Tectonic and palaeobiogeographic significance of the Chatham Islands, South Pacific, Late Cretaceous fauna

Abstract

Abstract The Cretaceous fauna of the Chatham Islands, South Pacific, is essentially 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 during the Late Cretaceous. At least 60 macroinvertebrate (predominantly Mollusca) taxa and one vertebrate have been recorded from the Kahuitara 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, along with changing oceanic circulation, lowering sea-surface temperatures and believed increasing temperature gradients, enhanced the distinctiveness of the fauna, as reflected in the strong species-level endemicity of the fossil record. Shoaling from volcanic activity in the Chatham 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 suspension feeders (ca. 30%), epifaunal browsers (ca. 14%), deposit feeders (ca. 8%), and carnivores (ca. 5%). The Kahuitara Tuff faunule is divided into four biogeographic groupings at genus- and subgenus-level: Indo-Pacific/Tethyan (ca. 37%), cosmopolitan (ca. 34%), palaeoaustral (ca. 28.5%), and endemic (ca. 8.5%). Palaeoaustral taxa are inclusive of endemic groups, in accordance with C.A. Fleming's original ideas. These percentages suggest an overall relatively warm-water, semi-global biogeographic flavour at this taxonomic level. Some 43% of taxa from the Kahuitara Tuff are found in mainland New Zealand coeval faunas and about 41% 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-Pacific/Tethyan and palaeoaustral genera/subgenera (83%) dominate, with negligible representation of widespread or cosmopolitan species and endemic species of endemic genera/subgenera. It is suggested that the Kahuitara Tuff faunule represents evolutionary divergence, reflecting range retractions of a former cosmopolitan, early to late Mesozoic world. Nearly all Kahuitara Tuff taxa are endemic to either the Chatham Islands or mainland New Zealand. Similarities of the fauna with other coeval faunas around the rim of the southern circum-Pacific are moderate to weak at genus- and subgenus-level, indicating a degree of provincialism and isolation, especially at species level. The Chatham Islands fauna probably belonged to the short-lived, Campanian to Masstrichtian, Weddellian Biotic Province of W.J. Zinsmeister. The Kahuitara Tuff fauna evolved from a mixture of Austral and Boreal elements during the Late Cretaceous with evidence of approximately 40% taxa having ancestors in the Chatham Islands-New Zealand region of the Gondwana supercontinent. Changes in composition across the KT boundary in the Chatham Islands were dramatic with very few genus- and subgenus-level taxa in the Kahuitara Tuff present in the Upper Palaeocene to Lower Eocene Red Bluff Tuff.