TAPHONOMY, SEDIMENTOLOGY, AND PALEOENVIRONMENTAL RECONSTRUCTION OF A UNIQUE TRICERATOPS SITE IN THE HELL CREEK, SOUTHEASTERN MONTANA

The latest Maastrichtian age Hell Creek Formation outcrops extensively in eastern Montana, western North Dakota, and South Dakota, and records the final stage of the Mesozoic Era. The Hell Creek contains a diverse and often well preserved Late Cretaceous vertebrate fauna. It is interpreted as being deposited on a broad floodplain, which bordered the western shoreline of the Cretaceous Western Interior Seaway that stretched from the Gulf of Mexico to the Arctic. The enclosing rock types vary throughout the formation and are dominated by unconsolidated siltstones, mudstones, and sandstones.

In the summer of 2005, the Burpee Museum of Natural History in Rockford, Illinois, sent a field crew to southeastern Montana to search for vertebrate fossils and to excavate previously located sites. In the waning days of the expedition, a crew member discovered the remains of what appeared to be a juvenile/sub-adult Triceratops. Excavation of the site the following summer revealed the remains of at least two individual sub-adult Triceratops specimens, totaling ~110 bone elements. The remains recovered, include skull elements, ribs, pectoral and pelvic girdles, and limb elements. While bonebeds of Ceratopsian dinosaurs are not uncommon in some late Cretaceous formations, this is the first known occurrence in the Hell Creek Formation and the first reported occurrence of more than one specimen of Triceratops at any single site. A detailed investigation of the site is needed to determine if the two individuals died together or if their remains were washed together following death. The bones were found in an organic rich mudstone that appears to be the result of an overbank flood. We present our preliminary findings of the taphonomic, stratigraphic, and sedimentological studies as well as the paleoenvironmental reconstruction based on fossil leaves and pollen collected from the site.