A new Late Cretaceous (late Campanian to early Maastrichtian) wood flora from southern Patagonia

Abstract

The Cerro Fortaleza Formation, in southernmost Patagonia (Argentina), contains a unique Late Cretaceous (Campanian) flora and fauna. This formation is characterized by lithified fluvial sands, overbank mud deposits, and paleosols 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 yet described.Fossil wood taxa from the Cerro Fortaleza Formation yields a diverse flora of gymnosperm wood but only two genera of angiosperm wood. Gymnosperm genera identified in this study include Agathoxylon, Planoxylon, Taxodioxylon, Cupressinoxylon, and Podocarpoxylon; and angiosperm genera 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 genera 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. The floral differences between these locations may be a relict from a widespread older flora that included Antarctica, regional floristic variations or a result of different depositional and/or taphonomic controls in discrete paleoenvironments.