THE FOSSIL RECORD AND AVIAN INTERCONTINENTAL DISPERSALS DURING PERIODS OF CLIMATE CHANGE IN THE CENOZOIC

Currently warming global climates have impacted the geographic distribution and migratory habits of birds, with largely northern shifts to their distributions in the Northern Hemisphere. The Cenozoic paleontological record and phylogenetic relationships among terrestrial vertebrates demonstrates that various taxa have moved among the continents that comprise the Holarctic realm. The roles of climatic and environmental change in facilitating those biogeographic movements are still unclear. By adding avian data to these biogeographic questions, broader conclusions about the biological response to climate change can be formulated. Large-scale movements of terrestrial vertebrates among North America (NA), Europe, and Asia are known in the early Eocene with its associated hyperthermal events. Eocene birds from Ellesmere Island (Canada) include specimens of Gastornis and Presbyornis. Their occurrence at high latitudes close to hypothesized land bridges suggests that their dispersals (along with coeval mammals) were facilitated by warm climates. The extinct duck Protomelanitta occurs in 11-12 Ma sediments in Mongolia and Nevada (USA) and demonstrate another past intercontinental dispersal event close in time to the Mi-5 isotope event, and the dispersal of Hipparion horses from NA to Asia. The cooling climate and proposed lowered sea levels at that time might have aided vertebrate movement via an expanded northern land bridge. Furthermore, stiff-tailed ducks (Oxyurinae) reached NA slightly before or during the Middle Miocene Climatic Optimum. Grouse (Tetraoninae) are well-known components of the ‘Mammoth Steppe’ ecosystem in the Pleistocene. Revision of the oldest supposed tetraonine fossils demonstrates that they are incorrectly identified. Thus, the oldest fossils of the clade in NA are instead middle Pleistocene (~1 Ma), younger than specimens from Eurasia. New data suggest a possible dispersal near the onset of Quaternary glaciation or later. The Cenozoic record of avian intercontinental dispersals does not exhibit a pattern consistent with climate regime (both cooling and warming). Future study of dispersal events requires increased phylogenetic resolution and improved chronostratigraphic data on a global scale, in addition to new fossils.