DATING JANE: PALYNOLOGY DETERMINES STRATIGRAPHIC POSITION AND GEOLOGIC AGE OF A JUVENILE TYRANNOSAURUS REX

In 2002, an expedition from the Burpee Museum of Natural History excavated a nearly complete and well-preserved skeleton of a dinosaur from the Hell Creek Formation (Upper Cretaceous) in southeastern Montana. The dinosaur, a juvenile tyrannosaurid (probably Tyrannosaurus rex) now on display at the Burpee Museum, has been named Jane. The discovery site is an isolated outcrop of the Hell Creek Formation, uncertain in stratigraphic position. Leaf impressions are present in a weakly cross-bedded sandstone, a point-bar deposit on the surface of which the tyrannosaurid skeleton was discovered. A poorly sorted deposit of siltstone, sandstone, and claystone balls above the sandstone had buried the bones. The claystone balls yielded an assemblage of pollen, spores, and algal cysts characteristic of the Wodehouseia spinata Assemblage Zone (upper Maastrichtian), which is present in the Hell Creek Formation in eastern Montana and western North Dakota. Key species of the W. spinata assemblage indicate the stratigraphic position of Jane and, in conjunction with data from the plant megafossils and magnetostratigraphy, serve to determine the geologic age of the specimen. In southwestern North Dakota, 75 km northeast of the Jane site, the W. spinata Assemblage Zone is informally subdivided, based on the stratigraphic ranges of ten pollen species. Applying this model to the palynomorph assemblage recovered from the Jane site, the presence of Aquilapollenites collaris, A. conatus, and A. marmarthensis and the absence of Liliacidites altimurus and Porosipollis porosis indicate a stratigraphic position 28—35 m below the K/T boundary, which is dated at 65.51 Ma, and only 4—11 m below the polarity subchron reversal C30n-C29r, which is dated at 65.84 Ma. Fossil leaves collected from the Jane site, which are characteristic of megafloral zone HCIIb, corroborate the stratigraphic position indicated by the pollen and support the determination of a geologic age range of 65.9 to 66.0 Ma for the Hell Creek strata at the Jane site.