First occurrence of the boreal fossil wood Xenoxylon meisteri from the Jurassic of North America: Morrison Formation of central Montana, USA
Résumé
Six specimens of the fossil wood Xenoxylon meisteri were discovered in the Upper Jurassic Morrison Formation of central Montana and are described herein. The discovery represents the first confirmed occurrence of the species X. meisteri in North America. In addition, we review all occurrences of petrified wood attributed to Xenoxylon from North America. Early to Late Jurassic X. phyllocladoides specimens of Greenland are accepted. Previously reported occurrences of Xenoxylon from the Lower Jurassic Navajo Sandstone of Utah, Xenoxylon morrisonense Medlyn and X. moorei Tidwell from the Upper Jurassic Morrison Formation of Utah, and fossil woods reported from the Lower Cretaceous Cloverly (Wyoming) and McMurray Sandstone (Alberta, Canada) formations have secondary xylem characteristics incongruous with the genus and are discounted. The remaining accepted specimens are Cretaceous in age from the Canadian Arctic (Valanginian) and the North Slope of Alaska (Albian and Maastrichtian). Thus, these new Montana specimens of Xenoxylon are the earliest and southernmost occurrences of the genus in continental North America. Global distribution and paleoecology strongly imply Xenoxylon was constrained to cool/wet climates. Thus, these lower latitude North American occurrences corroborate other proxies indicating a cooler wetter climate, at least locally/briefly, in the northern portion of the Upper Jurassic Morrison foreland basin.