DINOSAURS FROM MICROVERTEBRATE SITES IN THE UPPER CRETACEOUS FRUITLAND AND KIRTLAND FORMATIONS, SAN JUAN BASIN, NEW MEXICO

Vertebrate microfossil sites in the Fruitland and Kirtland Formations exposed in the Bisti/De-na-zin Wilderness and Fossil Forest Research Natural Area of the San Juan Basin, New Mexico include a diverse assemblage of dinosaurs, represented primarily by isolated teeth. The Fruitland and lower Kirtland localities (Hunter Wash, Fossil Forest, and Willow Wash local faunas) include hadrosaurids, ceratopsids, tyrannosaurids, and small, toothed theropods. Small theropods can often be identified at least to the level of genus and include, in decreasing order of abundance, Saurornitholestes, Paronychodon, Troodon, Richardoestesia, and Dromaeosaurus. These assemblages resemble those of late Campanian sites of the northern Rocky Mountain region. However, in northern assemblages, Dromaeosaurus tends to be relatively much more abundant.

At the top of the Kirtland Formation, the Naashoibito Member (Alamo Wash local fauna) has yielded teeth of, in decreasing order of abundance, ceratopsids, titanosaurids, hadrosaurids, tyrannosaurids including cf. T. rex, and species of Troodon and Richardoestesia distinct from those of from the Fruitland and lower Kirtland Formations. This assemblage closely resembles faunas described from late Maastrichtian (Lancian) localities of the northern Rocky Mountain region, except for the presence of sauropods, and supports a Lancian age for the Alamo Wash local fauna. This is in agreement with an age assignment based on mammals from the same microfossil sites.

Differences in dinosaur faunal composition between the Naashoibito Member and underlying sediments are consistent with the mammalian biostratigraphy of these units. When compared to contemporaneous northern dinosaur assemblages, those of the San Juan Basin are somewhat different in taxonomic composition and relative abundance of dinosaur taxa, probably reflecting biogeographic heterogeneity within the Late Cretaceous Western Interior.