ON THE STRATIGRAPHIC DISTRIBUTION OF TRICERATOPS IN THE LATEST MAASTRICHTIAN DEPOSITS OF THE UPPER HELL CREEK FORMATION IN GARFIELD COUNTY, MONTANA

The horned ornithischian dinosaur Triceratops is known to be the most abundant genus found in the terrestrial sediments of the latest Maastrichtian Upper Hell Creek Formation. Field work within the Hell Hollow and Trumbo Ranch USGS Quadrangles in eastern Montana over the past three years has yielded the discovery of thirty-six individual Triceratops localities, with a stratigraphic range spanning 60 m. Recording coordinates and elevation with a handheld GPS unit, and applying Steno's Principle of Superposition, the oldest specimen was located at 819.3 m above sea level while the youngest specimen was located at 879.3 m. Slightly over half (19 of 36) of the individual Triceratops were recorded between 838.1 and 853.4 m above sea level. Occurring within this 15 m “population peak,” the average stratigraphic position for Triceratops is at 35 m below the K/T Boundary. The average stratigraphic position of all thirty-six Triceratops localities is 849.1 m above sea level, with a standard deviation of 13.6 m. One third of the specimens occur above the population peak, within the top 30 m of the formation, which is also half of the total stratigraphic range. Preserving a decrease in two thirds of the sampled population occurring above the population peak, this data is regionally indicative of a gradual, rather then cataclysmic, extinction of Triceratops at the end of the Maastrichtian.