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 elements in the Late Cretaceous Hell Creek Formation of Montana and North Dakota, USA. A previously published data base consisting of 649 individuals (counted at the family level) was used to establish statistically fundamental 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 fluvial architectural elements. Flood-plain and channel deposits preserve the preponderance of the Hell Creek dinosaur fauna. Articulated fossils most commonly occur within floodplain and point-bar deposits. Floodplain and related deposits, however, preserve the highest dinosaur faunal diversities. The dinosaur sample inferred to be most representative of the original dinosaurian assemblage structure, therefore, is obtained from floodplain and genetically related deposits. These yield eight families of dinosaurs represented in the following proportions: Ceratopsidae, 61%; Hadrosauridae, 23%; Ornithomimidae, 5%; Tyrannosauridae, 4%; Hypsilophodontidae, 3%; Dromaeosauridae, 2%; Pachycephalosauridae, 1%; and Troodontidae, 1%. Among these groups, dromaeosaurs and troodontids are represented only by teeth, a circumstance attributed at least in part to thin-walled bones whose potential for preservation in an active fluvial system is jeopardized. Ornithomimids constitute 5% of the total assemblage, which makes them the third most common dinosaur in this study. Their relatively high abundance may suggest a herbivorous dietary preference