THE FIRST GLOBAL-SCALE PHYLOGENETIC NETWORK BIOGEOGRAPHY ANALYSIS OF LATE PALEOZOIC AND EARLY MESOZOIC TERRESTRIAL VERTEBRATES AND THE INFLUENCE OF MASS EXTINCTIONS ON ANIMAL DISTRIBUTION

The Triassic was a pivotal time in the evolution of life and is bracketed between the end Permian (PT) and end Triassic (TJ) mass extinctions. The presence of Pangaea complicates our understanding of biogeographic processes during this interval. It would mean few physical barriers to dispersal but climatic variation may have driven faunal provinciality. Overlap between contemporaneous faunas can be quantified as biogeographic connectedness (BC), the density of links in a taxon-locality network. Recent work has identified reduced BC in the Middle Triassic relative to the Late Permian in southern Gondwana. This was used to argue for breakup of a cosmopolitan ‘disaster fauna’, a result of the PT mass extinction, at this time. However, it is unknown if this local signal is representative of global trends and BC during the Early and Late Triassic has not been quantified. In addition, calculation of BC to-date has not included phylogenetic information.

We address these problems by employing a novel method including phylogenetic information in BC calculation by weighting links between taxa according to the phylogenetic distance between them. Bias due to increasing distance from the root through time is avoided by truncating the maximum length of branches prior to each time bin by a constant, k. This method is applied to a time-calibrated supertree and occurrence data of 1045 terrestrial amniote species, pooled into ten distinct biogeographic regions, to calculate global phylogenetic BC from the Late Permian through to the end of the Early Jurassic. Results are sensitive to variation of k, but consistent patterns emerge. BC increases over the PT boundary and remains high until the end of the Ladinian, longer than suggested by previous studies, suggesting discrepancy between local and global signals. BC then declines through the Late Triassic. An increase is observed across the TJ boundary, after which BC falls to return to Late Triassic levels by the end of the Early Jurassic.

An increase in cosmopolitanism is associated with both the PT and TJ mass extinctions. Whereas the TJ peak is short-lived, BC remains high for 25Ma following the more severe PT extinction event. This indicates that although biogeographic responses to mass extinctions in terrestrial faunas may be conserved, they are heavily influenced by extinction magnitude.