Although the Antarctic Sector of the Pacific appears little more than a windswept ocean with places of ornithological interest restricted to its periphery, it was not always so. The region was probably the cradle from which the early penguin/petrel stock began its evolutionary growth and diversification in the late Cretaceous and early Tertiary. That the penguins (Sphenisciformes) and the petrels (Procellariiformes) originated in the Late Cretaceous seems clear since none of the region's Tertiary penguin faunas show primitive features (e.g., Palaeeudyptes marplesi and Pachydyptes ponderosus from late Eocene at Kakanui, New Zealand (Simpson, 1975) and others from Australia, Patagonia and Seymour Island off the Antarctic Peninsula).

Harper (1978) proposed that the environmental trigger which helped initiate the mid-Tertiary radiation of the penguin and petrel faunas in the Pacific Sector was the major global cooling which began at the Eocene/Oligocene boundary (c. 38 m.yrs B.P.). Continuing crustal movements led to substantial climatic and oceanographic changes, the most pivotal of which was the northward displacement of the Australian continent and the development of the circumpolar Antarctic current during the late Oligocene 30-25 m.yrs B.P. (Kennett et al., 1975b). Changes in sea levels and land area in the New Zealand region through out the Tertiary and Recent (Fleming, 1979) created a range of marine habitats into which the oceanic birds diversified with considerable success. Recolonization of Antarctica by penguins, petrels, and skuas occurred only very recently, c. 6,500 years ago in McMurdo Sound, and a little earlier than this further north (Young, 1981).

According to Kennett et al. (1975b), the Polar Front became a prominent feature in the Pacific as long ago as the Miocene, so that its influence on the ecology and evolution of southern avifaunas has been important for some 15 million years. Penguins reached their maximum diversity during the Miocene and the sub-sequent extinctions, especially of the larger species as suggested by Simpson (1975) may have resulted from increasing competition from the rapidly-evolving marine mammals.

0 0

Post a comment