A PRELIMINARY DESCRIPTION OF THE FIRST CENTROSAURUS APERTUS (CERITOPSIDAE) BONE BED FROM THE OLDMAN FORMATION (CRETACEOUS, CAMPANIAN) OF SOUTH-EASTERN ALBERTA WITH IMPLICATIONS FOR THE LOCATION OF CERATOPSIAN NESTING SITES

The Belly River Group (Late Cretaceous) of southern Alberta consists of three formations; in ascending order, the Foremost, Oldman, and Dinosaur Park formations. The upper Oldman and Dinosaur Park formations represent a transgressive period of the Bearpaw Transgression-Regression sequence. A regionally extensive diachronous discontinuity exists between the more-inland Oldman and more-coastal Dinosaur Park formations that preserve more xeric and mesic environments, respectively. We report here a preliminary description of a monotaxic ceratopsid bone bed from the Oldman Formation that contains an unusually high percentage of juvenile and sub-adult-sized specimens. Based on mostly fragmentary diagnostic cranial material, the centrosaurine present can be identified as Centrosaurus apertus. The locality also preserves a small (<5%) amount of theropod material (mostly teeth) that are tentatively identified as Daspletosaurus sp. and Saurornitholestes sp. Additional material includes the remains of hadrosaurs, indeterminate reptiles, and fish. Bone beds of C. apertus are known in Alberta from the Dinosaur Park Formation of Dinosaur Provincial Park and the badlands adjacent to the South Saskatchewan River. Previously reported data from micro-vertebrate localities from the Belly River Group has been used to support a hypothesis that ceratopsids preferred a coastal environment; this hypothesis is also supported by the number of Centrosaurine bone beds within the Dinosaur Park Formation (20+ for C. apertus and at least 2 for Styracosaurus albertensis). However, the lack of any ceratopsid eggs or nests in any of the rich fossil localities in the Western Interior of North America implies that these dinosaurs may have had their nesting localities in dryer, more upland environments. The co-occurrence of C. apertus in time equivalent portions of the Oldman and Dinosaur Park formations suggests that the paleoecology of these animals may be more complex than previously believed. Although the high percentage of small C. apertus elements at the reported locality can be explained by a variety of taphonomic processes, it is suggestive that juvenile centrosaurs may have been more common in dryer environments, and this may be reflective of the relative proximity to their place of hatching.