HORNER, JOHN R.

Museum of the Rockies, Montana State University, Bozeman, MT 59717

Abstract: Dinosaur Biozones in the Campanian Clastic Wedge Complex of Montana, and their Biological Implications

Biostratigraphic analysis of the Upper Cretaceous (Campanian) Two Medicine/Judith River clastic wedge complex reveals discrete zonation of dinosaur assemblages with no evidence of vertical overlap. Five biozones (inclusive of both taxon-ranges and assemblage zones) are recognized in the upland Two Medicine Formation strata, and at least two biozones are identified in the lowland Judith River Formation. Biozonation resolution increases in younger strata, correlating with increased numbers of taxa, and greater numbers of specimens. In the top 100 meters of the Two Medicine Formation rapid, anagenetic species turnover correlates with seaway transgression.

Within each biozone dinosaur diversity is at the subfamily and family taxonomic levels. Only one representative of each group is present. There is no evidence of Serengeti-type diversity from the Upper Cretaceous coastal plain of Montana. Dinosaur faunas were less diverse, and species turnover much higher than previously hypothesized.

Comparing Montana and Alberta Campanian assemblages reveals that for the most part these two geographic areas are not time correlatives. The dinosaur assemblage found in the Dinosaur Park Formation is not represented in Montana.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana