Patterns of Late Cretaceous ammonite biogeography in southern high latitudes: the family Kossmaticeratidae in Antarctica

A refined biostratigraphy of the Antarctic kossmaticeratids from the James Ross Basin is contrasted against other Upper Cretaceous sections around sou thern Gondwana to inspect for possible biogeographic patterns. In the 3-km- thick Marambio Group two major unconformities are recognized at the base of the upper Campanian and Maastrichtian, respectively. The unconformities di vide the succession into a Santonian-lower Campanian Sequence (NS); an uppe r Campanian-lower Maastrichtian Sequence (NGS); and a Maastrichtian-Danian Sequence (MGS). Natalites in the NS; Neograhamites-Gunnarites in the NGS; a nd Maorites-Grossouvrites in the MGS are the most abundant kossmaticeratid ammonites, represented in parts by hundreds or thousands of specimens. Comp arison of the Antarctic fauna with that of eastern South Africa, Madagascar , southern India, Australia, New Zealand and southern South America suggest s three diversification phases for the Kossmaticeratinae and Brahmaitinae, occurring in different places at different times in southern Gondwana. Earl y diversification of the Kossmaticeratinae started in the Santonian of east ern Gondwana and continued into the early Campanian of southern high latitu des. Five kossmaticeratid genera, including the relatively long-ranging Mao rites and Grossouvrites, and a low-endemic ammonite fauna are recorded. The NS/NGS unconformity marks a major faunal change and the early disappearanc e in Antarctica of several mollusc groups that are known to range into the Maastrichtian in lower latitudes. Within the NGS, a new late Campanian-earl y Maastrichtian diversification phase took place in southern high latitudes and five Kossmaticeratinae, mostly endemic to the Weddellian Province, wer e added. Finally, the Maastrichtian NGS/MGS unconformity marks a drastic re duction of Weddellian Kossmaticeratinae, with only Grossouvrites and Maorit es reaching the topmost Maastrichtian in Antarctica. By contrast, the Maast richtian of eastern Gondwana saw a final diversification phase, with three new Brahmaitinae (present also in the Northern Hemisphere) and the persiste nce of five Campanian-early Maastrichtian Weddellian Kossmaticeratinae. The pattern of successive low endemism in the early Campanian NS, earlier excl usion of lower latitude Maastrichtian groups and higher endemism in the lat e Campanian-Maastrichtian NGS, and drastic diversity decrease in the Maastr ichtian MGS, is consistent with known calcareous microfossil biogeography a nd long-term cooling in southern high latitudes. During the final Maastrich tian diversification, the kossmaticeratids spread toward lower latitudes as a stenothermal group of ammonites moving toward their preferred seawater t emperature. (C) 2000 Academic Press.