TAPHONOMY AND SUGGESTED STRUCTURE OF THE DINOSAURIAN ASSEMBLAGE OF THE HELL CREEK FORMATION (MAASTRICHTIAN), EASTERN MONTANA AND WESTERN NORTH-DAKOTA

This study quantifies the taphonomic context of fossil dinosaur elemen ts in the Late Cretaceous Hell Creek Formation of Montana and North Da kota, USA. A previously published data base consisting of 649 individu als (counted at the family level) was wed to establish statistically f undamental associations between the vertebrate faunal elements and the fluvial architectural elements in which they were found. In the Hell Creek, preservation is not equally distributed among the various fluvi al architectural elements. Floodplain and channel deposits preserve th e preponderance of the Hell Creek dinosaur fauna. Articulated fossils most commonly occur within floodplain and point-bar deposits. Floodpla in. and related deposits, however preserve the highest dinosaur faunal diversities. The dinosaur sample inferred to be most representative o f the original dinosaurian assemblage structure, therefore, is obtaine d from floodplain and genetically related deposits. These yield eight families of dinosaurs represented in the following proportions: Cerato psidae, 61%; Hadrosauridae, 23%; Ornithomimidae, 5%; Tyrannosauridae, 4%; Hypsilophodontidae, 3%; Dromaeosauridae, 2%; Pachycephalosauridae, 2%; and Troodontidae, 1%. Among these groups, dromaeosaurs and troodo ntids are represented only by teeth, a circumstance attributed at leas t in part to thin-walled bones whose potential for preservation in an active fluvial system is jeopardized. Ornithomimids constitute 5% of t he total assemblage, which makes them the third most common dinosaur i n this study. Their relatively high abundance may suggest a herbivorou s dietary preference.