LATE CRETACEOUS WOOD FROM SOUTHERN PATAGONIA AND PALEOBIOGEOGRAPHIC IMPLICATIONS FOR SOUTH AMERICA AND ANTARCTICA

The Cerro Fortaleza Formation, in southern-most Patagonia (Argentina), contains a unique Late Cretaceous (Campanian) flora and fauna. This formation is characterized by lithified fluvial sands, overbank mud deposits, and palaeosols deposited in fluvial, fluvial-palustrine and coastal plain environments from the northeastern margin of the Austral (Magellanes) Basin. The overlying and underlying formations have been dated as Campanian and Maastrichtian, respectively. Therefore, the Cerro Fortaleza Formation putatively falls within the Late Campanian-Maastrichtian. This formation is known for its diverse fauna including dinosaurs, fishes, and turtles. Furthermore, poorly preserved leaf impressions from indeterminate conifers and cycads have also been discovered but not described.

Fossil wood samples have been collected and thin-sectioned from the Cerro Fortaleza Formation. Gymnosperm morphogenera identified in this study include Agathoxylon, Planoxylon, Taxodioxylon, Cupressinoxylon, and Podocarpoxylon; and angiosperm morphogenera identified include Hedycaryoxylon and Nothofagoxylon. This is the first record of Planoxylon, Taxodioxylon, Cupressinoxylon and Hedycaryoxylon from Argentina. Additionally this is the oldest occurrence of Nothofagoxylon in Argentina. Both the angiosperm and gymnosperm wood samples possess distinct growth rings, providing strong evidence for seasonal growth regimes in the region. All of the wood morphogenera from the Cerro Fortaleza Formation, except Planoxylon, have also been described from Late Cretaceous sediments of the Antarctic Peninsula. Thus, the presence of these taxa in both regions supports Late Cretaceous plant dispersal between them. Despite sharing the same taxa, the floras from the Cerro Fortaleza Formation and the Antarctic Peninsula exhibit strikingly different relative abundances. The ratio of gymnosperm to angiosperm wood in the Cerro Fortaleza Formation is 75:25; whereas, coeval floras from the Antarctic Peninsula are ~25:75. This floristic differentiation could be a function of regional floristic variations or a result of different depositional and/or taphonomic controls in discrete paleoenvironments.