FIRST NORTH AMERICAN OCCURRENCES OF THE FOSSIL WOOD XENOXYLON MEISTERI FROM THE UPPER JURASSIC MORRISON FORMATION OF CENTRAL MONTANA

Two specimens of the fossil wood Xenoxylon meisteri were discovered in the Upper Jurassic Morrison Formation of central Montana. The first specimen, TUFA, consists of a partial log incorporated in a Morrison-aged tufa deposit on the western flank of Spindletop Dome, a Laramide anticline. The second specimen, SQTR, is located on the southern flank of the anticline 2.3 kilometers to the southeast. TUFA is stratigraphically 20 meters below the overlying Lower Cretaceous Kootenai Formation. SQTR is stratigraphically 26 meters below the Kootenai Formation.

The earliest fossils of Xenoxylon are from the Late Triassic of the Far East and Western Europe. The geographical range of the genus expanded through Asia and Europe during the Jurassic and Cretaceous periods. Only a few authenticated occurrences of Xenoxylon for North America have been reported. These few validated specimens of X. latiporosum are Cretaceous in age from the Canadian Arctic (Valanginian) and the North Slope of Alaska (Albian and Maastrichtian). X. meisteri has not been previously reported from North America and is only known from Russia, China, and Japan.

Previously reported occurrences of Xenoxylon from the Lower Jurassic Navajo Sandstone, Xenoxylon morrisonense, and X. moorei 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 characteristics incongruous with the genus.

Uniseriate, flattened bordered pits in contact with one another in earlywood tracheids and fenestrate cross-field pitting are diagnostic characters of the genus. Median bordered pit width/height ratios are 0.57 and 0.56 (TUFA and SQTR respectively). Median ray cell size for both specimens is 26.4 μm. Median ray heights differ between the two specimens (218 μm and 176 μm TUFA and SQTR respectively). Abrupt transitions from earlywood to latewood are characteristic of Xenoxylon; nevertheless, an abrupt transition is an insufficient character to assign fossils to the genus.

Evidence strongly suggests the genus Xenoxylon was constrained to cool/wet climates. These southernmost North American occurrences of wood assignable to Xenoxylon have significant paleoclimatic implications for the northern portion of the Upper Jurassic Morrison foreland basin.