FAUNAL DYNAMICS OF THE LATE DEVONIAN BIODIVERSITY CRISIS: A COMPLEX INTERPLAY OF SPECIATION, EXTINCTION, AND BIOGEOGRAPHIC CHANGE

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. To unravel the faunal dynamics of this complex crisis, it is essential to examine shifting biogeographic ranges and speciation/extinction rates within an evolutionary context. In this analysis, phylogenetic analyses are used to constrain both timing and magnitude of biogeographic and diversity changes.

Four representative Devonian clades are included in this analysis: two articulate brachiopod lineages, Floweria and Schizophoria (Schizophoria), one bivalve clade, Leptodesma (Leiopteria), and the phyllocarid crustacean order Archaeostraca. Species-level phylogenetic hypotheses are used to constrain timing of speciation events using the ghost-lineage method. Speciation and extinction rates are calculated under a birth process model for the late Emsian to Tournaisian (Middle Devonian to earliest Mississippian) to assess rates operating during the both background and crisis intervals. Phylogenetic biogeographic analysis is used to examine the role of tectonic versus eustatic influences on evolutionary patterns and to characterize speciation events as resulting from vicariance or dispersal processes.

Results of speciation and extinction rate analyses indicate: (1) moderate rates of speciation in the Middle Devonian which decline to near zero in the Frasnian; (2) elevated extinction rates during the Givetian and Frasnian Stages. Extinction rates, while elevated during the crisis interval, do not exceed rates calculated for pre-crisis time. Thus, it is probable that speciation decline was the key determinant in net biodiversity loss. Speciation decline occurs, in part, due to biogeographic controls. Phylogenetic biogeographic analysis reveals an offset in timing between vicariance (Early to Middle Devonian) and geodispersal patterns (Late Devonian) that may indicate a fundamental change in the style of biogeographic patterns during the Middle Devonian. This shift may reflect earlier eustastic versus later tectonic controls on biogeographic patterns, possibly due to the relative intensity of Acadian orogenic events.