The Late Devonian was a time of profound evolutionary and environmental change associated with the Frasnian-Famennian biodiversity crisis including reduction in speciation rates, increased extinction rates, rampant species invasions, and ecosystem restructuring. The biodiversity crisis may have lasted as long as three million years with a final pulse of more severe crisis in the last few hundred thousand years of the Frasnian. To unravel the faunal dynamics of this complex crisis, it is critical to understand both the spatial and temporal patterns associated with biodiversity decline.

This analysis investigates the relationship between geographic range, speciation and extinction, and environmental and tectonic factors of pan-Laurentian brachiopod and bivalve species during the Givetian through Famennian stages (late Middle to terminal Devonian). Representatives from all major clades and paleocommunities were examined to provide a cross faunal comparison. Reconstruction of species ranges both spatially and temporally (at the level of conodont subzones) using GIS provide an understanding of the timing and extent of events such as species invasions into new tectonic basins and the importance of geographic range in determining species survival through the crisis interval.

At least four episodes of species invasion were determined from GIS analysis during the Late Devonian. These correspond to the beginning of the Frasnian, mid Frasnian, and two pulses in the late Frasnian, which relate to the onset and final stage of the biodiversity crisis. The first three pulses of invasion may also correlate to the start of T-R cycles IIb through IId of Johnson et al. (1985), while the last pulse may be associated with the second transgression within cycle IId. The invasion into new areas, and the concomitant expansion in geographic range, may also to confer an advantage in species survival through the crisis interval. Species that persisted into the Famennian have, on average, larger geographic ranges than those species that became extinct by the end of the Frasnian.