PRELIMINARY VERTEBRATE FAUNA OF THE MID CRETACEOUS BLACKLEAF FORMATION OF MONTANA

Although recognized nearly 90 years ago, the Cretaceous (Albian-Cenomanian) Blackleaf Formation of western Montana had until recently produced only fragmentary reptile bones. Discoveries from 2003-06 now document the presence of a moderately diverse vertebrate fauna. The Blackleaf Formation represents deposition within the western, proximal edge of the foreland basin and has been described as more terrestrial to the southwest. The vertebrate assemblage is consistent with this interpretation. Sparse northern localities yield primarily shark teeth, while 20 localities to the southwest produce a mix of freshwater and terrestrial forms. Here aquatic vertebrates (crocodilians, turtles, fish) and larger dinosaurs predominate in the lower portion of the formation. Large dinosaurs are represented only by isolated teeth. In contrast, two basal ornithopods dominate the assemblage of the upper Blackleaf. One small form represents a hypsilophodont akin to Zephyrosaurus and Orodromeus, while the other is a more advanced, slightly larger (femur length ~25cm) taxon. All identifiable bones and associated skeletons are assignable to one of these two taxa. This material occurs within green-gray, gray, and red-brown mudrocks with occasional horizons of nodular carbonate. Two localities include associated hypsilophodont skeletons preserved within large-diameter (~30 cm) burrows. Several varieties of eggshell likely represent additional dinosaur taxa. As suggested by Behrensmeyer and Hook (1992), well-drained paleosols may have favored preservation of small taxa. Although contemporaneous with the upper portions of the Cedar Mountain Formation, the Blackleaf vertebrate assemblage is most similar to the geographically proximal Wayan Formation of Idaho.