TECTONIC AND PALEOBIOGEOGRAPHIC SIGNIFICANCE OF THE CHATHAM ISLANDS, SOUTH-PACIFIC, LATE CRETACEOUS FAUNA

The Cretaceous fauna of the Chatham Islands, South Pacific, is essenti ally a tectonically controlled facies fauna, with origins relating to divergent plate motions and concomitant deposition of volcaniclastic, transgressive sediments in half-grabens in the Chatham Rise region dur ing the Late Cretaceous. Al least 60 macroinvertebrate (predominantly Mollusca) taxa and one Vertebrate have been recorded from the Kahuitar a Tuff (Campanian-lower Maastrichtian) of Pitt Island. The separation of the New Zealand subcontinent, including the Chatham Rise and Tasman Sea region, from the Marie Byrd Land sector of the Gondwana margin, a long with changing oceanic circulation, lowering sea-surface temperatu res and believed increasing temperature gradients, enhanced the distin ctiveness of the fauna, as reflected in the strong species-level endem icity of the fossil record. Shoaling from volcanic activity in the Cha tham Islands region created substrates suitable for colonization of a characteristic hardground community dominated by epifaunal suspension feeders (ca. 41%), followed by lower percentages of infaunal suspensio n feeders (ca. 30%) epifaunal browsers (ca. 14%), deposit feeders (ca. 8%), and carnivores (ca. 5%). The Kahuitara Tuff faunule is divided i nto four biogeographic groupings at genus-and subgenus-level: Indo-Pac ific/Tethyan (ca. 37%), cosmopolitan (ca. 34%), palaeoaustral (ca. 28. 5%), and endemic (ca. 8.5%). Palaeoaustral taxa are inclusive of endem ic groups, in accordance with C.A. Fleming's original ideas. These per centages suggest an overall relatively warm-water, semi-global biogeog raphic flavour at this taxonomic level. Some 43% of taxa from the Kahu itara Tuff are found in mainland New Zealand coeval faunas and about 4 1% are endemic to the Chatham Islands, differences being attributable largely to facies and to a much lesser degree to geographic isolation. At species level, endemic taxa of cosmopolitan or wide-ranging Indo-P acific/Tethyan and palaeoaustral genera/subgenera (83%) dominate, with negligible representation of widespread or cosmopolitan species and e ndemic species of endemic genera/subgenera. It is suggested that the K ahuitara Tuff faunule represents evolutionary divergence, reflecting r ange retractions of a former cosmopolitan, early to late Mesozoic worl d. Nearly all Kahuitara Tuff taxa are endemic to either the Chatham Is lands or mainland New Zealand. Similarities of the fauna with other co eval Faunas around the rim of the southern circum-Pacific are moderate to weak at genus-and subgenus-level, indicating a degree of provincia lism and isolation, especially at species level. The Chatham Islands f auna probably belonged to the short-lived, Campanian to Maastrichtian, Weddellian Biotic Province of W.J. Zinsmeister. The Kahuitara Tuff fa una evolved from a mixture of Austral and Boreal elements during the L ate Cretaceous with evidence of approximately 40% taxa having ancestor s in the Chatham Islands-New Zealand region of the Gondwana superconti nent. Changes in composition across the K-T boundary in the Chatham Is lands were dramatic with very few genus- and subgenus-level taxa in th e Kahuitara Tuff present in the Upper Palaeocene to Lower Eocene Red B luff Tuff. (C) 1997 Elsevier Science B.V.