tectonic basins correspond to episodes of relative sea level rise; and species survival through the crisis interval appears tied to depositional environment, as species inhabiting relatively deeper water environments survive preferentially compared with shallow water species. These patterns are also retrieved among clades with known phylogenetic relationships, allowing the placement of these patterns into additional evolutionary contexts. Leptodesma (Leiopteria) and Schizophoria (Schizophoria), a bivalve and brachiopod subgenus, respectively, show speciation and extinction patterns that follow the general pattern observed for the whole fauna and permit examination of specific examples. Another pattern recovered from phylogenetic analyses is the reduction in vicariant speciation compared to speciation by dispersal during the Late Devonian. A pattern that may be partly explained by the interaction of environment and sea level mentioned above.