Monday, July 6, 2015

Evolution of Large Flightless Birds

Looking at the current distribution of ratites (large flightless birds including ostriches, rheas, emus and cassowaries) and understanding the history of Continental Drift, one is inclined to assume that a common ancestor of these birds lived on Gondwanaland (the unified Southern Continents) after the breakup of Pangea and that they diversified as the component Continents split apart.  Indeed, this has been the accepted theory of ratite evolution since the dawn of plate tectonics.

Pangea, formed by the merger of all major land masses during the Permian Period, split into Laurasia (the Northern Continents) and Gondwanaland (the Southern Continents) during the Triassic Period (some 200 million years ago -MYA).  Africa (the current home of ostriches) and South America (now inhabited by rheas), broke from Gondwanaland about 140 MYA and split from one another 100 MYA.  Madagascar (the former home of the elephant bird, an extinct ratite) rifted from Africa 160 MYA, joining India-Antarctica-Australia; India-Madagascar split from Antarctica and Australia about 90 MYA.  Finally, Australia (now inhabited by emus and cassowaries) broke from Antarctica 55 MYA and Madagascar split from India 75 MYA, drifting back toward the southeastern Africa Coast.

Fossil and recent molecular evidence suggest that ratites evolved from flight-capable species about 60 MYA, soon after the demise of the dinosaurs.  This suggests that the various ratites evolved in geographic isolation though, apparently, from a common flying ancestor (or family of ancestors); no doubt, the fossils of early ratites also lie beneath the ice of Antarctica.  If nothing else, the unfolding natural history of these birds illustrates the fact that common sense, devoid of scientific evidence, can be misleading; nevertheless, I suspect that the mystery of ratite evolution has yet to be fully solved.